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diff --git a/docs/.gitignore b/docs/.gitignore new file mode 100644 index 0000000..ca35be0 --- /dev/null +++ b/docs/.gitignore @@ -0,0 +1 @@ +_site diff --git a/docs/AUTOMATIC_BOOT_ASSESSMENT.md b/docs/AUTOMATIC_BOOT_ASSESSMENT.md new file mode 100644 index 0000000..f6d63af --- /dev/null +++ b/docs/AUTOMATIC_BOOT_ASSESSMENT.md @@ -0,0 +1,207 @@ +--- +title: Automatic Boot Assessment +category: Booting +layout: default +--- + +# Automatic Boot Assessment + +systemd provides support for automatically reverting back to the previous +version of the OS or kernel in case the system consistently fails to boot. This +support is built into various of its components. When used together these +components provide a complete solution on UEFI systems, built as add-on to the +[Boot Loader Specification](https://systemd.io/BOOT_LOADER_SPECIFICATION). +However, the different components may also be used independently, and in +combination with other software, to implement similar schemes, for example with +other boot loaders or for non-UEFI systems. Here's a brief overview of the +complete set of components: + +* The + [`systemd-boot(7)`](https://www.freedesktop.org/software/systemd/man/systemd-boot.html) + boot loader optionally maintains a per-boot-loader-entry counter that is + decreased by one on each attempt to boot the entry, prioritizing entries that + have non-zero counters over those which already reached a counter of zero + when choosing the entry to boot. + +* The + [`systemd-bless-boot.service(8)`](https://www.freedesktop.org/software/systemd/man/systemd-bless-boot.service.html) + service automatically marks a boot loader entry, for which boot counting as + mentioned above is enabled, as "good" when a boot has been determined to be + successful, thus turning off boot counting for it. + +* The + [`systemd-bless-boot-generator(8)`](https://www.freedesktop.org/software/systemd/man/systemd-bless-boot-generator.html) + generator automatically pulls in `systemd-bless-boot.service` when use of + `systemd-boot` with boot counting enabled is detected. + +* The + [`systemd-boot-check-no-failures.service(8)`](https://www.freedesktop.org/software/systemd/man/systemd-boot-check-no-failures.service.html) + service is a simple health check tool that determines whether the boot + completed successfully. When enabled it becomes an indirect dependency of + `systemd-bless-boot.service` (by means of `boot-complete.target`, see + below), ensuring that the boot will not be considered successful if there are + any failed services. + +* The `boot-complete.target` target unit (see + [`systemd.special(7)`](https://www.freedesktop.org/software/systemd/man/systemd.special.html)) + serves as a generic extension point both for units that are necessary to + consider a boot successful (example: `systemd-boot-check-no-failures.service` + as described above), and units that want to act only if the boot is + successful (example: `systemd-bless-boot.service` as described above). + +* The + [`kernel-install(8)`](https://www.freedesktop.org/software/systemd/man/kernel-install.html) + script can optionally create boot loader entries that carry an initial boot + counter (the initial counter is configurable in `/etc/kernel/tries`). + +## Details + +The boot counting data `systemd-boot` and `systemd-bless-boot.service` +manage is stored in the name of the boot loader entries. If a boot loader entry +file name contains `+` followed by one or two numbers (if two numbers, then +those need to be separated by `-`) right before the `.conf` suffix, then boot +counting is enabled for it. The first number is the "tries left" counter +encoding how many attempts to boot this entry shall still be made. The second +number is the "tries done" counter, encoding how many failed attempts to boot +it have already been made. Each time a boot loader entry marked this way is +booted the first counter is decreased by one, and the second one increased by +one. (If the second counter is missing, then it is assumed to be equivalent to +zero.) If the "tries left" counter is above zero the entry is still considered +for booting (the entry's state is considered to be "indeterminate"), as soon as +it reached zero the entry is not tried anymore (entry state "bad"). If the boot +attempt completed successfully the entry's counters are removed from the name +(entry state "good"), thus turning off boot counting for the future. + +## Walkthrough + +Here's an example walkthrough of how this all fits together. + +1. The user runs `echo 3 > /etc/kernel/tries` to enable boot counting. + +2. A new kernel is installed. `kernel-install` is used to generate a new boot + loader entry file for it. Let's say the version string for the new kernel is + `4.14.11-300.fc27.x86_64`, a new boot loader entry + `/boot/loader/entries/4.14.11-300.fc27.x86_64+3.conf` is hence created. + +3. The system is booted for the first time after the new kernel is + installed. The boot loader now sees the `+3` counter in the entry file + name. It hence renames the file to `4.14.11-300.fc27.x86_64+2-1.conf` + indicating that at this point one attempt has started and thus only one less + is left. After the rename completed the entry is booted as usual. + +4. Let's say this attempt to boot fails. On the following boot the boot loader + will hence see the `+2-1` tag in the name, and hence rename the entry file to + `4.14.11-300.fc27.x86_64+1-2.conf`, and boot it. + +5. Let's say the boot fails again. On the subsequent boot the loader hence will + see the `+1-2` tag, and rename the file to + `4.14.11-300.fc27.x86_64+0-3.conf` and boot it. + +6. If this boot also fails, on the next boot the boot loader will see the + tag `+0-3`, i.e. the counter reached zero. At this point the entry will be + considered "bad", and ordered to the beginning of the list of entries. The + next newest boot entry is now tried, i.e. the system automatically reverted + back to an earlier version. + +The above describes the walkthrough when the selected boot entry continuously +fails. Let's have a look at an alternative ending to this walkthrough. In this +scenario the first 4 steps are the same as above: + +1. *as above* + +2. *as above* + +3. *as above* + +4. *as above* + +5. Let's say the second boot succeeds. The kernel initializes properly, systemd + is started and invokes all generators. + +6. One of the generators started is `systemd-bless-boot-generator` which + detects that boot counting is used. It hence pulls + `systemd-bless-boot.service` into the initial transaction. + +7. `systemd-bless-boot.service` is ordered after and `Requires=` the generic + `boot-complete.target` unit. This unit is hence also pulled into the initial + transaction. + +8. The `boot-complete.target` unit is ordered after and pulls in various units + that are required to succeed for the boot process to be considered + successful. One such unit is `systemd-boot-check-no-failures.service`. + +9. `systemd-boot-check-no-failures.service` is run after all its own + dependencies completed, and assesses that the boot completed + successfully. It hence exits cleanly. + +10. This allows `boot-complete.target` to be reached. This signifies to the + system that this boot attempt shall be considered successful. + +11. Which in turn permits `systemd-bless-boot.service` to run. It now + determines which boot loader entry file was used to boot the system, and + renames it dropping the counter tag. Thus + `4.14.11-300.fc27.x86_64+1-2.conf` is renamed to + `4.14.11-300.fc27.x86_64.conf`. From this moment boot counting is turned + off. + +12. On the following boot (and all subsequent boots after that) the entry is + now seen with boot counting turned off, no further renaming takes place. + +## How to adapt this scheme to other setups + +Of the stack described above many components may be replaced or augmented. Here +are a couple of recommendations. + +1. To support alternative boot loaders in place of `systemd-boot` two scenarios + are recommended: + + a. Boot loaders already implementing the Boot Loader Specification can simply + implement an equivalent file rename based logic, and thus integrate fully + with the rest of the stack. + + b. Boot loaders that want to implement boot counting and store the counters + elsewhere can provide their own replacements for + `systemd-bless-boot.service` and `systemd-bless-boot-generator`, but should + continue to use `boot-complete.target` and thus support any services + ordered before that. + +2. To support additional components that shall succeed before the boot is + considered successful, simply place them in units (if they aren't already) + and order them before the generic `boot-complete.target` target unit, + combined with `Requires=` dependencies from the target, so that the target + cannot be reached when any of the units fail. You may add any number of + units like this, and only if they all succeed the boot entry is marked as + good. Note that the target unit shall pull in these boot checking units, not + the other way around. + +3. To support additional components that shall only run on boot success, simply + wrap them in a unit and order them after `boot-complete.target`, pulling it + in. + +## FAQ + +1. *Why do you use file renames to store the counter? Why not a regular file?* + — Mainly two reasons: it's relatively likely that renames can be implemented + atomically even in simpler file systems, while writing to file contents has + a much bigger chance to be result in incomplete or corrupt data, as renaming + generally avoids allocating or releasing data blocks. Moreover it has the + benefit that the boot count metadata is directly attached to the boot loader + entry file, and thus the lifecycle of the metadata and the entry itself are + bound together. This means no additional clean-up needs to take place to + drop the boot loader counting information for an entry when it is removed. + +2. *Why not use EFI variables for storing the boot counter?* — The memory chips + used to back the persistent EFI variables are generally not of the highest + quality, hence shouldn't be written to more than necessary. This means we + can't really use it for changes made regularly during boot, but can use it + only for seldom made configuration changes. + +3. *I have a service which — when it fails — should immediately cause a + reboot. How does that fit in with the above?* — Well, that's orthogonal to + the above, please use `FailureAction=` in the unit file for this. + +4. *Under some condition I want to mark the current boot loader entry as bad + right-away, so that it never is tried again, how do I do that?* — You may + invoke `/usr/lib/systemd/systemd-bless-boot bad` at any time to mark the + current boot loader entry as "bad" right-away so that it isn't tried again + on later boots. diff --git a/docs/BLOCK_DEVICE_LOCKING.md b/docs/BLOCK_DEVICE_LOCKING.md new file mode 100644 index 0000000..82df155 --- /dev/null +++ b/docs/BLOCK_DEVICE_LOCKING.md @@ -0,0 +1,80 @@ +--- +title: Locking Block Device Access +category: Interfaces +layout: default +--- + +# Locking Block Device Access + +*TL;DR: Use BSD file locks +[(`flock(2)`)](http://man7.org/linux/man-pages/man2/flock.2.html) on block +device nodes to synchronize access for partitioning and file system formatting +tools.* + +`systemd-udevd` probes all block devices showing up for file system superblock +and partition table information (utilizing `libblkid`). If another program +concurrently modifies a superblock or partition table this probing might be +affected, which is bad in itself, but also might in turn result in undesired +effects in programs subscribing to `udev` events. + +Applications manipulating a block device can temporarily stop `systemd-udevd` +from processing rules on it — and thus bar it from probing the device — by +taking a BSD file lock on the block device node. Specifically, whenever +`systemd-udevd` starts processing a block device it takes a `LOCK_SH|LOCK_NB` +lock using [`flock(2)`](http://man7.org/linux/man-pages/man2/flock.2.html) on +the main block device (i.e. never on any partition block device, but on the +device the partition belongs to). If this lock cannot be taken (i.e. `flock()` +returns `EBUSY`), it refrains from processing the device. If it manages to take +the lock it is kept for the entire time the device is processed. + +Note that `systemd-udevd` also watches all block device nodes it manages for +`inotify()` `IN_CLOSE_WRITE` events: whenever such an event is seen, this is +used as trigger to re-run the rule-set for the device. + +These two concepts allow tools such as disk partitioners or file system +formatting tools to safely and easily take exclusive ownership of a block +device while operating: before starting work on the block device, they should +take an `LOCK_EX` lock on it. This has two effects: first of all, in case +`systemd-udevd` is still processing the device the tool will wait for it to +finish. Second, after the lock is taken, it can be sure that `systemd-udevd` +will refrain from processing the block device, and thus all other client +applications subscribed to it won't get device notifications from potentially +half-written data either. After the operation is complete the +partitioner/formatter can simply close the device node. This has two effects: +it implicitly releases the lock, so that `systemd-udevd` can process events on +the device node again. Secondly, it results an `IN_CLOSE_WRITE` event, which +causes `systemd-udevd` to immediately re-process the device — seeing all +changes the tool made — and notify subscribed clients about it. + +Ideally, `systemd-udevd` would explicitly watch block devices for `LOCK_EX` +locks being released. Such monitoring is not supported on Linux however, which +is why it watches for `IN_CLOSE_WRITE` instead, i.e. for `close()` calls to +writable file descriptors referring to the block device. In almost all cases, +the difference between these two events does not matter much, as any locks +taken are implicitly released by `close()`. However, it should be noted that if +an application unlocks a device after completing its work without closing it, +i.e. while keeping the file descriptor open for further, longer time, then +`systemd-udevd` will not notice this and not retrigger and thus reprobe the +device. + +Besides synchronizing block device access between `systemd-udevd` and such +tools this scheme may also be used to synchronize access between those tools +themselves. However, do note that `flock()` locks are advisory only. This means +if one tool honours this scheme and another tool does not, they will of course +not be synchronized properly, and might interfere with each other's work. + +Note that the file locks follow the usual access semantics of BSD locks: since +`systemd-udevd` never writes to such block devices it only takes a `LOCK_SH` +*shared* lock. A program intending to make changes to the block device should +take a `LOCK_EX` *exclusive* lock instead. For further details, see the +`flock(2)` man page. + +And please keep in mind: BSD file locks (`flock()`) and POSIX file locks +(`lockf()`, `F_SETLK`, …) are different concepts, and in their effect +orthogonal. The scheme discussed above uses the former and not the latter, +because these types of locks more closely match the required semantics. + +Summarizing: it is recommended to take `LOCK_EX` BSD file locks when +manipulating block devices in all tools that change file system block devices +(`mkfs`, `fsck`, …) or partition tables (`fdisk`, `parted`, …), right after +opening the node. diff --git a/docs/BOOT_LOADER_INTERFACE.md b/docs/BOOT_LOADER_INTERFACE.md new file mode 100644 index 0000000..15a575d --- /dev/null +++ b/docs/BOOT_LOADER_INTERFACE.md @@ -0,0 +1,152 @@ +--- +title: Boot Loader Interface +category: Booting +layout: default +--- + +# The Boot Loader Interface + +systemd can interface with the boot loader to receive performance data and +other information, and pass control information. This is only supported on EFI +systems. Data is transferred between the boot loader and systemd in EFI +variables. All EFI variables use the vendor UUID +`4a67b082-0a4c-41cf-b6c7-440b29bb8c4f`. + +* The EFI Variable `LoaderTimeInitUSec` contains the timestamp in microseconds + when the loader was initialized. This value is the time spent in the firmware + for initialization, it is formatted as numeric, NUL-terminated, decimal + string, in UTF-16. + +* The EFI Variable `LoaderTimeExecUSec` contains the timestamp in microseconds + when the loader finished its work and is about to execute the kernel. The + time spent in the loader is the difference between `LoaderTimeExecUSec` and + `LoaderTimeInitUSec`. This value is formatted the same way as + `LoaderTimeInitUSec`. + +* The EFI variable `LoaderDevicePartUUID` contains the partition GUID of the + ESP the boot loader was run from formatted as NUL-terminated UTF16 string, in + normal GUID syntax. + +* The EFI variable `LoaderConfigTimeout` contains the boot menu timeout + currently in use. It may be modified both by the boot loader and by the + host. The value should be formatted as numeric, NUL-terminated, decimal + string, in UTF-16. The time is specified in µs. + +* Similarly, the EFI variable `LoaderConfigTimeoutOneShot` contains a boot menu + timeout for a single following boot. It is set by the OS in order to request + display of the boot menu on the following boot. When set overrides + `LoaderConfigTimeout`. It is removed automatically after being read by the + boot loader, to ensure it only takes effect a single time. This value is + formatted the same way as `LoaderConfigTimeout`. If set to `0` the boot menu + timeout is turned off, and the menu is shown indefinitely. + +* The EFI variable `LoaderEntries` may contain a series of boot loader entry + identifiers, one after the other, each individually NUL terminated. This may + be used to let the OS know which boot menu entries were discovered by the + boot loader. A boot loader entry identifier should be a short, non-empty + alphanumeric string (possibly containing `-`, too). The list should be in the + order the entries are shown on screen during boot. See below regarding a + recommended vocabulary for boot loader entry identifiers. + +* The EFI variable `LoaderEntryDefault` contains the default boot loader entry + to use. It contains a NUL-terminated boot loader entry identifier. + +* Similarly, the EFI variable `LoaderEntryOneShot` contains the default boot + loader entry to use for a single following boot. It is set by the OS in order + to request booting into a specific menu entry on the following boot. When set + overrides `LoaderEntryDefault`. It is removed automatically after being read + by the boot loader, to ensure it only takes effect a single time. This value + is formatted the same way as `LoaderEntryDefault`. + +* The EFI variable `LoaderEntrySelected` contains the boot loader entry + identifier that was booted. It is set by the boot loader and read by + the OS in order to identify which entry has been used for the current boot. + +* The EFI variable `LoaderFeatures` contains a 64bit unsigned integer with a + number of flags bits that are set by the boot loader and passed to the OS and + indicate the features the boot loader supports. Specifically, the following + bits are defined: + + * `1 << 0` → The boot loader honours `LoaderConfigTimeout` when set. + * `1 << 1` → The boot loader honours `LoaderConfigTimeoutOneShot` when set. + * `1 << 2` → The boot loader honours `LoaderEntryDefault` when set. + * `1 << 3` → The boot loader honours `LoaderEntryOneShot` when set. + * `1 << 4` → The boot loader supports boot counting as described in [Automatic Boot Assessment](https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT). + * `1 << 5` → The boot loader supports looking for boot menu entries in the Extended Boot Loader Partition. + * `1 << 6` → The boot loader supports passing a random seed to the OS. + +* The EFI variable `LoaderRandomSeed` contains a binary random seed if set. It + is set by the boot loader to pass an entropy seed read from the ESP partition + to the OS. The system manager then credits this seed to the kernel's entropy + pool. It is the responsibility of the boot loader to ensure the quality and + integrity of the random seed. + +* The EFI variable `LoaderSystemToken` contains binary random data, + persistently set by the OS installer. Boot loaders that support passing + random seeds to the OS should use this data and combine it with the random + seed file read from the ESP. By combining this random data with the random + seed read off the disk before generating a seed to pass to the OS and a new + seed to store in the ESP the boot loader can protect itself from situations + where "golden" OS images that include a random seed are replicated and used + on multiple systems. Since the EFI variable storage is usually independent + (i.e. in physical NVRAM) of the ESP file system storage, and only the latter + is part of "golden" OS images, this ensures that different systems still come + up with different random seeds. Note that the `LoaderSystemToken` is + generally only written once, by the OS installer, and is usually not touched + after that. + +If `LoaderTimeInitUSec` and `LoaderTimeExecUSec` are set, `systemd-analyze` +will include them in its boot-time analysis. If `LoaderDevicePartUUID` is set, +systemd will mount the ESP that was used for the boot to `/boot`, but only if +that directory is empty, and only if no other file systems are mounted +there. The `systemctl reboot --boot-loader-entry=…` and `systemctl reboot +--boot-loader-menu=…` commands rely on the `LoaderFeatures` , +`LoaderConfigTimeoutOneShot`, `LoaderEntries`, `LoaderEntryOneShot` +variables. `LoaderRandomSeed` is read by PID during early boot and credited to +the kernel's random pool. + +## Boot Loader Entry Identifiers + +While boot loader entries may be named relatively freely, it's highly +recommended to follow the following rules when picking identifiers for the +entries, so that programs (and users) can derive basic context and meaning from +the identifiers as passed in `LoaderEntries`, `LoaderEntryDefault`, +`LoaderEntryOneShot`, `LoaderEntrySelected`, and possibly show nicely localized +names for them in UIs. + +1. When boot loader entries are defined through [Boot Loader + Specification](https://systemd.io/BOOT_LOADER_SPECIFICATION) drop-in files + the identifier should be derived directly from the drop-in snippet name, but + with the `.conf` (or `.efi` in case of Type #2 entries) suffix removed. + +2. Entries automatically discovered by the boot loader (as opposed to being + configured in configuration files) should generally have an identifier + prefixed with `auto-`. + +3. Boot menu entries referring to Microsoft Windows installations should either + use the identifier `windows` or use the `windows-` prefix for the + identifier. If a menu entry is automatically discovered, it should be + prefixed with `auto-`, see above (Example: this means an automatically + discovered Windows installation might have the identifier `auto-windows` or + `auto-windows-10` or so.). + +4. Similar, boot menu entries referring to Apple MacOS X installations should + use the identifier `osx` or one that is prefixed with `osx-`. If such an + entry is automatically discovered by the boot loader use `auto-osx` as + identifier, or `auto-osx-` as prefix for the identifier, see above. + +5. If a boot menu entry encapsulates the EFI shell program, it should use the + identifier `efi-shell` (or when automatically discovered: `auto-efi-shell`, + see above). + +6. If a boot menu entry encapsulates a reboot into EFI firmware setup feature, + it should use the identifier `reboot-to-firmware-setup` (or + `auto-reboot-to-firmware-setup` in case it is automatically discovered). + +## Links + +[Boot Loader Specification](https://systemd.io/BOOT_LOADER_SPECIFICATION)<br> +[Discoverable Partitions Specification](https://systemd.io/DISCOVERABLE_PARTITIONS)<br> +[systemd-boot(7)](https://www.freedesktop.org/software/systemd/man/systemd-boot.html)<br> +[bootctl(1)](https://www.freedesktop.org/software/systemd/man/bootctl.html)<br> +[systemd-gpt-auto-generator(8)](https://www.freedesktop.org/software/systemd/man/systemd-gpt-auto-generator.html) diff --git a/docs/BOOT_LOADER_SPECIFICATION.md b/docs/BOOT_LOADER_SPECIFICATION.md new file mode 100644 index 0000000..2909771 --- /dev/null +++ b/docs/BOOT_LOADER_SPECIFICATION.md @@ -0,0 +1,262 @@ +--- +title: Boot Loader Specification +category: Booting +layout: default +--- + +# The Boot Loader Specification + +_TL;DR: Currently there's no common boot scheme across architectures and +platforms for open-source operating systems. There's also little cooperation +between multiple distributions in dual-boot (or triple, … multi-boot) +setups. We'd like to improve this situation by getting everybody to commit to a +single boot configuration format that is based on drop-in files, and thus is +robust, simple, works without rewriting configuration files and is free of +namespace clashes._ + +The Boot Loader Specification defines a scheme how different operating systems +can cooperatively manage a boot loader configuration directory, that accepts +drop-in files for boot menu items that are defined in a format that is shared +between various boot loader implementations, operating systems, and userspace +programs. The same scheme can be used to prepare OS media for cases where the +firmware includes a boot loader. The target audience for this specification is: + +* Boot loader developers, to write a boot loader that directly reads its configuration at runtime from these drop-in snippets +* Firmware developers, to add generic boot loading support directly to the firmware itself +* Distribution and Core OS developers, in order to create these snippets at OS/kernel package installation time +* UI developers, for implementing a user interface that discovers the available boot options +* OS Installer developers, to prepare their installation media and for setting up the initial drop-in directory + +## Why is there a need for this specification? + +Of course, without this specification things already work mostly fine. But here's why we think this specification is needed: + +* To make the boot more robust, as no explicit rewriting of configuration files is required any more +* To allow an out of the box boot experience on any platform without the need of traditional firmware mechanisms (e.g. BIOS calls, UEFI Boot Services) +* To improve dual-boot scenarios. Currently, multiple Linux installations tend to fight over which boot loader becomes the primary one in possession of the MBR, and only that one installation can then update the boot loader configuration of it freely. Other Linux installs have to be manually configured to never touch the MBR and instead install a chain-loaded boot loader in their own partition headers. In this new scheme as all installations share a loader directory no manual configuration has to take place, and all participants implicitly cooperate due to removal of name collisions and can install/remove their own boot menu entries at free will, without interfering with the entries of other installed operating systems. +* Drop-in directories are otherwise now pretty ubiquitous on Linux as an easy way to extend configuration without having to edit, regenerate or manipulate configuration files. For the sake of uniformity, we should do the same for extending the boot menu. +* Userspace code can sanely parse boot loader configuration which is essential with modern BIOSes which do not necessarily initialize USB keyboards anymore during boot, which makes boot menus hard to reach for the user. If userspace code can parse the boot loader configuration, too, this allows for UIs that can select a boot menu item to boot into, before rebooting the machine, thus not requiring interactivity during early boot. +* To unify and thus simplify configuration of the various boot loaders around, which makes configuration of the boot loading process easier for users, administrators and developers alike. +* For boot loaders with configuration _scripts_ such as grub2, adopting this spec allows for mostly static scripts that are generated only once at first installation, but then do not need to be updated anymore as that is done via drop-in files exclusively. + +## Why not simply rely on the EFI boot menu logic? + +EFI is not ubiquitous, especially not in embedded systems. If you have an EFI +system, it provides a boot options logic that can offer similar +functionality. Here's why we think that it is not enough for our uses: + +* The various EFI implementations implement the boot order/boot item logic to different levels. Some firmware implementations do not offer a boot menu at all and instead unconditionally follow the EFI boot order, booting the first item that is working. +* If the firmware setup is used to reset all data usually all EFI boot entries are lost, making the system entirely unbootable, as the firmware setups generally do not offer a UI to define additional boot items. By placing the menu item information on disk, it is always available, regardless if the BIOS setup data is lost. +* Harddisk images should be movable between machines and be bootable without requiring explicit EFI variables to be set. This also requires that the list of boot options is defined on disk, and not in EFI variables alone. +* EFI is not universal yet (especially on non-x86 platforms), this specification is useful both for EFI and non-EFI boot loaders. +* Many EFI systems disable USB support during early boot to optimize boot times, thus making keyboard input unavailable in the EFI menu. It is thus useful if the OS UI has a standardized way to discover available boot options which can be booted to. + +## Technical Details + +Everything described below is located on a placeholder file system `$BOOT`. The installer program should pick `$BOOT` according to the following rules: + +* On disks with an MBR partition table: + * If the OS is installed on a disk with an MBR partition table, and a partition with the type id of 0xEA already exists it should be used as `$BOOT`. + * Otherwise, if the OS is installed on a disk with an MBR partition table, a new partition with type id of 0xEA shall be created, of a suitable size (let's say 500MB), and it should be used as `$BOOT`. +* On disks with GPT (GUID Partition Table) + * If the OS is installed on a disk with GPT, and an Extended Boot Loader Partition or XBOOTLDR partition for short, i.e. a partition with GPT type GUID of `bc13c2ff-59e6-4262-a352-b275fd6f7172`, already exists, it should be used as `$BOOT`. + * Otherwise, if the OS is installed on a disk with GPT, and an EFI System Partition or ESP for short, i.e. a partition with GPT type UID of `c12a7328-f81f-11d2-ba4b-00a0c93ec93b`) already exists and is large enough (let's say 250MB) and otherwise qualifies, it should be used as `$BOOT`. + * Otherwise, if the OS is installed on a disk with GPT, and if the ESP partition already exists but is too small, a new suitably sized (let's say 500MB) XBOOTLDR partition shall be created and used as `$BOOT`. + * Otherwise, if the OS is installed on a disk with GPT, and no ESP partition exists yet, a new suitably sized (let's say 500MB) ESP should be created and used as `$BOOT`. + +This placeholder file system shall be determined during _installation time_, and an fstab entry may be created. It should be mounted to either `/boot/` or `/efi/`. Additional locations like `/boot/efi/`, with `/boot/` being a separate file system, might be supported by implementations. This is not recommended because the mounting of `$BOOT` is then dependent on and requires the mounting of the intermediate file system. + +**Note:** _`$BOOT` should be considered **shared** among all OS installations of a system. Instead of maintaining one `$BOOT` per installed OS (as `/boot/` was traditionally handled), all installed OS share the same place to drop in their boot-time configuration._ + +For systems where the firmware is able to read file systems directly, `$BOOT` +must be a file system readable by the firmware. For other systems and generic +installation and live media, `$BOOT` must be a VFAT (16 or 32) file +system. Applications accessing `$BOOT` should hence not assume that fancier +file system features such as symlinks, hardlinks, access control or case +sensitivity are supported. + +This specification defines two types of boot loader entries. The first type is +text based, very simple and suitable for a variety of firmware, architecture +and image types ("Type #1"). The second type is specific to EFI, but allows +single-file images that embed all metadata in the kernel binary itself, which +is useful to cryptographically sign them as one file for the purpose of +SecureBoot ("Type #2"). + +Not all boot loader entries will apply to all systems. For example, Type #1 +entries that use the `efi` key and all Type #2 entries only apply to EFI +systems. Entries using the `architecture` key might specify an architecture that +doesn't match the local one. Boot loaders should ignore all entries that don't +match the local platform and what the boot loader can support, and hide them +from the user. Only entries matching the feature set of boot loader and system +shall be considered and displayed. This allows image builders to put together +images that transparently support multiple different architectures. + +Note that the `$BOOT` partition is not supposed to be exclusive territory of +this specification. This specification only defines semantics of the `/loader/` +directory inside the file system (see below), but it doesn't intend to define +ownership of the whole file system exclusively. Boot loaders, firmware, and +other software implementing this specification may choose to place other +files and directories in the same file system. For example, boot loaders that +implement this specification might install their own boot code into the `$BOOT` +partition. On systems where `$BOOT` is the ESP this is a particularly common +setup. Implementations of this specification must be able to operate correctly +if files or directories other than `/loader/` are found in the top level +directory. Implementations that add their own files or directories to the file +systems should use well-named directories, to make name collisions between +multiple users of the file system unlikely. + +### Type #1 Boot Loader Specification Entries + +We define two directories below `$BOOT`: + +* `$BOOT/loader/` is the directory containing all files needed for Type #1 entries +* `$BOOT/loader/entries/` is the directory containing the drop-in snippets. This directory contains one `.conf` file for each boot menu item. + +**Note:** _In all cases the `/loader/` directory should be located directly in the root of the file system. Specifically, if `$BOOT` is the ESP, then `/loader/` directory should be located directly in the root directory of the ESP, and not in the `/EFI/` subdirectory._ + +Inside the `$BOOT/loader/entries/` directory each OS vendor may drop one or +more configuration snippets with the suffix ".conf", one for each boot menu +item. The file name of the file is used for identification of the boot item but +shall never be presented to the user in the UI. The file name may be chosen +freely but should be unique enough to avoid clashes between OS +installations. More specifically it is suggested to include the machine ID +(`/etc/machine-id` or the D-Bus machine ID for OSes that lack +`/etc/machine-id`), the kernel version (as returned by `uname -r`) and an OS +identifier (The ID field of `/etc/os-release`). Example: +`$BOOT/loader/entries/6a9857a393724b7a981ebb5b8495b9ea-3.8.0-2.fc19.x86_64.conf`. + +In order to maximize compatibility with file system implementations and +restricted boot loader environments, and to minimize conflicting character use +with other programs, file names shall be chosen from a restricted character set: +ASCII upper and lower case characters, digits, "+", "-", "_" and ".". Also, the +file names should have a length of at least one and at most 255 characters +(including file name suffix). + +These configuration snippets shall be Unix-style text files (i.e. line separation with a single newline character), in the UTF-8 encoding. The configuration snippets are loosely inspired on Grub1's configuration syntax. Lines beginning with '#' shall be ignored and used for commenting. The first word of a line is used as key and shall be separated by one or more spaces from its value. The following keys are known: + +* `title` shall contain a human readable title string for this menu item. This will be displayed in the boot menu for the item. It is a good idea to initialize this from the `PRETTY_NAME` of `/etc/os-release`. This name should be descriptive and does not have to be unique. If a boot loader discovers two entries with the same title it is a good idea to show more than just the raw title in the UI, for example by appending the `version` field. This field is optional. Example: "Fedora 18 (Spherical Cow)". +* `version` shall contain a human readable version string for this menu item. This is usually the kernel version and is intended for use by OSes to install multiple kernel versions at the same time with the same `title` field. This field shall be in a syntax that is useful for Debian-style version sorts, so that the boot loader UI can determine the newest version easily and show it first or preselect it automatically. This field is optional. Example: `3.7.2-201.fc18.x86_64`. +* `machine-id` shall contain the machine ID of the OS `/etc/machine-id`. This is useful for boot loaders and applications to filter out boot entries, for example to show only a single newest kernel per OS, or to group items by OS, or to maybe filter out the currently booted OS in UIs that want to show only other installed operating systems. This ID shall be formatted as 32 lower case hexadecimal characters (i.e. without any UUID formatting). This key is optional. Example: `4098b3f648d74c13b1f04ccfba7798e8`. +* `linux` refers to the Linux kernel to spawn and shall be a path relative to the `$BOOT` directory. It is recommended that every distribution creates a machine id and version specific subdirectory below `$BOOT` and places its kernels and initial RAM disk images there. Example: `/6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/linux`. +* `initrd` refers to the initrd to use when executing the kernel. This also shall be a path relative to the `$BOOT` directory. This key is optional. This key may appear more than once in which case all specified images are used, in the order they are listed. Example: `6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/initrd`. +* `efi` refers to an arbitrary EFI program. This also takes a path relative to `$BOOT`. If this key is set, and the system is not an EFI system this entry should be hidden. +* `options` shall contain kernel parameters to pass to the Linux kernel to spawn. This key is optional and may appear more than once in which case all specified parameters are used in the order they are listed. +* `devicetree` refers to the binary device tree to use when executing the +kernel. This also shall be a path relative to the `$BOOT` directory. This +key is optional. Example: `6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.armv7hl/tegra20-paz00.dtb`. +* `devicetree-overlay` refers to a list of device tree overlays that should be +applied by the boot loader. Multiple overlays are separated by spaces and +applied in the same order as they are listed. This key is optional but depends +on the `devicetree` key. Example: +`/6a9857a393724b7a981ebb5b8495b9ea/overlays/overlay_A.dtbo +/6a9857a393724b7a981ebb5b8495b9ea/overlays/overlay_B.dtbo` +* `architecture` refers to the architecture this entry is defined for. The argument should be an architecture identifier, using the architecture vocabulary defined by the EFI specification (i.e. `IA32`, `x64`, `IA64`, `ARM`, `AA64`, …). If specified and this does not match (case insensitively) the local system architecture this entry should be hidden. + +Each configuration drop-in snippet must include at least a `linux` or an `efi` key and is otherwise not valid. Here's an example for a complete drop-in file: + + # /boot/loader/entries/6a9857a393724b7a981ebb5b8495b9ea-3.8.0-2.fc19.x86_64.conf + title Fedora 19 (Rawhide) + version 3.8.0-2.fc19.x86_64 + machine-id 6a9857a393724b7a981ebb5b8495b9ea + options root=UUID=6d3376e4-fc93-4509-95ec-a21d68011da2 + architecture x64 + linux /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/linux + initrd /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/initrd + +On EFI systems all Linux kernel images should be EFI images. In order to +increase compatibility with EFI systems it is highly recommended only to +install EFI kernel images, even on non-EFI systems, if that's applicable and +supported on the specific architecture. + +Conversely, in order to increase compatibility it is recommended to install +generic kernel images that make few assumptions about the firmware they run on, +i.e. it is a good idea that both images shipped as UEFI PE images and those +which are not don't make unnecessary assumption on the underlying firmware, +i.e. don't hard depend on legacy BIOS calls or UEFI boot services. + +Note that these configuration snippets may only reference kernels (and EFI programs) that reside on the same file system as the configuration snippets, i.e. everything referenced must be contained in the same file system. This is by design, as referencing other partitions or devices would require a non-trivial language for denoting device paths. If kernels/initrds are to be read from other partitions/disks the boot loader can do this in its own native configuration, using its own specific device path language, and this is out of focus for this specification. More specifically, on non-EFI systems configuration snippets following this specification cannot be used to spawn other operating systems (such as Windows). + +### Type #2 EFI Unified Kernel Images + +A unified kernel image is a single EFI PE executable combining an EFI stub +loader, a kernel image, an initramfs image, and the kernel command line. See +the description of the `--uefi` option in +[dracut(8)](http://man7.org/linux/man-pages/man8/dracut.8.html). Such unified +images will be searched for under `$BOOT/EFI/Linux/` and must have the +extension `.efi`. Support for images of this type is of course specific to +systems with EFI firmware. Ignore this section if you work on systems not +supporting EFI. + +Type #2 file names should be chosen from the same restricted character set as +Type #1 described above (but use a different file name suffix of `.efi` instead +of `.conf`). + +Images of this type have the advantage that all metadata and payload that makes +up the boot entry is monopolized in a single PE file that can be signed +cryptographically as one for the purpose of EFI SecureBoot. + +A valid unified kernel image must contain two PE sections: + +* `.cmdline` section with the kernel command line +* `.osrel` section with an embedded copy of the [os-release](https://www.freedesktop.org/software/systemd/man/os-release.html) file describing the image + +The `PRETTY_NAME=` and `VERSION_ID=` fields in the embedded os-release file are used the same as `title` and `version` in the "boot loader specification" entries. The `.cmdline` section is used instead of the `options` field. `linux` and `initrd` fields are not necessary, and there is no counterpart for the `machine-id` field. + +On EFI, any such images shall be added to the list of valid boot entries. + +### Additional notes + +Note that these configurations snippets do not need to be the only configuration source for a boot loader. It may extend this list of entries with additional items from other configuration files (for example its own native configuration files) or automatically detected other entries without explicit configuration. + +To make this explicitly clear: this specification is designed with "free" operating systems in mind, starting Windows or MacOS is out of focus with these configuration snippets, use boot-loader specific solutions for that. In the text above, if we say "OS" we hence imply "free", i.e. primarily Linux (though this could be easily be extended to the BSDs and whatnot). + +Note that all paths used in the configuration snippets use a Unix-style "/" as path separator. This needs to be converted to an EFI-style "\" separator in EFI boot loaders. + + +## Logic + +A _boot loader_ needs a file system driver to discover and read `$BOOT`, then +simply reads all files `$BOOT/loader/entries/*.conf`, and populates its boot +menu with this. On EFI, it then extends this with any unified kernel images +found in `$BOOT/EFI/Linux/*.efi`. It may also add additional entries, for +example a "Reboot into firmware" option. Optionally it may sort the menu based +on the `machine-id` and `version` fields, and possibly others. It uses the file +name to identify specific items, for example in case it supports storing away +default entry information somewhere. A boot loader should generally not modify +these files. + +For "Boot Loader Specification Entries" (Type #1), the _kernel package +installer_ installs the kernel and initrd images to `$BOOT` (it is recommended +to place these files in a vendor and OS and installation specific directory) +and then generates a configuration snippet for it, placing this in +`$BOOT/loader/entries/xyz.conf`, with xyz as concatenation of machine id and +version information (see above). The files created by a kernel package are +private property of the kernel package and should be removed along with it. + +For "EFI Unified Kernel Images" (Type #2), the vendor or kernel package +installer creates the combined image and drops it into `$BOOT/EFI/Linux/`. This +file is also private property of the kernel package and should be removed along +with it. + +A _UI application_ intended to show available boot options shall operate similar to a boot loader, but might apply additional filters, for example by filtering out the booted OS via the machine ID, or by suppressing all but the newest kernel versions. + +An _OS installer_ picks the right place for `$BOOT` as defined above (possibly creating a partition and file system for it) and pre-creates the `/loader/entries/` directory in it. It then installs an appropriate boot loader that can read these snippets. Finally, it installs one or more kernel packages. + + +## Out of Focus + +There are a couple of items that are out of focus for this specification: + +* If userspace can figure out the available boot options, then this is only useful so much: we'd still need to come up with a way how userspace could communicate to the boot loader the default boot loader entry temporarily or persistently. Defining a common scheme for this is certainly a good idea, but out of focus for this specification. +* This specification is just about "Free" Operating systems. Hooking in other operating systems (like Windows and macOS) into the boot menu is a different story and should probably happen outside of this specification. For example, boot loaders might choose to detect other available OSes dynamically at runtime without explicit configuration (like `systemd-boot` does it), or via native configuration (for example via explicit Grub2 configuration generated once at installation). +* This specification leaves undefined what to do about systems which are upgraded from an OS that does not implement this specification. As the previous boot loader logic was largely handled by in distribution-specific ways we probably should leave the upgrade path (and whether there actually is one) to the distributions. The simplest solution might be to simply continue with the old scheme for old installations and use this new scheme only for new installations. + + +## Links + +[GUID Partition Table](https://en.wikipedia.org/wiki/GUID_Partition_Table)<br> +[Boot Loader Interface](https://systemd.io/BOOT_LOADER_INTERFACE)<br> +[Discoverable Partitions Specification](https://systemd.io/DISCOVERABLE_PARTITIONS)<br> +[systemd-boot(7)](https://www.freedesktop.org/software/systemd/man/systemd-boot.html)<br> +[bootctl(1)](https://www.freedesktop.org/software/systemd/man/bootctl.html)<br> +[systemd-gpt-auto-generator(8)](https://www.freedesktop.org/software/systemd/man/systemd-gpt-auto-generator.html) diff --git a/docs/CGROUP_DELEGATION.md b/docs/CGROUP_DELEGATION.md new file mode 100644 index 0000000..4011f09 --- /dev/null +++ b/docs/CGROUP_DELEGATION.md @@ -0,0 +1,481 @@ +--- +title: Control Group APIs and Delegation +category: Interfaces +layout: default +--- + +# Control Group APIs and Delegation + +*Intended audience: hackers working on userspace subsystems that require direct +cgroup access, such as container managers and similar.* + +So you are wondering about resource management with systemd, you know Linux +control groups (cgroups) a bit and are trying to integrate your software with +what systemd has to offer there. Here's a bit of documentation about the +concepts and interfaces involved with this. + +What's described here has been part of systemd and documented since v205 +times. However, it has been updated and improved substantially, even +though the concepts stayed mostly the same. This is an attempt to provide more +comprehensive up-to-date information about all this, particular in light of the +poor implementations of the components interfacing with systemd of current +container managers. + +Before you read on, please make sure you read the low-level kernel +documentation about the +[unified cgroup hierarchy](https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html). +This document then adds in the higher-level view from systemd. + +This document augments the existing documentation we already have: + +* [The New Control Group Interfaces](https://www.freedesktop.org/wiki/Software/systemd/ControlGroupInterface/) +* [Writing VM and Container Managers](https://www.freedesktop.org/wiki/Software/systemd/writing-vm-managers/) + +These wiki documents are not as up to date as they should be, currently, but +the basic concepts still fully apply. You should read them too, if you do something +with cgroups and systemd, in particular as they shine more light on the various +D-Bus APIs provided. (That said, sooner or later we should probably fold that +wiki documentation into this very document, too.) + +## Two Key Design Rules + +Much of the philosophy behind these concepts is based on a couple of basic +design ideas of cgroup v2 (which we however try to adapt as far as we can to +cgroup v1 too). Specifically two cgroup v2 rules are the most relevant: + +1. The **no-processes-in-inner-nodes** rule: this means that it's not permitted +to have processes directly attached to a cgroup that also has child cgroups and +vice versa. A cgroup is either an inner node or a leaf node of the tree, and if +it's an inner node it may not contain processes directly, and if it's a leaf +node then it may not have child cgroups. (Note that there are some minor +exceptions to this rule, though. E.g. the root cgroup is special and allows +both processes and children — which is used in particular to maintain kernel +threads.) + +2. The **single-writer** rule: this means that each cgroup only has a single +writer, i.e. a single process managing it. It's OK if different cgroups have +different processes managing them. However, only a single process should own a +specific cgroup, and when it does that ownership is exclusive, and nothing else +should manipulate it at the same time. This rule ensures that various pieces of +software don't step on each other's toes constantly. + +These two rules have various effects. For example, one corollary of this is: if +your container manager creates and manages cgroups in the system's root cgroup +you violate rule #2, as the root cgroup is managed by systemd and hence off +limits to everybody else. + +Note that rule #1 is generally enforced by the kernel if cgroup v2 is used: as +soon as you add a process to a cgroup it is ensured the rule is not +violated. On cgroup v1 this rule didn't exist, and hence isn't enforced, even +though it's a good thing to follow it then too. Rule #2 is not enforced on +either cgroup v1 nor cgroup v2 (this is UNIX after all, in the general case +root can do anything, modulo SELinux and friends), but if you ignore it you'll +be in constant pain as various pieces of software will fight over cgroup +ownership. + +Note that cgroup v1 is currently the most deployed implementation, even though +it's semantically broken in many ways, and in many cases doesn't actually do +what people think it does. cgroup v2 is where things are going, and most new +kernel features in this area are only added to cgroup v2, and not cgroup v1 +anymore. For example cgroup v2 provides proper cgroup-empty notifications, has +support for all kinds of per-cgroup BPF magic, supports secure delegation of +cgroup trees to less privileged processes and so on, which all are not +available on cgroup v1. + +## Three Different Tree Setups 🌳 + +systemd supports three different modes how cgroups are set up. Specifically: + +1. **Unified** — this is the simplest mode, and exposes a pure cgroup v2 +logic. In this mode `/sys/fs/cgroup` is the only mounted cgroup API file system +and all available controllers are exclusively exposed through it. + +2. **Legacy** — this is the traditional cgroup v1 mode. In this mode the +various controllers each get their own cgroup file system mounted to +`/sys/fs/cgroup/<controller>/`. On top of that systemd manages its own cgroup +hierarchy for managing purposes as `/sys/fs/cgroup/systemd/`. + +3. **Hybrid** — this is a hybrid between the unified and legacy mode. It's set +up mostly like legacy, except that there's also an additional hierarchy +`/sys/fs/cgroup/unified/` that contains the cgroup v2 hierarchy. (Note that in +this mode the unified hierarchy won't have controllers attached, the +controllers are all mounted as separate hierarchies as in legacy mode, +i.e. `/sys/fs/cgroup/unified/` is purely and exclusively about core cgroup v2 +functionality and not about resource management.) In this mode compatibility +with cgroup v1 is retained while some cgroup v2 features are available +too. This mode is a stopgap. Don't bother with this too much unless you have +too much free time. + +To say this clearly, legacy and hybrid modes have no future. If you develop +software today and don't focus on the unified mode, then you are writing +software for yesterday, not tomorrow. They are primarily supported for +compatibility reasons and will not receive new features. Sorry. + +Superficially, in legacy and hybrid modes it might appear that the parallel +cgroup hierarchies for each controller are orthogonal from each other. In +systemd they are not: the hierarchies of all controllers are always kept in +sync (at least mostly: sub-trees might be suppressed in certain hierarchies if +no controller usage is required for them). The fact that systemd keeps these +hierarchies in sync means that the legacy and hybrid hierarchies are +conceptually very close to the unified hierarchy. In particular this allows us +to talk of one specific cgroup and actually mean the same cgroup in all +available controller hierarchies. E.g. if we talk about the cgroup `/foo/bar/` +then we actually mean `/sys/fs/cgroup/cpu/foo/bar/` as well as +`/sys/fs/cgroup/memory/foo/bar/`, `/sys/fs/cgroup/pids/foo/bar/`, and so on. +Note that in cgroup v2 the controller hierarchies aren't orthogonal, hence +thinking about them as orthogonal won't help you in the long run anyway. + +If you wonder how to detect which of these three modes is currently used, use +`statfs()` on `/sys/fs/cgroup/`. If it reports `CGROUP2_SUPER_MAGIC` in its +`.f_type` field, then you are in unified mode. If it reports `TMPFS_MAGIC` then +you are either in legacy or hybrid mode. To distinguish these two cases, run +`statfs()` again on `/sys/fs/cgroup/unified/`. If that succeeds and reports +`CGROUP2_SUPER_MAGIC` you are in hybrid mode, otherwise not. +From a shell, you can use check the `Type` in `stat -f /sys/fs/cgroup` and +`stat -f /sys/fs/cgroup/unified`. + +## systemd's Unit Types + +The low-level kernel cgroups feature is exposed in systemd in three different +"unit" types. Specifically: + +1. 💼 The `.service` unit type. This unit type is for units encapsulating + processes systemd itself starts. Units of these types have cgroups that are + the leaves of the cgroup tree the systemd instance manages (though possibly + they might contain a sub-tree of their own managed by something else, made + possible by the concept of delegation, see below). Service units are usually + instantiated based on a unit file on disk that describes the command line to + invoke and other properties of the service. However, service units may also + be declared and started programmatically at runtime through a D-Bus API + (which is called *transient* services). + +2. 👓 The `.scope` unit type. This is very similar to `.service`. The main + difference: the processes the units of this type encapsulate are forked off + by some unrelated manager process, and that manager asked systemd to expose + them as a unit. Unlike services, scopes can only be declared and started + programmatically, i.e. are always transient. That's because they encapsulate + processes forked off by something else, i.e. existing runtime objects, and + hence cannot really be defined fully in 'offline' concepts such as unit + files. + +3. 🔪 The `.slice` unit type. Units of this type do not directly contain any + processes. Units of this type are the inner nodes of part of the cgroup tree + the systemd instance manages. Much like services, slices can be defined + either on disk with unit files or programmatically as transient units. + +Slices expose the trunk and branches of a tree, and scopes and services are +attached to those branches as leaves. The idea is that scopes and services can +be moved around though, i.e. assigned to a different slice if needed. + +The naming of slice units directly maps to the cgroup tree path. This is not +the case for service and scope units however. A slice named `foo-bar-baz.slice` +maps to a cgroup `/foo.slice/foo-bar.slice/foo-bar-baz.slice/`. A service +`quux.service` which is attached to the slice `foo-bar-baz.slice` maps to the +cgroup `/foo.slice/foo-bar.slice/foo-bar-baz.slice/quux.service/`. + +By default systemd sets up four slice units: + +1. `-.slice` is the root slice. i.e. the parent of everything else. On the host + system it maps directly to the top-level directory of cgroup v2. + +2. `system.slice` is where system services are by default placed, unless + configured otherwise. + +3. `user.slice` is where user sessions are placed. Each user gets a slice of + its own below that. + +4. `machines.slice` is where VMs and containers are supposed to be + placed. `systemd-nspawn` makes use of this by default, and you're very welcome + to place your containers and VMs there too if you hack on managers for those. + +Users may define any amount of additional slices they like though, the four +above are just the defaults. + +## Delegation + +Container managers and suchlike often want to control cgroups directly using +the raw kernel APIs. That's entirely fine and supported, as long as proper +*delegation* is followed. Delegation is a concept we inherited from cgroup v2, +but we expose it on cgroup v1 too. Delegation means that some parts of the +cgroup tree may be managed by different managers than others. As long as it is +clear which manager manages which part of the tree each one can do within its +sub-graph of the tree whatever it wants. + +Only sub-trees can be delegated (though whoever decides to request a sub-tree +can delegate sub-sub-trees further to somebody else if they like). Delegation +takes place at a specific cgroup: in systemd there's a `Delegate=` property you +can set for a service or scope unit. If you do, it's the cut-off point for +systemd's cgroup management: the unit itself is managed by systemd, i.e. all +its attributes are managed exclusively by systemd, however your program may +create/remove sub-cgroups inside it freely, and those then become exclusive +property of your program, systemd won't touch them — all attributes of *those* +sub-cgroups can be manipulated freely and exclusively by your program. + +By turning on the `Delegate=` property for a scope or service you get a few +guarantees: + +1. systemd won't fiddle with your sub-tree of the cgroup tree anymore. It won't + change attributes of any cgroups below it, nor will it create or remove any + cgroups thereunder, nor migrate processes across the boundaries of that + sub-tree as it deems useful anymore. + +2. If your service makes use of the `User=` functionality, then the sub-tree + will be `chown()`ed to the indicated user so that it can correctly create + cgroups below it. Note however that systemd will do that only in the unified + hierarchy (in unified and hybrid mode) as well as on systemd's own private + hierarchy (in legacy and hybrid mode). It won't pass ownership of the legacy + controller hierarchies. Delegation to less privileges processes is not safe + in cgroup v1 (as a limitation of the kernel), hence systemd won't facilitate + access to it. + +3. Any BPF IP filter programs systemd installs will be installed with + `BPF_F_ALLOW_MULTI` so that your program can install additional ones. + +In unit files the `Delegate=` property is superficially exposed as +boolean. However, since v236 it optionally takes a list of controller names +instead. If so, delegation is requested for listed controllers +specifically. Note that this only encodes a request. Depending on various +parameters it might happen that your service actually will get fewer +controllers delegated (for example, because the controller is not available on +the current kernel or was turned off) or more. If no list is specified +(i.e. the property simply set to `yes`) then all available controllers are +delegated. + +Let's stress one thing: delegation is available on scope and service units +only. It's expressly not available on slice units. Why? Because slice units are +our *inner* nodes of the cgroup trees and we freely attach service and scopes +to them. If we'd allow delegation on slice units then this would mean that +both systemd and your own manager would create/delete cgroups below the slice +unit and that conflicts with the single-writer rule. + +So, if you want to do your own raw cgroups kernel level access, then allocate a +scope unit, or a service unit (or just use the service unit you already have +for your service code), and turn on delegation for it. + +(OK, here's one caveat: if you turn on delegation for a service, and that +service has `ExecStartPost=`, `ExecReload=`, `ExecStop=` or `ExecStopPost=` +set, then these commands will be executed within the `.control/` sub-cgroup of +your service's cgroup. This is necessary because by turning on delegation we +have to assume that the cgroup delegated to your service is now an *inner* +cgroup, which means that it may not directly contain any processes. Hence, if +your service has any of these four settings set, you must be prepared that a +`.control/` subcgroup might appear, managed by the service manager. This also +means that your service code should have moved itself further down the cgroup +tree by the time it notifies the service manager about start-up readiness, so +that the service's main cgroup is definitely an inner node by the time the +service manager might start `ExecStartPost=`.) + +## Three Scenarios + +Let's say you write a container manager, and you wonder what to do regarding +cgroups for it, as you want your manager to be able to run on systemd systems. + +You basically have three options: + +1. 😊 The *integration-is-good* option. For this, you register each container + you have either as a systemd service (i.e. let systemd invoke the executor + binary for you) or a systemd scope (i.e. your manager executes the binary + directly, but then tells systemd about it. In this mode the administrator + can use the usual systemd resource management and reporting commands + individually on those containers. By turning on `Delegate=` for these scopes + or services you make it possible to run cgroup-enabled programs in your + containers, for example a nested systemd instance. This option has two + sub-options: + + a. You transiently register the service or scope by directly contacting + systemd via D-Bus. In this case systemd will just manage the unit for you + and nothing else. + + b. Instead you register the service or scope through `systemd-machined` + (also via D-Bus). This mini-daemon is basically just a proxy for the same + operations as in a. The main benefit of this: this way you let the system + know that what you are registering is a container, and this opens up + certain additional integration points. For example, `journalctl -M` can + then be used to directly look into any container's journal logs (should + the container run systemd inside), or `systemctl -M` can be used to + directly invoke systemd operations inside the containers. Moreover tools + like "ps" can then show you to which container a process belongs (`ps -eo + pid,comm,machine`), and even gnome-system-monitor supports it. + +2. 🙁 The *i-like-islands* option. If all you care about is your own cgroup tree, + and you want to have to do as little as possible with systemd and no + interest in integration with the rest of the system, then this is a valid + option. For this all you have to do is turn on `Delegate=` for your main + manager daemon. Then figure out the cgroup systemd placed your daemon in: + you can now freely create sub-cgroups beneath it. Don't forget the + *no-processes-in-inner-nodes* rule however: you have to move your main + daemon process out of that cgroup (and into a sub-cgroup) before you can + start further processes in any of your sub-cgroups. + +3. 🙁 The *i-like-continents* option. In this option you'd leave your manager + daemon where it is, and would not turn on delegation on its unit. However, + as first thing you register a new scope unit with systemd, and that scope + unit would have `Delegate=` turned on, and then you place all your + containers underneath it. From systemd's PoV there'd be two units: your + manager service and the big scope that contains all your containers in one. + +BTW: if for whatever reason you say "I hate D-Bus, I'll never call any D-Bus +API, kthxbye", then options #1 and #3 are not available, as they generally +involve talking to systemd from your program code, via D-Bus. You still have +option #2 in that case however, as you can simply set `Delegate=` in your +service's unit file and you are done and have your own sub-tree. In fact, #2 is +the one option that allows you to completely ignore systemd's existence: you +can entirely generically follow the single rule that you just use the cgroup +you are started in, and everything below it, whatever that might be. That said, +maybe if you dislike D-Bus and systemd that much, the better approach might be +to work on that, and widen your horizon a bit. You are welcome. + +## Controller Support + +systemd supports a number of controllers (but not all). Specifically, supported +are: + +* on cgroup v1: `cpu`, `cpuacct`, `blkio`, `memory`, `devices`, `pids` +* on cgroup v2: `cpu`, `io`, `memory`, `pids` + +It is our intention to natively support all cgroup v2 controllers as they are +added to the kernel. However, regarding cgroup v1: at this point we will not +add support for any other controllers anymore. This means systemd currently +does not and will never manage the following controllers on cgroup v1: +`freezer`, `cpuset`, `net_cls`, `perf_event`, `net_prio`, `hugetlb`. Why not? +Depending on the case, either their API semantics or implementations aren't +really usable, or it's very clear they have no future on cgroup v2, and we +won't add new code for stuff that clearly has no future. + +Effectively this means that all those mentioned cgroup v1 controllers are up +for grabs: systemd won't manage them, and hence won't delegate them to your +code (however, systemd will still mount their hierarchies, simply because it +mounts all controller hierarchies it finds available in the kernel). If you +decide to use them, then that's fine, but systemd won't help you with it (but +also not interfere with it). To be nice to other tenants it might be wise to +replicate the cgroup hierarchies of the other controllers in them too however, +but of course that's between you and those other tenants, and systemd won't +care. Replicating the cgroup hierarchies in those unsupported controllers would +mean replicating the full cgroup paths in them, and hence the prefixing +`.slice` components too, otherwise the hierarchies will start being orthogonal +after all, and that's not really desirable. On more thing: systemd will clean +up after you in the hierarchies it manages: if your daemon goes down, its +cgroups will be removed too. You basically get the guarantee that you start +with a pristine cgroup sub-tree for your service or scope whenever it is +started. This is not the case however in the hierarchies systemd doesn't +manage. This means that your programs should be ready to deal with left-over +cgroups in them — from previous runs, and be extra careful with them as they +might still carry settings that might not be valid anymore. + +Note a particular asymmetry here: if your systemd version doesn't support a +specific controller on cgroup v1 you can still make use of it for delegation, +by directly fiddling with its hierarchy and replicating the cgroup tree there +as necessary (as suggested above). However, on cgroup v2 this is different: +separately mounted hierarchies are not available, and delegation has always to +happen through systemd itself. This means: when you update your kernel and it +adds a new, so far unseen controller, and you want to use it for delegation, +then you also need to update systemd to a version that groks it. + +## systemd as Container Payload + +systemd can happily run as a container payload's PID 1. Note that systemd +unconditionally needs write access to the cgroup tree however, hence you need +to delegate a sub-tree to it. Note that there's nothing too special you have to +do beyond that: just invoke systemd as PID 1 inside the root of the delegated +cgroup sub-tree, and it will figure out the rest: it will determine the cgroup +it is running in and take possession of it. It won't interfere with any cgroup +outside of the sub-tree it was invoked in. Use of `CLONE_NEWCGROUP` is hence +optional (but of course wise). + +Note one particular asymmetry here though: systemd will try to take possession +of the root cgroup you pass to it *in* *full*, i.e. it will not only +create/remove child cgroups below it, it will also attempt to manage the +attributes of it. OTOH as mentioned above, when delegating a cgroup tree to +somebody else it only passes the rights to create/remove sub-cgroups, but will +insist on managing the delegated cgroup tree's top-level attributes. Or in +other words: systemd is *greedy* when accepting delegated cgroup trees and also +*greedy* when delegating them to others: it insists on managing attributes on +the specific cgroup in both cases. A container manager that is itself a payload +of a host systemd which wants to run a systemd as its own container payload +instead hence needs to insert an extra level in the hierarchy in between, so +that the systemd on the host and the one in the container won't fight for the +attributes. That said, you likely should do that anyway, due to the +no-processes-in-inner-cgroups rule, see below. + +When systemd runs as container payload it will make use of all hierarchies it +has write access to. For legacy mode you need to make at least +`/sys/fs/cgroup/systemd/` available, all other hierarchies are optional. For +hybrid mode you need to add `/sys/fs/cgroup/unified/`. Finally, for fully +unified you (of course, I guess) need to provide only `/sys/fs/cgroup/` itself. + +## Some Dos + +1. ⚡ If you go for implementation option 1a or 1b (as in the list above), then + each of your containers will have its own systemd-managed unit and hence + cgroup with possibly further sub-cgroups below. Typically the first process + running in that unit will be some kind of executor program, which will in + turn fork off the payload processes of the container. In this case don't + forget that there are two levels of delegation involved: first, systemd + delegates a group sub-tree to your executor. And then your executor should + delegate a sub-tree further down to the container payload. Oh, and because + of the no-process-in-inner-nodes rule, your executor needs to migrate itself + to a sub-cgroup of the cgroup it got delegated, too. Most likely you hence + want a two-pronged approach: below the cgroup you got started in, you want + one cgroup maybe called `supervisor/` where your manager runs in and then + for each container a sibling cgroup of that maybe called `payload-xyz/`. + +2. ⚡ Don't forget that the cgroups you create have to have names that are + suitable as UNIX file names, and that they live in the same namespace as the + various kernel attribute files. Hence, when you want to allow the user + arbitrary naming, you might need to escape some of the names (for example, + you really don't want to create a cgroup named `tasks`, just because the + user created a container by that name, because `tasks` after all is a magic + attribute in cgroup v1, and your `mkdir()` will hence fail with `EEXIST`. In + systemd we do escaping by prefixing names that might collide with a kernel + attribute name with an underscore. You might want to do the same, but this + is really up to you how you do it. Just do it, and be careful. + +## Some Don'ts + +1. 🚫 Never create your own cgroups below arbitrary cgroups systemd manages, i.e + cgroups you haven't set `Delegate=` in. Specifically: 🔥 don't create your + own cgroups below the root cgroup 🔥. That's owned by systemd, and you will + step on systemd's toes if you ignore that, and systemd will step on + yours. Get your own delegated sub-tree, you may create as many cgroups there + as you like. Seriously, if you create cgroups directly in the cgroup root, + then all you do is ask for trouble. + +2. 🚫 Don't attempt to set `Delegate=` in slice units, and in particular not in + `-.slice`. It's not supported, and will generate an error. + +3. 🚫 Never *write* to any of the attributes of a cgroup systemd created for + you. It's systemd's private property. You are welcome to manipulate the + attributes of cgroups you created in your own delegated sub-tree, but the + cgroup tree of systemd itself is out of limits for you. It's fine to *read* + from any attribute you like however. That's totally OK and welcome. + +4. 🚫 When not using `CLONE_NEWCGROUP` when delegating a sub-tree to a + container payload running systemd, then don't get the idea that you can bind + mount only a sub-tree of the host's cgroup tree into the container. Part of + the cgroup API is that `/proc/$PID/cgroup` reports the cgroup path of every + process, and hence any path below `/sys/fs/cgroup/` needs to match what + `/proc/$PID/cgroup` of the payload processes reports. What you can do safely + however, is mount the upper parts of the cgroup tree read-only (or even + replace the middle bits with an intermediary `tmpfs` — but be careful not to + break the `statfs()` detection logic discussed above), as long as the path + to the delegated sub-tree remains accessible as-is. + +5. ⚡ Currently, the algorithm for mapping between slice/scope/service unit + naming and their cgroup paths is not considered public API of systemd, and + may change in future versions. This means: it's best to avoid implementing a + local logic of translating cgroup paths to slice/scope/service names in your + program, or vice versa — it's likely going to break sooner or later. Use the + appropriate D-Bus API calls for that instead, so that systemd translates + this for you. (Specifically: each Unit object has a `ControlGroup` property + to get the cgroup for a unit. The method `GetUnitByControlGroup()` may be + used to get the unit for a cgroup.) + +6. ⚡ Think twice before delegating cgroup v1 controllers to less privileged + containers. It's not safe, you basically allow your containers to freeze the + system with that and worse. Delegation is a strongpoint of cgroup v2 though, + and there it's safe to treat delegation boundaries as privilege boundaries. + +And that's it for now. If you have further questions, refer to the systemd +mailing list. + +— Berlin, 2018-04-20 diff --git a/docs/CNAME b/docs/CNAME new file mode 100644 index 0000000..cdcf4d9 --- /dev/null +++ b/docs/CNAME @@ -0,0 +1 @@ +systemd.io
\ No newline at end of file diff --git a/docs/CODE_OF_CONDUCT.md b/docs/CODE_OF_CONDUCT.md new file mode 100644 index 0000000..b906bf5 --- /dev/null +++ b/docs/CODE_OF_CONDUCT.md @@ -0,0 +1,20 @@ +--- +title: systemd Community Conduct Guidelines +category: Contributing +layout: default +--- + +# The systemd Community Conduct Guidelines + +This document provides community guidelines for a safe, respectful, productive, and collaborative place for any person who is willing to contribute to systemd. It applies to all “collaborative spaces”, which is defined as community communications channels (such as mailing lists, submitted patches, commit comments, etc.). + +- Participants will be tolerant of opposing views. +- Participants must ensure that their language and actions are free of personal attacks and disparaging personal remarks. +- When interpreting the words and actions of others, participants should always assume good intentions. +- Behaviour which can be reasonably considered harassment will not be tolerated. + +## Enforcement + +Instances of abusive, harassing, or otherwise unacceptable behavior may be reported by contacting the project team at systemd-conduct@googlegroups.com. This team currently consists of David Strauss <<systemd-conduct@davidstrauss.net>>, Ekaterina Gerasimova (Kat) <<Kittykat3756@gmail.com>>, and Zbigniew Jędrzejewski-Szmek <<zbyszek@in.waw.pl>>. In the unfortunate event that you wish to make a complaint against one of the members, you may instead contact any of the other members individually. + +All complaints will be reviewed and investigated and will result in a response that is deemed necessary and appropriate to the circumstances. The project team is obligated to maintain confidentiality with regard to the reporter of an incident. diff --git a/docs/CODE_QUALITY.md b/docs/CODE_QUALITY.md new file mode 100644 index 0000000..a724d66 --- /dev/null +++ b/docs/CODE_QUALITY.md @@ -0,0 +1,78 @@ +--- +title: Code Quality Tools +category: Contributing +layout: default +--- + +# Code Quality Tools + +The systemd project has a number of code quality tools set up in the source +tree and on the github infrastructure. Here's an incomprehensive list of the +available functionality: + +1. Use `ninja -C build test` to run the unit tests. Some tests are skipped if + no privileges are available, hence consider also running them with `sudo + ninja -C build test`. A couple of unit tests are considered "unsafe" (as + they change system state); to run those too, build with `meson + -Dtests=unsafe`. Finally, some unit tests are considered to be very slow, + build them too with `meson -Dslow-tests=true`. (Note that there are a couple + of manual tests in addition to these unit tests.) + +2. Use `./test/run-integration-tests.sh` to run the full integration test + suite. This will build OS images with a number of integration tests and run + them in nspawn and qemu. Requires root. + +3. Use `./coccinelle/run-coccinelle.sh` to run all + [Coccinelle](http://coccinelle.lip6.fr/) semantic patch scripts we ship. The + output will show false positives, hence take it with a pinch of salt. + +4. Use `./tools/find-double-newline.sh recdiff` to find double newlines. Use + `./tools/find-double-newline.sh recpatch` to fix them. Take this with a grain + of salt, in particular as we generally leave foreign header files we include in + our tree unmodified, if possible. + +5. Similar use `./tools/find-tabs.sh recdiff` to find TABs, and + `./tools/find-tabs.sh recpatch` to fix them. (Again, grain of salt, foreign + headers should usually be left unmodified.) + +6. Use `ninja -C build check-api-docs` to compare the list of exported + symbols of `libsystemd.so` and `libudev.so` with the list of man pages. Symbols + lacking documentation are highlighted. + +7. Use `ninja -C build hwdb-update` to automatically download and import the + PCI, USB and OUI databases into hwdb. + +8. Use `ninja -C build man/update-man-rules` to update the meson rules for + building man pages automatically from the docbook XML files included in + `man/`. + +9. There are multiple CI systems in use that run on every github PR submission. + +10. [Coverity](https://scan.coverity.com/) is analyzing systemd master in + regular intervals. The reports are available + [online](https://scan.coverity.com/projects/systemd). + +11. [oss-fuzz](https://oss-fuzz.com/) is continuously fuzzing the + codebase. Reports are available + [online](https://oss-fuzz.com/v2/testcases?project=systemd). + +12. Our tree includes `.editorconfig`, `.dir-locals.el` and `.vimrc` files, to + ensure that editors follow the right indentiation styles automatically. + +13. When building systemd from a git checkout the build scripts will + automatically enable a git commit hook that ensures whitespace cleanliness. + +14. [LGTM](https://lgtm.com/) analyzes every commit pushed to master. The list + of active alerts can be found + [here](https://lgtm.com/projects/g/systemd/systemd/alerts/?mode=list). + +15. Each PR is automatically tested with [Address Sanitizer](https://clang.llvm.org/docs/AddressSanitizer.html) + and [Undefined Behavior Sanitizer](https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html). + See [Testing systemd using sanitizers](https://systemd.io/TESTING_WITH_SANITIZERS) + for more information. + +16. Fossies provides [source code misspelling reports](https://fossies.org/features.html#codespell). + The systemd report can be found [here](https://fossies.org/linux/test/systemd-master.tar.gz/codespell.html). + +Access to Coverity and oss-fuzz reports is limited. Please reach out to the +maintainers if you need access. diff --git a/docs/CODING_STYLE.md b/docs/CODING_STYLE.md new file mode 100644 index 0000000..851676b --- /dev/null +++ b/docs/CODING_STYLE.md @@ -0,0 +1,596 @@ +--- +title: Coding Style +category: Contributing +layout: default +--- + +# Coding Style + +## Formatting + +- 8ch indent, no tabs, except for files in `man/` which are 2ch indent, and + still no tabs, and shell scripts, which are 4ch indent, and no tabs either. + +- We prefer `/* comments */` over `// comments` in code you commit, + please. This way `// comments` are left for developers to use for local, + temporary commenting of code for debug purposes (i.e. uncommittable stuff), + making such comments easily discernible from explanatory, documenting code + comments (i.e. committable stuff). + +- Don't break code lines too eagerly. We do **not** force line breaks at 80ch, + all of today's screens should be much larger than that. But then again, don't + overdo it, ~109ch should be enough really. The `.editorconfig`, `.vimrc` and + `.dir-locals.el` files contained in the repository will set this limit up for + you automatically, if you let them (as well as a few other things). Please + note that emacs loads `.dir-locals.el` automatically, but vim needs to be + configured to load `.vimrc`, see that file for instructions. + +- If you break a function declaration over multiple lines, do it like this: + + ```c + void some_function( + int foo, + bool bar, + char baz) { + + int a, b, c; + ``` + + (i.e. use double indentation — 16 spaces — for the parameter list.) + +- Try to write this: + + ```c + void foo() { + } + ``` + + instead of this: + + ```c + void foo() + { + } + ``` + +- Single-line `if` blocks should not be enclosed in `{}`. Write this: + + ```c + if (foobar) + waldo(); + ``` + + instead of this: + + ```c + if (foobar) { + waldo(); + } + ``` + +- Do not write `foo ()`, write `foo()`. + +## Code Organization and Semantics + +- Please name structures in `PascalCase` (with exceptions, such as public API + structs), variables and functions in `snake_case`. + +- Avoid static variables, except for caches and very few other cases. Think + about thread-safety! While most of our code is never used in threaded + environments, at least the library code should make sure it works correctly + in them. Instead of doing a lot of locking for that, we tend to prefer using + TLS to do per-thread caching (which only works for small, fixed-size cache + objects), or we disable caching for any thread that is not the main + thread. Use `is_main_thread()` to detect whether the calling thread is the + main thread. + +- Do not write functions that clobber call-by-reference variables on + failure. Use temporary variables for these cases and change the passed in + variables only on success. The rule is: never clobber return parameters on + failure, always initialize return parameters on success. + +- Typically, function parameters fit into three categories: input parameters, + mutable objects, and call-by-reference return parameters. Input parameters + should always carry suitable "const" declarators if they are pointers, to + indicate they are input-only and not changed by the function. Return + parameters are best prefixed with "ret_", to clarify they are return + parameters. (Conversely, please do not prefix parameters that aren't + output-only with "ret_", in particular not mutable parameters that are both + input as well as output). Example: + + ```c + static int foobar_frobnicate( + Foobar* object, /* the associated mutable object */ + const char *input, /* immutable input parameter */ + char **ret_frobnicated) { /* return parameter */ + … + return 0; + } + ``` + +- The order in which header files are included doesn't matter too + much. systemd-internal headers must not rely on an include order, so it is + safe to include them in any order possible. However, to not clutter global + includes, and to make sure internal definitions will not affect global + headers, please always include the headers of external components first + (these are all headers enclosed in <>), followed by our own exported headers + (usually everything that's prefixed by `sd-`), and then followed by internal + headers. Furthermore, in all three groups, order all includes alphabetically + so duplicate includes can easily be detected. + +- Please avoid using global variables as much as you can. And if you do use + them make sure they are static at least, instead of exported. Especially in + library-like code it is important to avoid global variables. Why are global + variables bad? They usually hinder generic reusability of code (since they + break in threaded programs, and usually would require locking there), and as + the code using them has side-effects make programs non-transparent. That + said, there are many cases where they explicitly make a lot of sense, and are + OK to use. For example, the log level and target in `log.c` is stored in a + global variable, and that's OK and probably expected by most. Also in many + cases we cache data in global variables. If you add more caches like this, + please be careful however, and think about threading. Only use static + variables if you are sure that thread-safety doesn't matter in your + case. Alternatively, consider using TLS, which is pretty easy to use with + gcc's `thread_local` concept. It's also OK to store data that is inherently + global in global variables, for example data parsed from command lines, see + below. + +- You might wonder what kind of common code belongs in `src/shared/` and what + belongs in `src/basic/`. The split is like this: anything that is used to + implement the public shared object we provide (sd-bus, sd-login, sd-id128, + nss-systemd, nss-mymachines, nss-resolve, nss-myhostname, pam_systemd), must + be located in `src/basic` (those objects are not allowed to link to + libsystemd-shared.so). Conversely, anything which is shared between multiple + components and does not need to be in `src/basic/`, should be in + `src/shared/`. + + To summarize: + + `src/basic/` + - may be used by all code in the tree + - may not use any code outside of `src/basic/` + + `src/libsystemd/` + - may be used by all code in the tree, except for code in `src/basic/` + - may not use any code outside of `src/basic/`, `src/libsystemd/` + + `src/shared/` + - may be used by all code in the tree, except for code in `src/basic/`, + `src/libsystemd/`, `src/nss-*`, `src/login/pam_systemd.*`, and files under + `src/journal/` that end up in `libjournal-client.a` convenience library. + - may not use any code outside of `src/basic/`, `src/libsystemd/`, `src/shared/` + +- Our focus is on the GNU libc (glibc), not any other libcs. If other libcs are + incompatible with glibc it's on them. However, if there are equivalent POSIX + and Linux/GNU-specific APIs, we generally prefer the POSIX APIs. If there + aren't, we are happy to use GNU or Linux APIs, and expect non-GNU + implementations of libc to catch up with glibc. + +## Using C Constructs + +- Preferably allocate local variables on the top of the block: + + ```c + { + int a, b; + + a = 5; + b = a; + } + ``` + +- Do not mix function invocations with variable definitions in one line. Wrong: + + ```c + { + int a = foobar(); + uint64_t x = 7; + } + ``` + + Right: + + ```c + { + int a; + uint64_t x = 7; + + a = foobar(); + } + ``` + +- Use `goto` for cleaning up, and only use it for that. i.e. you may only jump + to the end of a function, and little else. Never jump backwards! + +- To minimize strict aliasing violations, we prefer unions over casting. + +- Instead of using `memzero()`/`memset()` to initialize structs allocated on + the stack, please try to use c99 structure initializers. It's short, prettier + and actually even faster at execution. Hence: + + ```c + struct foobar t = { + .foo = 7, + .bar = "bazz", + }; + ``` + + instead of: + + ```c + struct foobar t; + zero(t); + t.foo = 7; + t.bar = "bazz"; + ``` + +- To implement an endless loop, use `for (;;)` rather than `while (1)`. The + latter is a bit ugly anyway, since you probably really meant `while + (true)`. To avoid the discussion what the right always-true expression for an + infinite while loop is, our recommendation is to simply write it without any + such expression by using `for (;;)`. + +- To determine the length of a constant string `"foo"`, don't bother with + `sizeof("foo")-1`, please use `strlen()` instead (both gcc and clang optimize + the call away for fixed strings). The only exception is when declaring an + array. In that case use STRLEN, which evaluates to a static constant and + doesn't force the compiler to create a VLA. + +- Please use C's downgrade-to-bool feature only for expressions that are + actually booleans (or "boolean-like"), and not for variables that are really + numeric. Specifically, if you have an `int b` and it's only used in a boolean + sense, by all means check its state with `if (b) …` — but if `b` can actually + have more than two semantic values, and you want to compare for non-zero, + then please write that explicitly with `if (b != 0) …`. This helps readability + as the value range and semantical behaviour is directly clear from the + condition check. As a special addition: when dealing with pointers which you + want to check for non-NULL-ness, you may also use downgrade-to-bool feature. + +- Please do not use yoda comparisons, i.e. please prefer the more readable `if + (a == 7)` over the less readable `if (7 == a)`. + +## Destructors + +- The destructors always deregister the object from the next bigger object, not + the other way around. + +- For robustness reasons, destructors should be able to destruct + half-initialized objects, too. + +- When you define a destructor or `unref()` call for an object, please accept a + `NULL` object and simply treat this as NOP. This is similar to how libc + `free()` works, which accepts `NULL` pointers and becomes a NOP for them. By + following this scheme a lot of `if` checks can be removed before invoking + your destructor, which makes the code substantially more readable and robust. + +- Related to this: when you define a destructor or `unref()` call for an + object, please make it return the same type it takes and always return `NULL` + from it. This allows writing code like this: + + ```c + p = foobar_unref(p); + ``` + + which will always work regardless if `p` is initialized or not, and + guarantees that `p` is `NULL` afterwards, all in just one line. + +## Error Handling + +- Error codes are returned as negative `Exxx`. e.g. `return -EINVAL`. There are + some exceptions: for constructors, it is OK to return `NULL` on OOM. For + lookup functions, `NULL` is fine too for "not found". + + Be strict with this. When you write a function that can fail due to more than + one cause, it *really* should have an `int` as the return value for the error + code. + +- Do not bother with error checking whether writing to stdout/stderr worked. + +- Do not log errors from "library" code, only do so from "main program" + code. (With one exception: it is OK to log with DEBUG level from any code, + with the exception of maybe inner loops). + +- In public API calls, you **must** validate all your input arguments for + programming error with `assert_return()` and return a sensible return + code. In all other calls, it is recommended to check for programming errors + with a more brutal `assert()`. We are more forgiving to public users than for + ourselves! Note that `assert()` and `assert_return()` really only should be + used for detecting programming errors, not for runtime errors. `assert()` and + `assert_return()` by usage of `_likely_()` inform the compiler that he should + not expect these checks to fail, and they inform fellow programmers about the + expected validity and range of parameters. + +- When you invoke certain calls like `unlink()`, or `mkdir_p()` and you know it + is safe to ignore the error it might return (because a later call would + detect the failure anyway, or because the error is in an error path and you + thus couldn't do anything about it anyway), then make this clear by casting + the invocation explicitly to `(void)`. Code checks like Coverity understand + that, and will not complain about ignored error codes. Hence, please use + this: + + ```c + (void) unlink("/foo/bar/baz"); + ``` + + instead of just this: + + ```c + unlink("/foo/bar/baz"); + ``` + + When returning from a `void` function, you may also want to shorten the error + path boilerplate by returning a function invocation cast to `(void)` like so: + + ```c + if (condition_not_met) + return (void) log_tests_skipped("Cannot run ..."); + ``` + + Don't cast function calls to `(void)` that return no error + conditions. Specifically, the various `xyz_unref()` calls that return a + `NULL` object shouldn't be cast to `(void)`, since not using the return value + does not hide any errors. + +- When returning a return code from `main()`, please preferably use + `EXIT_FAILURE` and `EXIT_SUCCESS` as defined by libc. + +## Logging + +- For every function you add, think about whether it is a "logging" function or + a "non-logging" function. "Logging" functions do (non-debug) logging on their + own, "non-logging" function never log on their own (except at debug level) + and expect their callers to log. All functions in "library" code, i.e. in + `src/shared/` and suchlike must be "non-logging". Every time a "logging" + function calls a "non-logging" function, it should log about the resulting + errors. If a "logging" function calls another "logging" function, then it + should not generate log messages, so that log messages are not generated + twice for the same errors. (Note that debug level logging — at syslog level + `LOG_DEBUG` — is not considered logging in this context, debug logging is + generally always fine and welcome.) + +- If possible, do a combined log & return operation: + + ```c + r = operation(...); + if (r < 0) + return log_(error|warning|notice|...)_errno(r, "Failed to ...: %m"); + ``` + + If the error value is "synthetic", i.e. it was not received from + the called function, use `SYNTHETIC_ERRNO` wrapper to tell the logging + system to not log the errno value, but still return it: + + ```c + n = read(..., s, sizeof s); + if (n != sizeof s) + return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to read ..."); + ``` + +## Memory Allocation + +- Always check OOM. There is no excuse. In program code, you can use + `log_oom()` for then printing a short message, but not in "library" code. + +- Avoid fixed-size string buffers, unless you really know the maximum size and + that maximum size is small. They are a source of errors, since they possibly + result in truncated strings. It is often nicer to use dynamic memory, + `alloca()` or VLAs. If you do allocate fixed-size strings on the stack, then + it is probably only OK if you either use a maximum size such as `LINE_MAX`, + or count in detail the maximum size a string can have. (`DECIMAL_STR_MAX` and + `DECIMAL_STR_WIDTH` macros are your friends for this!) + + Or in other words, if you use `char buf[256]` then you are likely doing + something wrong! + +- Make use of `_cleanup_free_` and friends. It makes your code much nicer to + read (and shorter)! + +- Use `alloca()`, but never forget that it is not OK to invoke `alloca()` + within a loop or within function call parameters. `alloca()` memory is + released at the end of a function, and not at the end of a `{}` block. Thus, + if you invoke it in a loop, you keep increasing the stack pointer without + ever releasing memory again. (VLAs have better behavior in this case, so + consider using them as an alternative.) Regarding not using `alloca()` + within function parameters, see the BUGS section of the `alloca(3)` man page. + +- If you want to concatenate two or more strings, consider using `strjoina()` + or `strjoin()` rather than `asprintf()`, as the latter is a lot slower. This + matters particularly in inner loops (but note that `strjoina()` cannot be + used there). + +## Runtime Behaviour + +- Avoid leaving long-running child processes around, i.e. `fork()`s that are + not followed quickly by an `execv()` in the child. Resource management is + unclear in this case, and memory CoW will result in unexpected penalties in + the parent much, much later on. + +- Don't block execution for arbitrary amounts of time using `usleep()` or a + similar call, unless you really know what you do. Just "giving something some + time", or so is a lazy excuse. Always wait for the proper event, instead of + doing time-based poll loops. + +- Whenever installing a signal handler, make sure to set `SA_RESTART` for it, + so that interrupted system calls are automatically restarted, and we minimize + hassles with handling `EINTR` (in particular as `EINTR` handling is pretty + broken on Linux). + +- When applying C-style unescaping as well as specifier expansion on the same + string, always apply the C-style unescaping first, followed by the specifier + expansion. When doing the reverse, make sure to escape `%` in specifier-style + first (i.e. `%` → `%%`), and then do C-style escaping where necessary. + +- Be exceptionally careful when formatting and parsing floating point + numbers. Their syntax is locale dependent (i.e. `5.000` in en_US is generally + understood as 5, while in de_DE as 5000.). + +- Make sure to enforce limits on every user controllable resource. If the user + can allocate resources in your code, your code must enforce some form of + limits after which it will refuse operation. It's fine if it is hard-coded + (at least initially), but it needs to be there. This is particularly + important for objects that unprivileged users may allocate, but also matters + for everything else any user may allocated. + +## Types + +- Think about the types you use. If a value cannot sensibly be negative, do not + use `int`, but use `unsigned`. + +- Use `char` only for actual characters. Use `uint8_t` or `int8_t` when you + actually mean a byte-sized signed or unsigned integers. When referring to a + generic byte, we generally prefer the unsigned variant `uint8_t`. Do not use + types based on `short`. They *never* make sense. Use `int`, `long`, `long + long`, all in unsigned and signed fashion, and the fixed-size types + `uint8_t`, `uint16_t`, `uint32_t`, `uint64_t`, `int8_t`, `int16_t`, `int32_t` + and so on, as well as `size_t`, but nothing else. Do not use kernel types + like `u32` and so on, leave that to the kernel. + +- Stay uniform. For example, always use `usec_t` for time values. Do not mix + `usec` and `msec`, and `usec` and whatnot. + +- Never use the `off_t` type, and particularly avoid it in public APIs. It's + really weirdly defined, as it usually is 64-bit and we don't support it any + other way, but it could in theory also be 32-bit. Which one it is depends on + a compiler switch chosen by the compiled program, which hence corrupts APIs + using it unless they can also follow the program's choice. Moreover, in + systemd we should parse values the same way on all architectures and cannot + expose `off_t` values over D-Bus. To avoid any confusion regarding conversion + and ABIs, always use simply `uint64_t` directly. + +- Unless you allocate an array, `double` is always a better choice than + `float`. Processors speak `double` natively anyway, so there is no speed + benefit, and on calls like `printf()` `float`s get promoted to `double`s + anyway, so there is no point. + +- Use the bool type for booleans, not integers. One exception: in public + headers (i.e those in `src/systemd/sd-*.h`) use integers after all, as `bool` + is C99 and in our public APIs we try to stick to C89 (with a few extension). + +## Deadlocks + +- Do not issue NSS requests (that includes user name and hostname lookups) + from PID 1 as this might trigger deadlocks when those lookups involve + synchronously talking to services that we would need to start up. + +- Do not synchronously talk to any other service from PID 1, due to risk of + deadlocks. + +## File Descriptors + +- When you allocate a file descriptor, it should be made `O_CLOEXEC` right from + the beginning, as none of our files should leak to forked binaries by + default. Hence, whenever you open a file, `O_CLOEXEC` must be specified, + right from the beginning. This also applies to sockets. Effectively, this + means that all invocations to: + + - `open()` must get `O_CLOEXEC` passed, + - `socket()` and `socketpair()` must get `SOCK_CLOEXEC` passed, + - `recvmsg()` must get `MSG_CMSG_CLOEXEC` set, + - `F_DUPFD_CLOEXEC` should be used instead of `F_DUPFD`, and so on, + - invocations of `fopen()` should take `e`. + +- It's a good idea to use `O_NONBLOCK` when opening 'foreign' regular files, + i.e. file system objects that are supposed to be regular files whose paths + where specified by the user and hence might actually refer to other types of + file system objects. This is a good idea so that we don't end up blocking on + 'strange' file nodes, for example if the user pointed us to a FIFO or device + node which may block when opening. Moreover even for actual regular files + `O_NONBLOCK` has a benefit: it bypasses any mandatory lock that might be in + effect on the regular file. If in doubt consider turning off `O_NONBLOCK` + again after opening. + +## Command Line + +- If you parse a command line, and want to store the parsed parameters in + global variables, please consider prefixing their names with `arg_`. We have + been following this naming rule in most of our tools, and we should continue + to do so, as it makes it easy to identify command line parameter variables, + and makes it clear why it is OK that they are global variables. + +- Command line option parsing: + - Do not print full `help()` on error, be specific about the error. + - Do not print messages to stdout on error. + - Do not POSIX_ME_HARDER unless necessary, i.e. avoid `+` in option string. + +## Exporting Symbols + +- Variables and functions **must** be static, unless they have a prototype, and + are supposed to be exported. + +- Public API calls (i.e. functions exported by our shared libraries) + must be marked `_public_` and need to be prefixed with `sd_`. No + other functions should be prefixed like that. + +- When exposing public C APIs, be careful what function parameters you make + `const`. For example, a parameter taking a context object should probably not + be `const`, even if you are writing an otherwise read-only accessor function + for it. The reason is that making it `const` fixates the contract that your + call won't alter the object ever, as part of the API. However, that's often + quite a promise, given that this even prohibits object-internal caching or + lazy initialization of object variables. Moreover, it's usually not too + useful for client applications. Hence, please be careful and avoid `const` on + object parameters, unless you are very sure `const` is appropriate. + +## Referencing Concepts + +- When referring to a configuration file option in the documentation and such, + please always suffix it with `=`, to indicate that it is a configuration file + setting. + +- When referring to a command line option in the documentation and such, please + always prefix with `--` or `-` (as appropriate), to indicate that it is a + command line option. + +- When referring to a file system path that is a directory, please always + suffix it with `/`, to indicate that it is a directory, not a regular file + (or other file system object). + +## Functions to Avoid + +- Use `memzero()` or even better `zero()` instead of `memset(..., 0, ...)` + +- Please use `streq()` and `strneq()` instead of `strcmp()`, `strncmp()` where + applicable (i.e. wherever you just care about equality/inequality, not about + the sorting order). + +- Never use `strtol()`, `atoi()` and similar calls. Use `safe_atoli()`, + `safe_atou32()` and suchlike instead. They are much nicer to use in most + cases and correctly check for parsing errors. + +- `htonl()`/`ntohl()` and `htons()`/`ntohs()` are weird. Please use `htobe32()` + and `htobe16()` instead, it's much more descriptive, and actually says what + really is happening, after all `htonl()` and `htons()` don't operate on + `long`s and `short`s as their name would suggest, but on `uint32_t` and + `uint16_t`. Also, "network byte order" is just a weird name for "big endian", + hence we might want to call it "big endian" right-away. + +- Please never use `dup()`. Use `fcntl(fd, F_DUPFD_CLOEXEC, 3)` instead. For + two reasons: first, you want `O_CLOEXEC` set on the new `fd` (see + above). Second, `dup()` will happily duplicate your `fd` as 0, 1, 2, + i.e. stdin, stdout, stderr, should those `fd`s be closed. Given the special + semantics of those `fd`s, it's probably a good idea to avoid + them. `F_DUPFD_CLOEXEC` with `3` as parameter avoids them. + +- Don't use `fgets()`, it's too hard to properly handle errors such as overly + long lines. Use `read_line()` instead, which is our own function that handles + this much more nicely. + +- Don't invoke `exit()`, ever. It is not replacement for proper error + handling. Please escalate errors up your call chain, and use normal `return` + to exit from the main function of a process. If you `fork()`ed off a child + process, please use `_exit()` instead of `exit()`, so that the exit handlers + are not run. + +- We never use the POSIX version of `basename()` (which glibc defines it in + `libgen.h`), only the GNU version (which glibc defines in `string.h`). The + only reason to include `libgen.h` is because `dirname()` is needed. Every + time you need that please immediately undefine `basename()`, and add a + comment about it, so that no code ever ends up using the POSIX version! + +## Committing to git + +- Commit message subject lines should be prefixed with an appropriate component + name of some kind. For example "journal: ", "nspawn: " and so on. + +- Do not use "Signed-Off-By:" in your commit messages. That's a kernel thing we + don't do in the systemd project. diff --git a/docs/CONTAINER_INTERFACE.md b/docs/CONTAINER_INTERFACE.md new file mode 100644 index 0000000..980fce3 --- /dev/null +++ b/docs/CONTAINER_INTERFACE.md @@ -0,0 +1,377 @@ +--- +title: Container Interface +category: Interfaces +layout: default +--- + +# The Container Interface + +Also consult [Writing Virtual Machine or Container +Managers](http://www.freedesktop.org/wiki/Software/systemd/writing-vm-managers). + +systemd has a number of interfaces for interacting with container managers, +when systemd is used inside of an OS container. If you work on a container +manager, please consider supporting the following interfaces. + +## Execution Environment + +1. If the container manager wants to control the hostname for a container + running systemd it may just set it before invoking systemd, and systemd will + leave it unmodified when there is no hostname configured in `/etc/hostname` + (that file overrides whatever is pre-initialized by the container manager). + +2. Make sure to pre-mount `/proc/`, `/sys/`, and `/sys/fs/selinux/` before + invoking systemd, and mount `/proc/sys/`, `/sys/`, and `/sys/fs/selinux/` + read-only in order to prevent the container from altering the host kernel's + configuration settings. (As a special exception, if your container has + network namespaces enabled, feel free to make `/proc/sys/net/` writable). + systemd and various other subsystems (such as the SELinux userspace) have + been modified to behave accordingly when these file systems are read-only. + (It's OK to mount `/sys/` as `tmpfs` btw, and only mount a subset of its + sub-trees from the real `sysfs` to hide `/sys/firmware/`, `/sys/kernel/` and + so on. If you do that, make sure to mark `/sys/` read-only, as that + condition is what systemd looks for, and is what is considered to be the API + in this context.) + +3. Pre-mount `/dev/` as (container private) `tmpfs` for the container and bind + mount some suitable TTY to `/dev/console`. Also, make sure to create device + nodes for `/dev/null`, `/dev/zero`, `/dev/full`, `/dev/random`, + `/dev/urandom`, `/dev/tty`, `/dev/ptmx` in `/dev/`. It is not necessary to + create `/dev/fd` or `/dev/stdout`, as systemd will do that on its own. Make + sure to set up a `BPF_PROG_TYPE_CGROUP_DEVICE` BPF program — on cgroupv2 — + or the `devices` cgroup controller — on cgroupv1 — so that no other devices + but these may be created in the container. Note that many systemd services + use `PrivateDevices=`, which means that systemd will set up a private + `/dev/` for them for which it needs to be able to create these device nodes. + Dropping `CAP_MKNOD` for containers is hence generally not advisable, but + see below. + +4. `systemd-udevd` is not available in containers (and refuses to start), and + hence device dependencies are unavailable. The `systemd-udevd` unit files + will check for `/sys/` being read-only, as an indication whether device + management can work. Therefore make sure to mount `/sys/` read-only in the + container (see above). Various clients of `systemd-udevd` also check the + read-only state of `/sys/`, including PID 1 itself and `systemd-networkd`. + +5. If systemd detects it is run in a container it will spawn a single shell on + `/dev/console`, and not care about VTs or multiple gettys on VTs. (But see + `$container_ttys` below.) + +6. Either pre-mount all cgroup hierarchies in full into the container, or leave + that to systemd which will do so if they are missing. Note that it is + explicitly *not* OK to just mount a sub-hierarchy into the container as that + is incompatible with `/proc/$PID/cgroup` (which lists full paths). Also the + root-level cgroup directories tend to be quite different from inner + directories, and that distinction matters. It is OK however, to mount the + "upper" parts read-only of the hierarchies, and only allow write-access to + the cgroup sub-tree the container runs in. It's also a good idea to mount + all controller hierarchies with exception of `name=systemd` fully read-only + (this only applies to cgroupv1, of course), to protect the controllers from + alteration from inside the containers. Or to turn this around: only the + cgroup sub-tree of the container itself (on cgroupv2 in the unified + hierarchy, and on cgroupv1 in the `name=systemd` hierarchy) may be writable + to the container. + +7. Create the control group root of your container by either running your + container as a service (in case you have one container manager instance per + container instance) or creating one scope unit for each container instance + via systemd's transient unit API (in case you have one container manager + that manages all instances. Either way, make sure to set `Delegate=yes` in + it. This ensures that the unit you created will be part of all cgroup + controllers (or at least the ones systemd understands). The latter may also + be done via `systemd-machined`'s `CreateMachine()` API. Make sure to use the + cgroup path systemd put your process in for all operations of the container. + Do not add new cgroup directories to the top of the tree. This will not only + confuse systemd and the admin, but also prevent your implementation from + being "stackable". + +## Environment Variables + +1. To allow systemd (and other programs) to identify that it is executed within + a container, please set the `$container` environment variable for PID 1 in + the container to a short lowercase string identifying your + implementation. With this in place the `ConditionVirtualization=` setting in + unit files will work properly. Example: `container=lxc-libvirt` + +2. systemd has special support for allowing container managers to initialize + the UUID for `/etc/machine-id` to some manager supplied value. This is only + enabled if `/etc/machine-id` is empty (i.e. not yet set) at boot time of the + container. The container manager should set `$container_uuid` as environment + variable for the container's PID 1 to the container UUID. (This is similar + to the effect of `qemu`'s `-uuid` switch). Note that you should pass only a + UUID here that is actually unique (i.e. only one running container should + have a specific UUID), and gets changed when a container gets duplicated. + Also note that systemd will try to persistently store the UUID in + `/etc/machine-id` (if writable) when this option is used, hence you should + always pass the same UUID here. Keeping the externally used UUID for a + container and the internal one in sync is hopefully useful to minimize + surprise for the administrator. + +3. systemd can automatically spawn login gettys on additional ptys. A container + manager can set the `$container_ttys` environment variable for the + container's PID 1 to tell it on which ptys to spawn gettys. The variable + should take a space separated list of pty names, without the leading `/dev/` + prefix, but with the `pts/` prefix included. Note that despite the + variable's name you may only specify ptys, and not other types of ttys. Also + you need to specify the pty itself, a symlink will not suffice. This is + implemented in + [systemd-getty-generator(8)](https://www.freedesktop.org/software/systemd/man/systemd-getty-generator.html). + Note that this variable should not include the pty that `/dev/console` maps + to if it maps to one (see below). Example: if the container receives + `container_ttys=pts/7 pts/8 pts/14` it will spawn three additional login + gettys on ptys 7, 8, and 14. + +4. To allow applications to detect the OS version and other metadata of the host + running the container manager, if this is considered desirable, please parse + the host's `/etc/os-release` and set a `$container_host_<key>=<VALUE>` + environment variable for the ID fields described by the [os-release + interface](https://www.freedesktop.org/software/systemd/man/os-release.html), eg: + `$container_host_id=debian` + `$container_host_build_id=2020-06-15` + `$container_host_variant_id=server` + `$container_host_version_id=10` + +5. systemd supports passing immutable binary data blobs with limited size and + restricted access to services via the `LoadCredential=` and `SetCredential=` + settings. The same protocol may be used to pass credentials from the + container manager to systemd itself. The credential data should be placed in + some location (ideally a read-only and non-swappable file system, like + 'ramfs'), and the absolute path to this directory exported in the + `$CREDENTIALS_DIRECTORY` environment variable. If the container managers + does this, the credentials passed to the service manager can be propagated + to services via `LoadCredential=` (see ...). The container manager can + choose any path, but `/run/host/credentials` is recommended." + +## Advanced Integration + +1. Consider syncing `/etc/localtime` from the host file system into the + container. Make it a relative symlink to the containers's zoneinfo dir, as + usual. Tools rely on being able to determine the timezone setting from the + symlink value, and making it relative looks nice even if people list the + container's `/etc/` from the host. + +2. Make the container journal available in the host, by automatically + symlinking the container journal directory into the host journal directory. + More precisely, link `/var/log/journal/<container-machine-id>` of the + container into the same dir of the host. Administrators can then + automatically browse all container journals (correctly interleaved) by + issuing `journalctl -m`. The container machine ID can be determined from + `/etc/machine-id` in the container. + +3. If the container manager wants to cleanly shutdown the container, it might + be a good idea to send `SIGRTMIN+3` to its init process. systemd will then + do a clean shutdown. Note however, that since only systemd understands + `SIGRTMIN+3` like this, this might confuse other init systems. + +4. To support [Socket Activated + Containers](http://0pointer.de/blog/projects/socket-activated-containers.html) + the container manager should be capable of being run as a systemd + service. It will then receive the sockets starting with FD 3, the number of + passed FDs in `$LISTEN_FDS` and its PID as `$LISTEN_PID`. It should take + these and pass them on to the container's init process, also setting + $LISTEN_FDS and `$LISTEN_PID` (basically, it can just leave the FDs and + `$LISTEN_FDS` untouched, but it needs to adjust `$LISTEN_PID` to the + container init process). That's all that's necessary to make socket + activation work. The protocol to hand sockets from systemd to services is + hence the same as from the container manager to the container systemd. For + further details see the explanations of + [sd_listen_fds(1)](http://0pointer.de/public/systemd-man/sd_listen_fds.html) + and the [blog story for service + developers](http://0pointer.de/blog/projects/socket-activation.html). + +5. Container managers should stay away from the cgroup hierarchy outside of the + unit they created for their container. That's private property of systemd, + and no other code should modify it. + +6. systemd running inside the container can report when boot-up is complete + using the usual `sd_notify()` protocol that is also used when a service + wants to tell the service manager about readiness. A container manager can + set the `$NOTIFY_SOCKET` environment variable to a suitable socket path to + make use of this functionality. (Also see information about + `/run/host/notify` below.) + +## Networking + +1. Inside of a container, if a `veth` link is named `host0`, `systemd-networkd` + running inside of the container will by default run DHCPv4, DHCPv6, and + IPv4LL clients on it. It is thus recommended that container managers that + add a `veth` link to a container name it `host0`, to get an automatically + configured network, with no manual setup. + +2. Outside of a container, if a `veth` link is prefixed "ve-", `systemd-networkd` + will by default run DHCPv4 and DHCPv6 servers on it, as well as IPv4LL. It + is thus recommended that container managers that add a `veth` link to a + container name the external side `ve-` + the container name. + +3. It is recommended to configure stable MAC addresses for container `veth` + devices, for example hashed out of the container names. That way it is more + likely that DHCP and IPv4LL will acquire stable addresses. + +## The `/run/host/` Hierarchy + +Container managers may place certain resources the manager wants to provide to +the container payload below the `/run/host/` hierarchy. This hierarchy should +be mostly immutable (possibly some subdirs might be writable, but the top-level +hierarchy — and probably most subdirs should be read-only to the +container). Note that this hierarchy is used by various container managers, and +care should be taken to avoid naming conflicts. `systemd` (and in particular +`systemd-nspawn`) use the hierarchy for the following resources: + +1. The `/run/host/incoming/` directory mount point is configured for `MS_SLAVE` + mount propagation with the host, and is used as intermediary location for + mounts to establish in the container, for the implementation of `machinectl + bind`. Container payload should usually not directly interact with this + directory: it's used by code outside the container to insert mounts inside + it only, and is mostly an internal vehicle to achieve this. Other container + managers that want to implement similar functionality might consider using + the same directory. + +2. The `/run/host/inaccessible/` directory may be set up by the container + manager to include six file nodes: `reg`, `dir`, `fifo`, `sock`, `chr`, + `blk`. These nodes correspond with the six types of file nodes Linux knows + (with the exceptions of symlinks). Each node should be of the specific type + and have an all zero access mode, i.e. be inaccessible. The two device node + types should have major and minor of zero (which are unallocated devices on + Linux). These nodes are used as mount source for implementing the + `InaccessiblePath=` setting of unit files, i.e. file nodes to mask this way + are overmounted with these "inaccessible" inodes, guaranteeing that the file + node type does not change this way but the nodes still become + inaccessible. Note that systemd when run as PID 1 in the container payload + will create these nodes on its own if not passed in by the container + manager. However, in that case it likely lacks the privileges to create the + character and block devices nodes (there are fallbacks for this case). + +3. The `/run/host/notify` path is a good choice to place the `sd_notify()` + socket in, that may be used for the container's PID 1 to report to the + container manager when boot-up is complete. The path used for this doesn't + matter much as it is communicated via the `$NOTIFY_SOCKET` environment + variable, following the usual protocol for this, however it's suitable, and + recommended place for this socket in case ready notification is desired. + +4. The `/run/host/os-release` file contains the `/etc/os-release` file of the + host, i.e. may be used by the container payload to gather limited + information about the host environment, on top of what `uname -a` reports. + +5. The `/run/host/container-manager` file may be used to pass the same + information as the `$container` environment variable (see above), i.e. a + short string identifying the container manager implementation. This file + should be newline terminated. Passing this information via this file has the + benefit that payload code can easily access it, even when running + unprivileged without access to the container PID1's environment block. + +6. The `/run/host/container-uuid` file may be used to pass the same information + as the `$container_uuid` environment variable (see above). This file should + be newline terminated. + +7. The `/run/host/credentials/` directory is a good place to pass credentials + into the container, using the `$CREDENTIALS_DIRECTORY` protocol, see above. + +## What You Shouldn't Do + +1. Do not drop `CAP_MKNOD` from the container. `PrivateDevices=` is a commonly + used service setting that provides a service with its own, private, minimal + version of `/dev/`. To set this up systemd in the container needs this + capability. If you take away the capability than all services that set this + flag will cease to work. Use `BPF_PROG_TYPE_CGROUP_DEVICE` BPF programs — on + cgroupv2 — or the `devices` controller — on cgroupv1 — to restrict what + device nodes the container can create instead of taking away the capability + wholesale. (Also see the section about fully unprivileged containers below.) + +2. Do not drop `CAP_SYS_ADMIN` from the container. A number of the most + commonly used file system namespacing related settings, such as + `PrivateDevices=`, `ProtectHome=`, `ProtectSystem=`, `MountFlags=`, + `PrivateTmp=`, `ReadWriteDirectories=`, `ReadOnlyDirectories=`, + `InaccessibleDirectories=`, and `MountFlags=` need to be able to open new + mount namespaces and the mount certain file systems into them. You break all + services that make use of these options if you drop the capability. Also + note that logind mounts `XDG_RUNTIME_DIR` as `tmpfs` for all logged in users + and that won't work either if you take away the capability. (Also see + section about fully unprivileged containers below.) + +3. Do not cross-link `/dev/kmsg` with `/dev/console`. They are different things, + you cannot link them to each other. + +4. Do not pretend that the real VTs are available in the container. The VT + subsystem consists of all the devices `/dev/tty*`, `/dev/vcs*`, `/dev/vcsa*` + plus their `sysfs` counterparts. They speak specific `ioctl()`s and + understand specific escape sequences, that other ptys don't understand. + Hence, it is explicitly not OK to mount a pty to `/dev/tty1`, `/dev/tty2`, + `/dev/tty3`. This is explicitly not supported. + +5. Don't pretend that passing arbitrary devices to containers could really work + well. For example, do not pass device nodes for block devices to the + container. Device access (with the exception of network devices) is not + virtualized on Linux. Enumeration and probing of meta information from + `/sys/` and elsewhere is not possible to do correctly in a container. Simply + adding a specific device node to a container's `/dev/` is *not* *enough* to + do the job, as `systemd-udevd` and suchlike are not available at all, and no + devices will appear available or enumerable, inside the container. + +6. Don't mount only a sub-tree of the `cgroupfs` into the container. This will not + work as `/proc/$PID/cgroup` lists full paths and cannot be matched up with + the actual `cgroupfs` tree visible, then. (You may "prune" some branches + though, see above.) + +7. Do not make `/sys/` writable in the container. If you do, + `systemd-udevd.service` is started to manage your devices — inside the + container, but that will cause conflicts and errors given that the Linux + device model is not virtualized for containers on Linux and thus the + containers and the host would try to manage the same devices, fighting for + ownership. Multiple other subsystems of systemd similarly test for `/sys/` + being writable to decide whether to use `systemd-udevd` or assume that + device management is properly available on the instance. Among them + `systemd-networkd` and `systemd-logind`. The conditionalization on the + read-only state of `/sys/` enables a nice automatism: as soon as `/sys/` and + the Linux device model are changed to be virtualized properly the container + payload can make use of that, simply by marking `/sys/` writable. (Note that + as special exception, the devices in `/sys/class/net/` are virtualized + already, if network namespacing is used. Thus it is OK to mount the relevant + sub-directories of `/sys/` writable, but make sure to leave the root of + `/sys/` read-only.) + +## Fully Unprivileged Container Payload + +First things first, to make this clear: Linux containers are not a security +technology right now. There are more holes in the model than in swiss cheese. + +For example: if you do not use user namespacing, and share root and other users +between container and host, the `struct user` structures will be shared between +host and container, and hence `RLIMIT_NPROC` and so of the container users +affect the host and other containers, and vice versa. This is a major security +hole, and actually is a real-life problem: since Avahi sets `RLIMIT_NPROC` of +its user to 2 (to effectively disallow `fork()`ing) you cannot run more than +one Avahi instance on the entire system... + +People have been asking to be able to run systemd without `CAP_SYS_ADMIN` and +`CAP_SYS_MKNOD` in the container. This is now supported to some level in +systemd, but we recommend against it (see above). If `CAP_SYS_ADMIN` and +`CAP_SYS_MKNOD` are missing from the container systemd will now gracefully turn +off `PrivateTmp=`, `PrivateNetwork=`, `ProtectHome=`, `ProtectSystem=` and +others, because those capabilities are required to implement these options. The +services using these settings (which include many of systemd's own) will hence +run in a different, less secure environment when the capabilities are missing +than with them around. + +With user namespacing in place things get much better. With user namespaces the +`struct user` issue described above goes away, and containers can keep +`CAP_SYS_ADMIN` safely for the user namespace, as capabilities are virtualized +and having capabilities inside a container doesn't mean one also has them +outside. + +## Final Words + +If you write software that wants to detect whether it is run in a container, +please check `/proc/1/environ` and look for the `container=` environment +variable. Do not assume the environment variable is inherited down the process +tree. It generally is not. Hence check the environment block of PID 1, not your +own. Note though that this file is only accessible to root. systemd hence early +on also copies the value into `/run/systemd/container`, which is readable for +everybody. However, that's a systemd-specific interface and other init systems +are unlikely to do the same. + +Note that it is our intention to make systemd systems work flawlessly and +out-of-the-box in containers. In fact we are interested to ensure that the same +OS image can be booted on a bare system, in a VM and in a container, and behave +correctly each time. If you notice that some component in systemd does not work +in a container as it should, even though the container manager implements +everything documented above, please contact us. diff --git a/docs/CONTRIBUTING.md b/docs/CONTRIBUTING.md new file mode 100644 index 0000000..cab0074 --- /dev/null +++ b/docs/CONTRIBUTING.md @@ -0,0 +1,44 @@ +--- +title: Contributing +category: Contributing +layout: default +--- + +# Contributing + +We welcome contributions from everyone. However, please follow the following guidelines when posting a GitHub Pull Request or filing a GitHub Issue on the systemd project: + +## Filing Issues + +* We use [GitHub Issues](https://github.com/systemd/systemd/issues) **exclusively** for tracking **bugs** and **feature** **requests** (RFEs) of systemd. If you are looking for help, please contact [systemd-devel mailing list](https://lists.freedesktop.org/mailman/listinfo/systemd-devel) instead. +* We only track bugs in the **two** **most** **recently** **released** (non-rc) **versions** of systemd in the GitHub Issue tracker. If you are using an older version of systemd, please contact your distribution's bug tracker instead (see below). See [GitHub Release Page](https://github.com/systemd/systemd/releases) for the list of most recent releases. +* When filing a feature request issue (RFE), please always check first if the newest upstream version of systemd already implements the feature, and whether there's already an issue filed for your feature by someone else. +* When filing an issue, specify the **systemd** **version** you are experiencing the issue with. Also, indicate which **distribution** you are using. +* Please include an explanation how to reproduce the issue you are pointing out. + +Following these guidelines makes it easier for us to process your issue, and ensures we won't close your issue right-away for being misfiled. + +### Older downstream versions +For older versions that are still supported by your distribution please use respective downstream tracker: +* **Fedora** - [bugzilla](https://bugzilla.redhat.com/enter_bug.cgi?product=Fedora&component=systemd) +* **RHEL/CentOS** - [bugzilla](https://bugzilla.redhat.com/) or [systemd-rhel github](https://github.com/systemd-rhel/) +* **Debian** - [bugs.debian.org](https://bugs.debian.org/cgi-bin/pkgreport.cgi?pkg=systemd) + +## Security vulnerability reports + +See [reporting of security vulnerabilities](SECURITY.md). + +## Posting Pull Requests + +* Make sure to post PRs only relative to a very recent git master. +* Follow our [Coding Style](CODING_STYLE.md) when contributing code. This is a requirement for all code we merge. +* Please make sure to test your change before submitting the PR. See the [Hacking guide](HACKING.md) for details on how to do this. +* Make sure to run the test suite locally, before posting your PR. We use a CI system, meaning we don't even look at your PR, if the build and tests don't pass. +* If you need to update the code in an existing PR, force-push into the same branch, overriding old commits with new versions. +* After you have pushed a new version, add a comment about the new version (no notification is sent just for the commits, so it's easy to miss the update without an explicit comment). If you are a member of the systemd project on GitHub, remove the `reviewed/needs-rework` label. + +## Final Words + +We'd like to apologize in advance if we are not able to process and reply to your issue or PR right-away. We have a lot of work to do, but we are trying our best! + +Thank you very much for your contributions! diff --git a/docs/CONVERTING_TO_HOMED.md b/docs/CONVERTING_TO_HOMED.md new file mode 100644 index 0000000..78b6c61 --- /dev/null +++ b/docs/CONVERTING_TO_HOMED.md @@ -0,0 +1,135 @@ +--- +title: Converting Existing Users to systemd-homed +category: Users, Groups and Home Directories +layout: default +--- + +# Converting Existing Users to systemd-homed managed Users + +Traditionally on most Linux distributions, regular (human) users are managed +via entries in `/etc/passwd`, `/etc/shadow`, `/etc/group` and +`/etc/gshadow`. With the advent of +[`systemd-homed`](https://www.freedesktop.org/software/systemd/man/systemd-homed.service.html) +it might be desirable to convert an existing, traditional user account to a +`systemd-homed` managed one. Below is a brief guide how to do that. + +Before continuing, please read up on these basic concepts: + +* [Home Directories](https://systemd.io/HOME_DIRECTORY) +* [JSON User Records](https://systemd.io/USER_RECORD) +* [JSON Group Records](https://systemd.io/GROUP_RECORD) +* [User/Group Record Lookup API via Varlink](https://systemd.io/USER_GROUP_API) + +## Caveat + +This is a manual process, and possibly a bit fragile. Hence, do this at your +own risk, read up beforehand, and make a backup first. You know what's at +stake: your own home directory, i.e. all your personal data. + +## Step-By-Step + +Here's the step-by-step guide: + +0. Preparations: make sure you run a distribution that has `systemd-homed` + enabled and properly set up, including the necessary PAM and NSS + configuration updates. Make sure you have enough disk space in `/home/` for + a (temporary) second copy of your home directory. Make sure to backup your + home directory. Make sure to log out of your user account fully. Then log in + as root on the console. + +1. Rename your existing home directory to something safe. Let's say your user + ID is `foobar`. Then do: + + ``` + mv /home/foobar /home/foobar.saved + ``` + +2. Have a look at your existing user record, as stored in `/etc/passwd` and + related files. We want to use the same data for the new record, hence it's good + looking at the old data. Use commands such as: + + ``` + getent passwd foobar + getent shadow foobar + ``` + + This will tell you the `/etc/passwd` and `/etc/shadow` entries for your + user. For details about the fields, see the respective man pages + [passwd(5)](http://man7.org/linux/man-pages/man5/passwd.5.html) and + [shadow(5)](http://man7.org/linux/man-pages/man5/shadow.5.html). + + The fourth field in the `getent passwd foobar` output tells you the GID of + your user's main group. Depending on your distribution it's a group private + to the user, or a group shared by most local, regular users. Let's say the + GID reported is 1000, let's then query its details: + + ``` + getent group 1000 + ``` + + This will tell you the name of that group. If the name is the same as your + user name your distribution apparently provided you with a private group for + your user. If it doesn't match (and is something like `users`) it apparently + didn't. Note that `systemd-homed` will always manage a private group for + each user under the same name, hence if your distribution is one of the + latter kind, then there's a (minor) mismatch in structure when converting. + + Save the information reported by these three commands somewhere, for later + reference. + +3. Now edit your `/etc/passwd` file and remove your existing record + (i.e. delete a single line, the one of your user's account, leaving all + other lines unmodified). Similar for `/etc/shadow`, `/etc/group` (in case + you have a private group for your user) and `/etc/gshadow`. Most + distributions provide you with a tool for that, that adds safe + synchronization for these changes: `vipw`, `vipw -s`, `vigr` and `vigr -s`. + +4. At this point the old user account vanished, while the home directory still + exists safely under the `/home/foobar.saved` name. Let's now create a new + account with `systemd-homed`, using the same username and UID as before: + + ``` + homectl create foobar --uid=$UID --real-name=$GECOS + ``` + + In this command line, replace `$UID` by the UID you previously used, + i.e. the third field of the `getent passwd foobar` output above. Similar, + replace `$GECOS` by the GECOS field of your old account, i.e the fifth field + of the old output. If your distribution traditionally does not assign a + private group to regular user groups, then consider adding `--member-of=` + with the group name to get a modicum of compatibility with the status quo + ante: this way your new user account will still not have the old primary + group as new primary group, but will have it as auxiliary group. + + Consider reading through the + [homectl(1)](https://www.freedesktop.org/software/systemd/man/homectl.html) + manual page at this point, maybe there are a couple of other settings you + want to set for your new account. In particular, look at `--storage=` and + `--disk-size=`, in order to change how your home directory shall be stored + (the default `luks` storage is recommended). + +5. Your new user account exists now, but it has an empty home directory. Let's + now migrate your old home directory into it. For that let's mount the new + home directory temporarily and copy the data in. + + ``` + homectl with foobar -- rsync -aHAXv --remove-source-files /home/foobar.saved/ . + ``` + + This mounts the home directory of the user, and then runs the specified + `rsync` command which copies the contents of the old home directory into the + new. The new home directory is the working directory of the invoked `rsync` + process. We are invoking this command as root, hence the `rsync` runs as + root too. When the `rsync` command completes the home directory is + automatically unmounted again. Since we used `--remove-source-files` all files + copied are removed from the old home directory as the copy progresses. After + the command completes the old home directory should be empty. Let's remove + it hence: + + ``` + rmdir /home/foobar.saved + ``` + +And that's it, we are done already. You can log out now and should be able to +log in under your user account as usual, but now with `systemd-homed` managing +your home directory. diff --git a/docs/DESKTOP_ENVIRONMENTS.md b/docs/DESKTOP_ENVIRONMENTS.md new file mode 100644 index 0000000..9ae1aef --- /dev/null +++ b/docs/DESKTOP_ENVIRONMENTS.md @@ -0,0 +1,117 @@ +--- +title: Desktop Environment Integration +category: Concepts +layout: default +--- + +# Desktop Environments + +NOTE: This document is a work-in-progress. + +## Single Graphical Session + +systemd only supports running one graphical session per user at a time. +While this might not have always been the case historically, having multiple +sessions for one user running at the same time is problematic. +The DBus session bus is shared between all the logins, and services that are +started must be implicitly assigned to the user's current graphical session. + +In principle it is possible to run a single graphical session across multiple +logind seats, and this could be a way to use more than one display per user. +When a user logs in to a second seat, the seat resources could be assigned +to the existing session, allowing the graphical environment to present it +is a single seat. +Currently nothing like this is supported or even planned. + +## Pre-defined systemd units + +[`systemd.special(7)`](https://www.freedesktop.org/software/systemd/man/systemd.special.html) +defines the `graphical-session.target` and `graphical-session-pre.target` to +allow cross-desktop integration. Furthermore, systemd defines the three base +slices `background`, `app` and `session`. +All units should be placed into one of these slices depending on their purposes: + + * `session.slice`: Contains only processes essential to run the user's graphical session + * `app.slice`: Contains all normal applications that the user is running + * `background.slice`: Useful for low-priority background tasks + +The purpose of this grouping is to assign different priorities to the +applications. +This could e.g. mean reserving memory to session processes, +preferentially killing background tasks in out-of-memory situations +or assigning different memory/CPU/IO priorities to ensure that the session +runs smoothly under load. + +TODO: Will there be a default to place units into e.g. `app.slice` by default +rather than the root slice? + +## XDG standardization for applications + +To ensure cross-desktop compatibility and encourage sharing of good practices, +desktop environments should adhere to the following conventions: + + * Application units should follow the scheme `app[-<launcher>]-<ApplicationID>[@<RANDOM>].service` + or `app[-<launcher>]-<ApplicationID>-<RANDOM>.scope` + e.g: + - `app-gnome-org.gnome.Evince@12345.service` + - `app-flatpak-org.telegram.desktop@12345.service` + - `app-KDE-org.kde.okular@12345.service` + - `app-org.kde.amarok.service` + - `app-org.gnome.Evince-12345.scope` + * Using `.service` units instead of `.scope` units, i.e. allowing systemd to + start the process on behalf of the caller, + instead of the caller starting the process and letting systemd know about it, + is encouraged. + * The RANDOM should be a string of random characters to ensure that multiple instances + of the application can be launched. + It can be omitted in the case of a non-transient application services which can ensure + multiple instances are not spawned, such as a DBus activated application. + * If no application ID is available, the launcher should generate a reasonable + name when possible (e.g. using `basename(argv[0])`). This name must not + contain a `-` character. + +This has the following advantages: + * Using the `app-<launcher>-` prefix means that the unit defaults can be + adjusted using desktop environment specific drop-in files. + * The application ID can be retrieved by stripping the prefix and postfix. + This in turn should map to the corresponding `.desktop` file when available + +TODO: Define the name of slices that should be used. +This could be `app-<launcher>-<ApplicationID>-<RANDOM>.slice`. + +TODO: Does it really make sense to insert the `<launcher>`? In GNOME I am +currently using a drop-in to configure `BindTo=graphical-session.target`, +`CollectMode=inactive-or-failed` and `TimeoutSec=5s`. I feel that such a +policy makes sense, but it may make much more sense to just define a +global default for all (graphical) applications. + + * Should application lifetime be bound to the session? + * May the user have applications that do not belong to the graphical session (e.g. launched from SSH)? + * Could we maybe add a default `app-.service.d` drop-in configuration? + +## XDG autostart integration + +To allow XDG autostart integration, systemd ships a cross-desktop generator +to create appropriate units for the autostart directory +(`systemd-xdg-autostart-generator`). +Desktop Environments can opt-in to using this by starting +`xdg-desktop-autostart.target`. The systemd generator correctly handles +`OnlyShowIn=` and `NotShowin=`. It also handles the KDE and GNOME specific +`X-KDE-autostart-condition=` and `AutostartCondition=` by using desktop +environment provided binaries in an `ExecCondition=` line. + +However, this generator is somewhat limited in what it supports. For example, +all generated units will have `After=graphical-session.target` set on them, +it may therefore not be useful to start session services. + +Desktop files can be marked to be explicitly excluded from the generator using the line +`X-systemd-skip=true`. This should be set if an application provides its own +systemd service file for startup. + +## Startup and shutdown best practices + +Question here are: + + * Are there strong opinions on how the session-leader process should watch the user's session units? + * Should systemd/logind/… provide an integrated way to define a session in terms of a running *user* unit? + * Is having `gnome-session-shutdown.target` that is run with `replace-irreversibly` considered a good practice? diff --git a/docs/DISCOVERABLE_PARTITIONS.md b/docs/DISCOVERABLE_PARTITIONS.md new file mode 100644 index 0000000..fd8e927 --- /dev/null +++ b/docs/DISCOVERABLE_PARTITIONS.md @@ -0,0 +1,234 @@ +--- +title: Discoverable Partitions Specification +category: Concepts +layout: default +--- +# The Discoverable Partitions Specification + +_TL;DR: Let's automatically discover, mount and enable the root partition, +`/home/`, `/srv/`, `/var/` and `/var/tmp/` and the swap partitions based on +GUID Partition Tables (GPT)!_ + +This specification describes the use of GUID Partition Table (GPT) UUIDs to +enable automatic discovery of partitions and their intended mountpoints. +Traditionally Linux has made little use of partition types, mostly just +defining one UUID for file system/data partitions and another one for swap +partitions. With this specification, we introduce additional partition types +for specific uses. This has many benefits: + +* OS installers can automatically discover and make sense of partitions of + existing Linux installations. +* The OS can discover and mount the necessary file systems with a non-existent + or incomplete `/etc/fstab` file and without the `root=` kernel command line + option. +* Container managers (such as nspawn and libvirt-lxc) can introspect and set up + file systems contained in GPT disk images automatically and mount them to the + right places, thus allowing booting the same, identical images on bare metal + and in Linux containers. This enables true, natural portability of disk + images between physical machines and Linux containers. +* As a help to administrators and users partition manager tools can show more + descriptive information about partitions tables. + +Note that the OS side of this specification is currently implemented in +[systemd](http://systemd.io/) 211 and newer in the +[systemd-gpt-auto-generator(8)](http://www.freedesktop.org/software/systemd/man/systemd-gpt-auto-generator.html) +generator tool. Note that automatic discovery of the root only works if the +boot loader communicates this information to the OS, by implementing the [Boot +Loader +Interface](https://systemd.io/BOOT_LOADER_INTERFACE). + +## Defined Partition Type UUIDs + +| Partition Type UUID | Name | Allowed File Systems | Explanation | +|---------------------|------|----------------------|-------------| +| `44479540-f297-41b2-9af7-d131d5f0458a` | _Root Partition (x86)_ | Any native, optionally in LUKS | On systems with matching architecture, the first partition with this type UUID on the disk containing the active EFI ESP is automatically mounted to the root directory <tt>/</tt>. If the partition is encrypted with LUKS or has dm-verity integrity data (see below), the device mapper file will be named `/dev/mapper/root`. | +| `4f68bce3-e8cd-4db1-96e7-fbcaf984b709` | _Root Partition (x86-64)_ | ditto | ditto | +| `69dad710-2ce4-4e3c-b16c-21a1d49abed3` | _Root Partition (32-bit ARM)_ | ditto | ditto | +| `b921b045-1df0-41c3-af44-4c6f280d3fae` | _Root Partition (64-bit ARM/AArch64)_ | ditto | ditto | +| `993d8d3d-f80e-4225-855a-9daf8ed7ea97` | _Root Partition (Itanium/IA-64)_ | ditto | ditto | +| `60d5a7fe-8e7d-435c-b714-3dd8162144e1` | _Root Partition (RISC-V 32-bit)_ | ditto | ditto | +| `72ec70a6-cf74-40e6-bd49-4bda08e8f224` | _Root Partition (RISC-V 64-bit)_ | ditto | ditto | +| `d13c5d3b-b5d1-422a-b29f-9454fdc89d76` | _Root Verity Partition (x86)_ | A dm-verity superblock followed by hash data | On systems with matching architecture, contains dm-verity integrity hash data for the matching root partition. If this feature is used the partition UUID of the root partition should be the first 128bit of the root hash of the dm-verity hash data, and the partition UUID of this dm-verity partition should be the final 128bit of it, so that the root partition and its verity partition can be discovered easily, simply by specifying the root hash. | +| `2c7357ed-ebd2-46d9-aec1-23d437ec2bf5` | _Root Verity Partition (x86-64)_ | ditto | ditto | +| `7386cdf2-203c-47a9-a498-f2ecce45a2d6` | _Root Verity Partition (32-bit ARM)_ | ditto | ditto | +| `df3300ce-d69f-4c92-978c-9bfb0f38d820` | _Root Verity Partition (64-bit ARM/AArch64)_ | ditto | ditto | +| `86ed10d5-b607-45bb-8957-d350f23d0571` | _Root Verity Partition (Itanium/IA-64)_ | ditto | ditto | +| `ae0253be-1167-4007-ac68-43926c14c5de` | _Root Verity Partition (RISC-V 32-bit)_ | ditto | ditto | +| `b6ed5582-440b-4209-b8da-5ff7c419ea3d` | _Root Verity Partition (RISC-V 64-bit)_ | ditto | ditto | +| `75250d76-8cc6-458e-bd66-bd47cc81a812` | _`/usr/` Partition (x86)_ | Any native, optionally in LUKS | Similar semantics to root partition, but just the `/usr/` partition. | +| `8484680c-9521-48c6-9c11-b0720656f69e` | _`/usr/` Partition (x86-64)_ | ditto | ditto | +| `7d0359a3-02b3-4f0a-865c-654403e70625` | _`/usr/` Partition (32-bit ARM)_ | ditto | ditto | +| `b0e01050-ee5f-4390-949a-9101b17104e9` | _`/usr/` Partition (64-bit ARM/AArch64)_ | ditto | ditto | +| `4301d2a6-4e3b-4b2a-bb94-9e0b2c4225ea` | _`/usr/` Partition (Itanium/IA-64)_ | ditto | ditto | +| `b933fb22-5c3f-4f91-af90-e2bb0fa50702` | _`/usr/` Partition (RISC-V 32-bit)_ | ditto | ditto | +| `beaec34b-8442-439b-a40b-984381ed097d` | _`/usr/` Partition (RISC-V 64-bit)_ | ditto | ditto | +| `8f461b0d-14ee-4e81-9aa9-049b6fb97abd` | _`/usr/` Verity Partition (x86)_ | Any native, optionally in LUKS | Similar semantics to root Verity partition, but just for the `/usr/` partition. | +| `77ff5f63-e7b6-4633-acf4-1565b864c0e6` | _`/usr/` Verity Partition (x86-64)_ | ditto | ditto | +| `c215d751-7bcd-4649-be90-6627490a4c05` | _`/usr/` Verity Partition (32-bit ARM)_ | ditto | ditto | +| `6e11a4e7-fbca-4ded-b9e9-e1a512bb664e` | _`/usr/` Verity Partition (64-bit ARM/AArch64)_ | ditto | ditto | +| `6a491e03-3be7-4545-8e38-83320e0ea880` | _`/usr/` Verity Partition (Itanium/IA-64)_ | ditto | ditto | +| `cb1ee4e3-8cd0-4136-a0a4-aa61a32e8730` | _`/usr/` Verity Partition (RISC-V 32-bit)_ | ditto | ditto | +| `8f1056be-9b05-47c4-81d6-be53128e5b54` | _`/usr/` Verity Partition (RISC-V 64-bit)_ | ditto | ditto | +| `933ac7e1-2eb4-4f13-b844-0e14e2aef915` | _Home Partition_ | Any native, optionally in LUKS | The first partition with this type UUID on the disk containing the root partition is automatically mounted to `/home/`. If the partition is encrypted with LUKS, the device mapper file will be named `/dev/mapper/home`. | +| `3b8f8425-20e0-4f3b-907f-1a25a76f98e8` | _Server Data Partition_ | Any native, optionally in LUKS | The first partition with this type UUID on the disk containing the root partition is automatically mounted to `/srv/`. If the partition is encrypted with LUKS, the device mapper file will be named `/dev/mapper/srv`. | +| `4d21b016-b534-45c2-a9fb-5c16e091fd2d` | _Variable Data Partition_ | Any native, optionally in LUKS | The first partition with this type UUID on the disk containing the root partition is automatically mounted to `/var/` — under the condition that its partition UUID matches the first 128 bit of `HMAC-SHA256(machine-id, 0x4d21b016b53445c2a9fb5c16e091fd2d)` (i.e. the SHA256 HMAC hash of the binary type UUID keyed by the machine ID as read from [`/etc/machine-id`](https://www.freedesktop.org/software/systemd/man/machine-id.html). This special requirement is made because `/var/` (unlike the other partition types listed here) is inherently private to a specific installation and cannot possibly be shared between multiple OS installations on the same disk, and thus should be bound to a specific instance of the OS, identified by its machine ID. If the partition is encrypted with LUKS, the device mapper file will be named `/dev/mapper/var`. | +| `7ec6f557-3bc5-4aca-b293-16ef5df639d1` | _Temporary Data Partition_ | Any native, optionally in LUKS | The first partition with this type UUID on the disk containing the root partition is automatically mounted to `/var/tmp/`. If the partition is encrypted with LUKS, the device mapper file will be named `/dev/mapper/tmp`. Note that the intended mount point is indeed `/var/tmp/`, not `/tmp/`. The latter is typically maintained in memory via <tt>tmpfs</tt> and does not require a partition on disk. In some cases it might be desirable to make `/tmp/` persistent too, in which case it is recommended to make it a symlink or bind mount to `/var/tmp/`, thus not requiring its own partition type UUID. | +| `0657fd6d-a4ab-43c4-84e5-0933c84b4f4f` | _Swap_ | Swap | All swap partitions on the disk containing the root partition are automatically enabled. | +| `c12a7328-f81f-11d2-ba4b-00a0c93ec93b` | _EFI System Partition_ | VFAT | The ESP used for the current boot is automatically mounted to `/efi/` (or `/boot/` as fallback), unless a different partition is mounted there (possibly via `/etc/fstab`, or because the Extended Boot Loader Partition — see below — exists) or the directory is non-empty on the root disk. This partition type is defined by the [UEFI Specification](http://www.uefi.org/specifications). | +| `bc13c2ff-59e6-4262-a352-b275fd6f7172` | _Extended Boot Loader Partition_ | Typically VFAT | The Extended Boot Loader Partition (XBOOTLDR) used for the current boot is automatically mounted to <tt>/boot/</tt>, unless a different partition is mounted there (possibly via <tt>/etc/fstab</tt>) or the directory is non-empty on the root disk. This partition type is defined by the [Boot Loader Specification](https://systemd.io/BOOT_LOADER_SPECIFICATION). | +| `0fc63daf-8483-4772-8e79-3d69d8477de4` | _Other Data Partitions_ | Any native, optionally in LUKS | No automatic mounting takes place for other Linux data partitions. This partition type should be used for all partitions that carry Linux file systems. The installer needs to mount them explicitly via entries in <tt>/etc/fstab</tt>. Optionally, these partitions may be encrypted with LUKS. | + +Other GPT type IDs might be used on Linux, for example to mark software RAID or +LVM partitions. The definitions of those GPT types is outside of the scope of +this specification. + +[systemd-id128(1)](http://www.freedesktop.org/software/systemd/man/systemd-id128.html) +may be used to list those UUIDs. + +## Partition Names + +For partitions of the types listed above it is recommended to use +human-friendly, descriptive partition names in the GPT partition table, for +example "*Home*", "*Server* *Data*", "*Fedora* *Root*" and similar, possibly +localized. + +## Partition Flags + +For the root, `/usr/`, server data, home, variable data, temporary data and swap +partitions, the partition flag bit 63 ("*no-auto*") may be used to turn off +auto-discovery for the specific partition. If set, the partition will not be +automatically mounted or enabled. + +For the root, `/usr/` server data, home, variable data and temporary data +partitions, the partition flag bit 60 ("*read-only*") may be used to mark a +partition for read-only mounts only. If set, the partition will be mounted +read-only instead of read-write. Note that the variable data partition and the +temporary data partition will generally not be able to serve their purpose if +marked read-only, since by their very definition they are supposed to be +mutable. (The home and server data partitions are generally assumed to be +mutable as well, but the requirement for them is not equally strong.) Because +of that, while the read-only flag is defined and supported, it's almost never a +good idea to actually use it for these partitions. + +Note that these two flag definitions happen to map nicely to the ones used by +Microsoft Basic Data Partitions. + +## Suggested Mode of Operation + +An *installer* that repartitions the hard disk _should_ use the above UUID +partition types for appropriate partitions it creates. + +An *installer* which supports a "manual partitioning" interface _may_ choose to +pre-populate the interface with swap, `/home/`, `/srv/`, `/var/tmp/` partitions +of pre-existing Linux installations, identified with the GPT type UUIDs +above. The installer should not pre-populate such an interface with any +identified root, `/usr` or `/var/` partition unless the intention is to +overwrite an existing operating system that might be installed. + +An *installer* _may_ omit creating entries in `/etc/fstab` for root, `/home/`, +`/srv/`, `/var/`, `/var/tmp` and for the swap partitions if they use these UUID +partition types, and are the first partitions on the disk of each type. If the +ESP shall be mounted to `/efi/` (or `/boot/`), it may additionally omit +creating the entry for it in `/etc/fstab`. If an extended boot partition is +used, or if the EFI partition shall not be mounted to `/efi/` or `/boot/`, it +_must_ create `/etc/fstab` entries for them. If other partitions are used (for +example for `/usr/` or `/var/lib/mysql/`), the installer _must_ register these +in `/etc/fstab`. The `root=` parameter passed to the kernel by the boot loader +may be omitted if the root partition is the first one on the disk of its type. +If the root partition is not the first one on the disk, the `root=` parameter +_must_ be passed to the kernel by the boot loader. An installer that mounts a +root, `/usr/`, `/home/`, `/srv/`, `/var/`, or `/var/tmp/` file system with the +partition types defined as above which contains a LUKS header _must_ call the +device mapper device "root", "usr", "home", "srv", "var" or "tmp", +respectively. This is necessary to ensure that the automatic discovery will +never result in different device mapper names than any static configuration by +the installer, thus eliminating possible naming conflicts and ambiguities. + +An *operating* *system* _should_ automatically discover and mount the first +root partition that does not have the no-auto flag set (as described above) by +scanning the disk containing the currently used EFI ESP. It _should_ +automatically discover and mount the first `/usr/`, `/home/`, `/srv/`, `/var/`, +`/var/tmp/` and swap partitions that do not have the no-auto flag set by +scanning the disk containing the discovered root partition. It should +automatically discover and mount the partition containing the currently used +EFI ESP to `/efi/` (or `/boot/` as fallback). It should automatically discover +and mount the partition containing the currently used Extended Boot Loader +Partition to `/boot/`. It _should not_ discover or automatically mount +partitions with other UUID partition types, or partitions located on other +disks, or partitions with the no-auto flag set. User configuration shall +always override automatic discovery and mounting. If a root, `/usr/`, +`/home/`, `/srv/`, `/boot/`, `/var/`, `/var/tmp/`, `/efi/`, `/boot/` or swap +partition is listed in `/etc/fstab` or with `root=` on the kernel command line, +it _must_ take precedence over automatically discovered partitions. If a +`/home/`, `/usr/`, `/srv/`, `/boot/`, `/var/`, `/var/tmp/`, `/efi/` or `/boot/` +directory is found to be populated already in the root partition, the automatic +discovery _must not_ mount any discovered file system over it. + +A *container* *manager* should automatically discover and mount the root, +`/usr/`, `/home/`, `/srv/`, `/var/`, `/var/tmp/` partitions inside a container +disk image. It may choose to mount any discovered ESP and/or XBOOOTLDR +partition to `/efi/` or `/boot/`. It should ignore any swap should they be +included in a container disk image. + +If a btrfs file system is automatically discovered and mounted by the operating +system/container manager it will be mounted with its *default* subvolume. The +installer should make sure to set the default subvolume correctly using "btrfs +subvolume set-default". + +## Sharing of File Systems between Installations + +If two Linux-based operating systems are installed on the same disk, the scheme +above suggests that they may share the swap, `/home/`, `/srv/`, `/var/tmp/`, +ESP, XBOOTLDR. However, they should each have their own root, `/usr/` and +`/var/` partition. + +## Frequently Asked Questions + +### Why are you taking my `/etc/fstab` away? + +We are not. `/etc/fstab` always overrides automatic discovery and is indeed +mentioned in the specifications. We are simply trying to make the boot and +installation processes of Linux a bit more robust and self-descriptive. + +### Why did you only define the root partition for x86, x86-64, ARM, ARM64, ia64? + +The automatic discovery of the root partition is defined to operate on the disk +containing the current EFI System Partition (ESP). Since EFI only exists on +x86, x86-64, ia64, and ARM so far, we only defined root partition UUIDs for +these architectures. Should EFI become more common on other architectures, we +can define additional UUIDs for them. + +### Why define distinct root partition UUIDs for the various architectures? + +This allows disk images that may be booted on multiple architectures to use +discovery of the appropriate root partition on each architecture. + +### Doesn't this break multi-boot scenarios? + +No, it doesn't. The specification says that installers may not stop creating +`/etc/fstab` or stop including `root=` on the kernel command line, unless the used +partitions are the first ones of their type on the disk. Additionally, +`/etc/fstab` and `root=` both override automatic discovery. Multi-boot is hence +well supported, since it doesn't change anything for anything but the first +installation. + +That all said, it's not expected that generic installers generally stop setting +`root=` and creating `/etc/fstab` anyway. The option to drop these configuration +bits is primarily something for appliance-like devices. However, generic +installers should *still* set the right GPT partition types for the partitions +they create so that container managers, partition tools and administrators can +benefit. Phrased differently, this specification introduces A) the +*recommendation* to use the newly defined partition types to tag things +properly and B) the *option* to then drop `root=` and `/etc/fstab`. While we +advertise A) to *all* installers, we only propose B) for simpler, +appliance-like installations. + +### What partitioning tools will create a DPS-compliant partition table? + +As of util-linux 2.25.2, the fdisk tool provides type codes to create the root, +home, and swap partitions that the DPS expects, but the gdisk tool (version +0.8.10) and its variants do not support creation of a root file system with a +matching type code. By default, fdisk will create an old-style MBR, not a GPT, +so typing 'l' to list partition types will not show the choices that the root +partition with the correct UUID. You must first create an empty GPT and then +type 'l' in order for the DPS-compliant type codes to be available. diff --git a/docs/DISTRO_PORTING.md b/docs/DISTRO_PORTING.md new file mode 100644 index 0000000..2e4782f --- /dev/null +++ b/docs/DISTRO_PORTING.md @@ -0,0 +1,94 @@ +--- +title: Porting systemd To New Distributions +category: Concepts +layout: default +--- + +# Porting systemd To New Distributions + +## HOWTO + +You need to make the follow changes to adapt systemd to your +distribution: + +1. Find the right configure parameters for: + + * `-Drootprefix=` + * `-Dsysvinit-path=` + * `-Dsysvrcnd-path=` + * `-Drc-local=` + * `-Dloadkeys-path=` + * `-Dsetfont-path=` + * `-Dtty-gid=` + * `-Dntp-servers=` + * `-Ddns-servers=` + * `-Dsupport-url=` + +2. Try it out. + + Play around (as an ordinary user) with + `/usr/lib/systemd/systemd --test --system` for a test run + of systemd without booting. This will read the unit files and + print the initial transaction it would execute during boot-up. + This will also inform you about ordering loops and suchlike. + +## Compilation options + +The default configuration does not enable any optimization or hardening +options. This is suitable for development and testing, but not for end-user +installations. + +For deployment, optimization (`-O2` or `-O3` compiler options), link time +optimization (`-Db_lto=true` meson option), and hardening (e.g. +`-D_FORTIFY_SOURCE=2`, `-fstack-protector-strong`, `-fstack-clash-protection`, +`-fcf-protection`, `-pie` compiler options, and `-z relro`, `-z now`, +`--as-needed` linker options) are recommended. The most appropriate set of +options depends on the architecture and distribution specifics so no default is +provided. + +## NTP Pool + +By default, systemd-timesyncd uses the Google Public NTP servers +`time[1-4].google.com`, if no other NTP configuration is available. +They serve time that uses a +[leap second smear](https://developers.google.com/time/smear) +and can be up to .5s off from servers that use stepped leap seconds. + +If you prefer to use leap second steps, please register your own +vendor pool at ntp.org and make it the built-in default by +passing `-Dntp-servers=` to meson. Registering vendor +pools is [free](http://www.pool.ntp.org/en/vendors.html). + +Use `-Dntp-servers=` to direct systemd-timesyncd to different fallback +NTP servers. + +## DNS Servers + +By default, systemd-resolved uses Cloudflare and Google Public DNS servers +`1.1.1.1`, `8.8.8.8`, `1.0.0.1`, `8.8.4.4`, `2606:4700:4700::1111`, `2001:4860:4860::8888`, `2606:4700:4700::1001`, `2001:4860:4860::8844` +as fallback, if no other DNS configuration is available. + +Use `-Ddns-servers=` to direct systemd-resolved to different fallback +DNS servers. + +## PAM + +The default PAM config shipped by systemd is really bare bones. +It does not include many modules your distro might want to enable +to provide a more seamless experience. For example, limits set in +`/etc/security/limits.conf` will not be read unless you load `pam_limits`. +Make sure you add modules your distro expects from user services. + +Pass `-Dpamconfdir=no` to meson to avoid installing this file and +instead install your own. + +## Contributing Upstream + +We generally do no longer accept distribution-specific patches to +systemd upstream. If you have to make changes to systemd's source code +to make it work on your distribution, unless your code is generic +enough to be generally useful, we are unlikely to merge it. Please +always consider adopting the upstream defaults. If that is not +possible, please maintain the relevant patches downstream. + +Thank you for understanding. diff --git a/docs/ENVIRONMENT.md b/docs/ENVIRONMENT.md new file mode 100644 index 0000000..74a71bb --- /dev/null +++ b/docs/ENVIRONMENT.md @@ -0,0 +1,262 @@ +--- +title: Known Environment Variables +category: Interfaces +layout: default +--- + +# Known Environment Variables + +A number of systemd components take additional runtime parameters via +environment variables. Many of these environment variables are not supported at +the same level as command line switches and other interfaces are: we don't +document them in the man pages and we make no stability guarantees for +them. While they generally are unlikely to be dropped any time soon again, we +do not want to guarantee that they stay around for good either. + +Below is an (incomprehensive) list of the environment variables understood by +the various tools. Note that this list only covers environment variables not +documented in the proper man pages. + +All tools: + +* `$SYSTEMD_OFFLINE=[0|1]` — if set to `1`, then `systemctl` will + refrain from talking to PID 1; this has the same effect as the historical + detection of `chroot()`. Setting this variable to `0` instead has a similar + effect as `SYSTEMD_IGNORE_CHROOT=1`; i.e. tools will try to + communicate with PID 1 even if a `chroot()` environment is detected. + You almost certainly want to set this to `1` if you maintain a package build system + or similar and are trying to use a modern container system and not plain + `chroot()`. + +* `$SYSTEMD_IGNORE_CHROOT=1` — if set, don't check whether being invoked in a + `chroot()` environment. This is particularly relevant for systemctl, as it + will not alter its behaviour for `chroot()` environments if set. Normally it + refrains from talking to PID 1 in such a case; turning most operations such + as `start` into no-ops. If that's what's explicitly desired, you might + consider setting `SYSTEMD_OFFLINE=1`. + +* `$SD_EVENT_PROFILE_DELAYS=1` — if set, the sd-event event loop implementation + will print latency information at runtime. + +* `$SYSTEMD_PROC_CMDLINE` — if set, the contents are used as the kernel command + line instead of the actual one in /proc/cmdline. This is useful for + debugging, in order to test generators and other code against specific kernel + command lines. + +* `$SYSTEMD_FSTAB` — if set, use this path instead of /etc/fstab. Only useful + for debugging. + +* `$SYSTEMD_CRYPTTAB` — if set, use this path instead of /etc/crypttab. Only + useful for debugging. Currently only supported by systemd-cryptsetup-generator. + +* `$SYSTEMD_EFI_OPTIONS` — if set, used instead of the string in the + SystemdOptions EFI variable. Analogous to `$SYSTEMD_PROC_CMDLINE`. + +* `$SYSTEMD_IN_INITRD` — takes a boolean. If set, overrides initrd detection. + This is useful for debugging and testing initrd-only programs in the main + system. + +* `$SYSTEMD_BUS_TIMEOUT=SECS` — specifies the maximum time to wait for method call + completion. If no time unit is specified, assumes seconds. The usual other units + are understood, too (us, ms, s, min, h, d, w, month, y). If it is not set or set + to 0, then the built-in default is used. + +* `$SYSTEMD_MEMPOOL=0` — if set, the internal memory caching logic employed by + hash tables is turned off, and libc malloc() is used for all allocations. + +* `$SYSTEMD_EMOJI=0` — if set, tools such as "systemd-analyze security" will + not output graphical smiley emojis, but ASCII alternatives instead. Note that + this only controls use of Unicode emoji glyphs, and has no effect on other + Unicode glyphs. + +* `$RUNTIME_DIRECTORY` — various tools use this variable to locate the + appropriate path under /run. This variable is also set by the manager when + RuntimeDirectory= is used, see systemd.exec(5). + +* `$SYSTEMD_CRYPT_PREFIX` — if set configures the hash method prefix to use for + UNIX crypt() when generating passwords. By default the system's "preferred + method" is used, but this can be overridden with this environment + variable. Takes a prefix such as `$6$` or `$y$`. (Note that this is only + honoured on systems built with libxcrypt and is ignored on systems using + glibc's original, internal crypt() implementation.) + +* `$SYSTEMD_RDRAND=0` — if set, the RDRAND instruction will never be used, + even if the CPU supports it. + +* `$SYSTEMD_SECCOMP=0` – if set, seccomp filters will not be enforced, even if + support for it is compiled in and available in the kernel. + +* `$SYSTEMD_LOG_SECCOMP=1` — if set, system calls blocked by seccomp filtering, + for example in systemd-nspawn, will be logged to the audit log, if the current + kernel version supports this. + +systemctl: + +* `$SYSTEMCTL_FORCE_BUS=1` — if set, do not connect to PID1's private D-Bus + listener, and instead always connect through the dbus-daemon D-bus broker. + +* `$SYSTEMCTL_INSTALL_CLIENT_SIDE=1` — if set, enable or disable unit files on + the client side, instead of asking PID 1 to do this. + +* `$SYSTEMCTL_SKIP_SYSV=1` — if set, do not call out to SysV compatibility hooks. + +systemd-nspawn: + +* `$SYSTEMD_NSPAWN_UNIFIED_HIERARCHY=1` — if set, force nspawn into unified + cgroup hierarchy mode. + +* `$SYSTEMD_NSPAWN_API_VFS_WRITABLE=1` — if set, make /sys and /proc/sys and + friends writable in the container. If set to "network", leave only + /proc/sys/net writable. + +* `$SYSTEMD_NSPAWN_CONTAINER_SERVICE=…` — override the "service" name nspawn + uses to register with machined. If unset defaults to "nspawn", but with this + variable may be set to any other value. + +* `$SYSTEMD_NSPAWN_USE_CGNS=0` — if set, do not use cgroup namespacing, even if + it is available. + +* `$SYSTEMD_NSPAWN_LOCK=0` — if set, do not lock container images when running. + +* `$SYSTEMD_NSPAWN_TMPFS_TMP=0` — if set, do not overmount /tmp in the + container with a tmpfs, but leave the directory from the image in place. + +systemd-logind: + +* `$SYSTEMD_BYPASS_HIBERNATION_MEMORY_CHECK=1` — if set, report that + hibernation is available even if the swap devices do not provide enough room + for it. + +systemd-udevd: + +* `$NET_NAMING_SCHEME=` – if set, takes a network naming scheme (i.e. one of + "v238", "v239", "v240"…, or the special value "latest") as parameter. If + specified udev's net_id builtin will follow the specified naming scheme when + determining stable network interface names. This may be used to revert to + naming schemes of older udev versions, in order to provide more stable naming + across updates. This environment variable takes precedence over the kernel + command line option `net.naming-scheme=`, except if the value is prefixed + with `:` in which case the kernel command line option takes precedence, if it + is specified as well. + +* `$SYSTEMD_REBOOT_TO_FIRMWARE_SETUP` — if set overrides systemd-logind's + built-in EFI logic of requesting a reboot into the firmware. Takes a + boolean. If set to false the functionality is turned off entirely. If set to + true instead of requesting a reboot into the firmware setup UI through EFI a + file `/run/systemd/reboot-to-firmware-setup` is created whenever this is + requested. This file may be checked for by services run during system + shutdown in order to request the appropriate operation from the firmware in + an alternative fashion. + +* `$SYSTEMD_REBOOT_TO_BOOT_LOADER_MENU` — similar to the above, allows + overriding of systemd-logind's built-in EFI logic of requesting a reboot into + the boot loader menu. Takes a boolean. If set to false the functionality is + turned off entirely. If set to true instead of requesting a reboot into the + boot loader menu through EFI a file `/run/systemd/reboot-to-boot-loader-menu` + is created whenever this is requested. The file contains the requested boot + loader menu timeout in µs, formatted in ASCII decimals, or zero in case no + timeout is requested. This file may be checked for by services run during + system shutdown in order to request the appropriate operation from the boot + loader in an alternative fashion. + +* `$SYSTEMD_REBOOT_TO_BOOT_LOADER_ENTRY` — similar to the above, allows + overriding of systemd-logind's built-in EFI logic of requesting a reboot into + a specific boot loader entry. Takes a boolean. If set to false the + functionality is turned off entirely. If set to true instead of requesting a + reboot into a specific boot loader entry through EFI a file + `/run/systemd/reboot-to-boot-loader-entry` is created whenever this is + requested. The file contains the requested boot loader entry identifier. This + file may be checked for by services run during system shutdown in order to + request the appropriate operation from the boot loader in an alternative + fashion. Note that by default only boot loader entries which follow the [Boot + Loader Specification](https://systemd.io/BOOT_LOADER_SPECIFICATION) and are + placed in the ESP or the Extended Boot Loader partition may be selected this + way. However, if a directory `/run/boot-loader-entries/` exists, the entries + are loaded from there instead. The directory should contain the usual + directory hierarchy mandated by the Boot Loader Specification, i.e. the entry + drop-ins should be placed in + `/run/boot-loader-entries/loader/entries/*.conf`, and the files referenced by + the drop-ins (including the kernels and initrds) somewhere else below + `/run/boot-loader-entries/`. Note that all these files may be (and are + supposed to be) symlinks. systemd-logind will load these files on-demand, + these files can hence be updated (ideally atomically) whenever the boot + loader configuration changes. A foreign boot loader installer script should + hence synthesize drop-in snippets and symlinks for all boot entries at boot + or whenever they change if it wants to integrate with systemd-logind's APIs. + +installed systemd tests: + +* `$SYSTEMD_TEST_DATA` — override the location of test data. This is useful if + a test executable is moved to an arbitrary location. + +nss-systemd: + +* `$SYSTEMD_NSS_BYPASS_SYNTHETIC=1` — if set, `nss-systemd` won't synthesize + user/group records for the `root` and `nobody` users if they are missing from + `/etc/passwd`. + +* `$SYSTEMD_NSS_DYNAMIC_BYPASS=1` — if set, `nss-systemd` won't return + user/group records for dynamically registered service users (i.e. users + registered through `DynamicUser=1`). + +* `$SYSTEMD_NSS_BYPASS_BUS=1` — if set, `nss-systemd` won't use D-Bus to do + dynamic user lookups. This is primarily useful to make `nss-systemd` work + safely from within `dbus-daemon`. + +systemd-timedated: + +* `$SYSTEMD_TIMEDATED_NTP_SERVICES=…` — colon-separated list of unit names of + NTP client services. If set, `timedatectl set-ntp on` enables and starts the + first existing unit listed in the environment variable, and + `timedatectl set-ntp off` disables and stops all listed units. + +systemd-sulogin-shell: + +* `$SYSTEMD_SULOGIN_FORCE=1` — This skips asking for the root password if the + root password is not available (such as when the root account is locked). + See `sulogin(8)` for more details. + +bootctl and other tools that access the EFI System Partition (ESP): + +* `$SYSTEMD_RELAX_ESP_CHECKS=1` — if set, the ESP validation checks are + relaxed. Specifically, validation checks that ensure the specified ESP path + is a FAT file system are turned off, as are checks that the path is located + on a GPT partition with the correct type UUID. + +* `$SYSTEMD_ESP_PATH=…` — override the path to the EFI System Partition. This + may be used to override ESP path auto detection, and redirect any accesses to + the ESP to the specified directory. Not that unlike with bootctl's --path= + switch only very superficial validation of the specified path is done when + this environment variable is used. + +systemd itself: + +* `$SYSTEMD_ACTIVATION_UNIT` — set for all NSS and PAM module invocations that + are done by the service manager on behalf of a specific unit, in child + processes that are later (after execve()) going to become unit + processes. Contains the full unit name (e.g. "foobar.service"). NSS and PAM + modules can use this information to determine in which context and on whose + behalf they are being called, which may be useful to avoid deadlocks, for + example to bypass IPC calls to the very service that is about to be + started. Note that NSS and PAM modules should be careful to only rely on this + data when invoked privileged, or possibly only when getppid() returns 1, as + setting environment variables is of course possible in any even unprivileged + contexts. + +* `$SYSTEMD_ACTIVATION_SCOPE` — closely related to `$SYSTEMD_ACTIVATION_UNIT`, + it is either set to `system` or `user` depending on whether the NSS/PAM + module is called by systemd in `--system` or `--user` mode. + +systemd-remount-fs: + +* `$SYSTEMD_REMOUNT_ROOT_RW=1` — if set and no entry for the root directory + exists in /etc/fstab (this file always takes precedence), then the root + directory is remounted writable. This is primarily used by + systemd-gpt-auto-generator to ensure the root partition is mounted writable + in accordance to the GPT partition flags. + +systemd-firstboot and localectl: + +* `SYSTEMD_LIST_NON_UTF8_LOCALES=1` – if set non-UTF-8 locales are listed among + the installed ones. By default non-UTF-8 locales are suppressed from the + selection, since we are living in the 21st century. diff --git a/docs/GROUP_RECORD.md b/docs/GROUP_RECORD.md new file mode 100644 index 0000000..0180049 --- /dev/null +++ b/docs/GROUP_RECORD.md @@ -0,0 +1,162 @@ +--- +title: JSON Group Records +category: Users, Groups and Home Directories +layout: default +--- + +# JSON Group Records + +Long story short: JSON Group Records are to `struct group` what [JSON User +Records](https://systemd.io/USER_RECORD.md) are to `struct passwd`. + +Conceptually, much of what applies to JSON user records also applies to JSON +group records. They also consist of seven sections, with similar properties and +they carry some identical (or at least very similar) fields. + +## Fields in the `regular` section + +`groupName` → A string with the UNIX group name. Matches the `gr_name` field of +UNIX/glibc NSS `struct group`, or the shadow structure `struct sgrp`'s +`sg_namp` field. + +`realm` → The "realm" the group belongs to, conceptually identical to the same +field of user records. A string in DNS domain name syntax. + +`description` → A descriptive string for the group. This is similar to the +`realName` field of user records, and accepts arbitrary strings, as long as +they follow the same GECOS syntax requirements as `realName`. + +`disposition` → The disposition of the group, conceptually identical to the +same field of user records. A string. + +`service` → A string, an identifier for the service managing this group record +(this field is typically in reverse domain name syntax.) + +`lastChangeUSec` → An unsigned 64bit integer, a timestamp (in µs since the UNIX +epoch 1970) of the last time the group record has been modified. (Covers only +the `regular`, `perMachine` and `privileged` sections). + +`gid` → An unsigned integer in the range 0…4294967295: the numeric UNIX group +ID (GID) to use for the group. This corresponds to the `gr_gid` field of +`struct group`. + +`members` → An array of strings, listing user names that are members of this +group. Note that JSON user records also contain a `memberOf` field, or in other +words a group membership can either be denoted in the JSON user record or in +the JSON group record, or in both. The list of memberships should be determined +as the combination of both lists (plus optionally others). If a user is listed +as member of a group and doesn't exist it should be ignored. This field +corresponds to the `gr_mem` field of `struct group` and the `sg_mem` field of +`struct sgrp`. + +`administrators` → Similarly, an array of strings, listing user names that +shall be considered "administrators" of this group. This field corresponds to +the `sg_adm` field of `struct sgrp`. + +`privileged`/`perMachine`/`binding`/`status`/`signature`/`secret` → The +objects/arrays for the other six group record sections. These are organized the +same way as for the JSON user records, and have the same semantics. + +## Fields in the `privileged` section + +The following fields are defined: + +`hashedPassword` → An array of strings with UNIX hashed passwords; see the +matching field for user records for details. This field corresponds to the +`sg_passwd` field of `struct sgrp` (and `gr_passwd` of `struct group` in a +way). + +## Fields in the `perMachine` section + +`matchMachineId`/`matchHostname` → Strings, match expressions similar as for +user records, see the user record documentation for details. + +The following fields are defined for the `perMachine` section and are defined +equivalent to the fields of the same name in the `regular` section, and +override those: + +`gid`, `members`, `administrators` + +## Fields in the `binding` section + +The following fields are defined for the `binding` section, and are equivalent +to the fields of the same name in the `regular` and `perMachine` sections: + +`gid` + +## Fields in the `status` section + +The following fields are defined in the `status` section, and are mostly +equivalent to the fields of the same name in the `regular` section, though with +slightly different conceptual semantics, see the same fields in the user record +documentation: + +`service` + +## Fields in the `signature` section + +The fields in this section are defined identically to those in the matching +section in the user record. + +## Fields in the `secret` section + +Currently no fields are defined in this section for group records. + +## Mapping to `struct group` and `struct sgrp` + +When mapping classic UNIX group records (i.e. `struct group` and `struct sgrp`) +to JSON group records the following mappings should be applied: + +| Structure | Field | Section | Field | Condition | +|----------------|-------------|--------------|------------------|----------------------------| +| `struct group` | `gr_name` | `regular` | `groupName` | | +| `struct group` | `gr_passwd` | `privileged` | `password` | (See notes below) | +| `struct group` | `gr_gid` | `regular` | `gid` | | +| `struct group` | `gr_mem` | `regular` | `members` | | +| `struct sgrp` | `sg_namp` | `regular` | `groupName` | | +| `struct sgrp` | `sg_passwd` | `privileged` | `password` | (See notes below) | +| `struct sgrp` | `sg_adm` | `regular` | `administrators` | | +| `struct sgrp` | `sg_mem` | `regular` | `members` | | + +At this time almost all Linux machines employ shadow passwords, thus the +`gr_passwd` field in `struct group` is set to `"x"`, and the actual password +is stored in the shadow entry `struct sgrp`'s field `sg_passwd`. + +## Extending These Records + +The same logic and recommendations apply as for JSON user records. + +## Examples + +A reasonable group record for a system group might look like this: + +```json +{ + "groupName" : "systemd-resolve", + "gid" : 193, + "status" : { + "6b18704270e94aa896b003b4340978f1" : { + "service" : "io.systemd.NameServiceSwitch" + } + } +} +``` + +And here's a more complete one for a regular group: + +```json +{ + "groupName" : "grobie", + "binding" : { + "6b18704270e94aa896b003b4340978f1" : { + "gid" : 60232 + } + }, + "disposition" : "regular", + "status" : { + "6b18704270e94aa896b003b4340978f1" : { + "service" : "io.systemd.Home" + } + } +} +``` diff --git a/docs/HACKING.md b/docs/HACKING.md new file mode 100644 index 0000000..7ec3dde --- /dev/null +++ b/docs/HACKING.md @@ -0,0 +1,138 @@ +--- +title: Hacking on systemd +category: Contributing +layout: default +--- + +# Hacking on systemd + +We welcome all contributions to systemd. If you notice a bug or a missing +feature, please feel invited to fix it, and submit your work as a GitHub Pull +Request (PR) at https://github.com/systemd/systemd/pull/new. + +Please make sure to follow our [Coding Style](CODING_STYLE.md) when submitting patches. +Also have a look at our [Contribution Guidelines](CONTRIBUTING.md). + +When adding new functionality, tests should be added. For shared functionality +(in `src/basic/` and `src/shared/`) unit tests should be sufficient. The general +policy is to keep tests in matching files underneath `src/test/`, +e.g. `src/test/test-path-util.c` contains tests for any functions in +`src/basic/path-util.c`. If adding a new source file, consider adding a matching +test executable. For features at a higher level, tests in `src/test/` are very +strongly recommended. If that is not possible, integration tests in `test/` are +encouraged. + +Please also have a look at our list of [code quality tools](CODE_QUALITY.md) we have setup for systemd, +to ensure our codebase stays in good shape. + +Please always test your work before submitting a PR. For many of the components +of systemd testing is straight-forward as you can simply compile systemd and +run the relevant tool from the build directory. + +For some components (most importantly, systemd/PID1 itself) this is not +possible, however. In order to simplify testing for cases like this we provide +a set of `mkosi` build files directly in the source tree. `mkosi` is a tool for +building clean OS images from an upstream distribution in combination with a +fresh build of the project in the local working directory. To make use of this, +please acquire `mkosi` from https://github.com/systemd/mkosi first, unless your +distribution has packaged it already and you can get it from there. After the +tool is installed, symlink the settings file for your distribution of choice from +.mkosi/ to mkosi.default in the project root directory (note that the package +manager for this distro needs to be installed on your host system). After doing +that, it is sufficient to type `mkosi` in the systemd project directory to +generate a disk image `image.raw` you can boot either in `systemd-nspawn` or in +an UEFI-capable VM: + +``` +# systemd-nspawn -bi image.raw +``` + +or: + +``` +# qemu-system-x86_64 -enable-kvm -m 512 -smp 2 -bios /usr/share/edk2/ovmf/OVMF_CODE.fd -hda image.raw +``` + +Every time you rerun the `mkosi` command a fresh image is built, incorporating +all current changes you made to the project tree. + +Alternatively, you may install the systemd version from your git check-out +directly on top of your host system's directory tree. This mostly works fine, +but of course you should know what you are doing as you might make your system +unbootable in case of a bug in your changes. Also, you might step into your +package manager's territory with this. Be careful! + +And never forget: most distributions provide very simple and convenient ways to +install all development packages necessary to build systemd. For example, on +Fedora the following command line should be sufficient to install all of +systemd's build dependencies: + +``` +# dnf builddep systemd +``` + +Putting this all together, here's a series of commands for preparing a patch +for systemd (this example is for Fedora): + +```sh +$ sudo dnf builddep systemd # install build dependencies +$ sudo dnf install mkosi # install tool to quickly build images +$ git clone https://github.com/systemd/systemd.git +$ cd systemd +$ vim src/core/main.c # or wherever you'd like to make your changes +$ meson build # configure the build +$ ninja -C build # build it locally, see if everything compiles fine +$ ninja -C build test # run some simple regression tests +$ ln -s .mkosi/mkosi.fedora mkosi.default # Configure mkosi to build a fedora image +$ (umask 077; echo 123 > mkosi.rootpw) # set root password used by mkosi +$ sudo mkosi # build a test image +$ sudo systemd-nspawn -bi image.raw # boot up the test image +$ git add -p # interactively put together your patch +$ git commit # commit it +$ git push REMOTE HEAD:refs/heads/BRANCH + # where REMOTE is your "fork" on GitHub + # and BRANCH is a branch name. +``` + +And after that, head over to your repo on GitHub and click "Compare & pull request" + +Happy hacking! + + +## Developer and release modes + +In the default meson configuration (`-Dmode=developer`), certain checks are +enabled that are suitable when hacking on systemd (such as internal +documentation consistency checks). Those are not useful when compiling for code +for distribution and can be disabled by setting `-Dmode=release`. + +## Fuzzers + +systemd includes fuzzers in `src/fuzz/` that use libFuzzer and are automatically +run by [OSS-Fuzz](https://github.com/google/oss-fuzz) with sanitizers. +To add a fuzz target, create a new `src/fuzz/fuzz-foo.c` file with a `LLVMFuzzerTestOneInput` +function and add it to the list in `src/fuzz/meson.build`. + +Whenever possible, a seed corpus and a dictionary should also be added with new +fuzz targets. The dictionary should be named `src/fuzz/fuzz-foo.dict` and the seed +corpus should be built and exported as `$OUT/fuzz-foo_seed_corpus.zip` in +`tools/oss-fuzz.sh`. + +The fuzzers can be built locally if you have libFuzzer installed by running +`tools/oss-fuzz.sh`. You should also confirm that the fuzzer runs in the +OSS-Fuzz environment by checking out the OSS-Fuzz repo, and then running +commands like this: + +``` +python infra/helper.py build_image systemd +python infra/helper.py build_fuzzers --sanitizer memory systemd ../systemd +python infra/helper.py run_fuzzer systemd fuzz-foo +``` + +If you find a bug that impacts the security of systemd, please follow the +guidance in [CONTRIBUTING.md](CONTRIBUTING.md) on how to report a security vulnerability. + +For more details on building fuzzers and integrating with OSS-Fuzz, visit: + +- [Setting up a new project - OSS-Fuzz](https://google.github.io/oss-fuzz/getting-started/new-project-guide/) +- [Tutorials - OSS-Fuzz](https://google.github.io/oss-fuzz/reference/useful-links/#tutorials) diff --git a/docs/HOME_DIRECTORY.md b/docs/HOME_DIRECTORY.md new file mode 100644 index 0000000..a3eabb7 --- /dev/null +++ b/docs/HOME_DIRECTORY.md @@ -0,0 +1,177 @@ +--- +title: Home Directories +category: Users, Groups and Home Directories +layout: default +--- + +# Home Directories + +[`systemd-homed.service(8)`](https://www.freedesktop.org/software/systemd/man/systemd-homed.service.html) +manages home directories of regular ("human") users. Each directory it manages +encapsulates both the data store and the user record of the user so that it +comprehensively describes the user account, and is thus naturally portable +between systems without any further, external metadata. This document describes +the format used by these home directories, in context of the storage mechanism +used. + +## General Structure + +Inside of the home directory a file `~/.identity` contains the JSON formatted +user record of the user. It follows the format defined in [`JSON User +Records`](https://systemd.io/USER_RECORD). It is recommended to bring the +record into 'normalized' form (i.e. all objects should contain their fields +sorted alphabetically by their key) before storing it there, though this is not +required nor enforced. Since the user record is cryptographically signed the +user cannot make modifications to the file on their own (at least not without +corrupting it, or knowing the private key used for signing the record). Note +that user records are stored here without their `binding`, `status` and +`secret` sections, i.e. only with the sections included in the signature plus +the signature section itself. + +## Storage Mechanism: Plain Directory/`btrfs` Subvolume + +If the plain directory or `btrfs` subvolume storage mechanism of +`systemd-homed` is used (i.e. `--storage=directory` or `--storage=subvolume` on +the +[`homectl(1)`](https://www.freedesktop.org/software/systemd/man/homectl.html) +command line) the home directory requires no special set-up besides including +the user record in the `~/.identity` file. + +It is recommended to name home directories managed this way by +`systemd-homed.service` by the user name, suffixed with `.homedir` (example: +`lennart.homedir` for a user `lennart`) but this is not enforced. When the user +is logged in the directory is generally mounted to `/home/$USER` (in our +example: `/home/lennart`), thus dropping the suffix while the home directory is +active. `systemd-homed` will automatically discover home directories named this +way in `/home/*.homedir` and synthesize NSS user records for them as they show +up. + +## Storage Mechanism: `fscrypt` Directories + +This storage mechanism is mostly identical to the plain directory storage +mechanism, except that the home directory is encrypted using `fscrypt`. (Use +`--storage=fscrypt` on the `homectl` command line.) Key management is +implemented via extended attributes on the directory itself: for each password +an extended attribute `trusted.fscrypt_slot0`, `trusted.fscrypt_slot1`, +`trusted.fscrypt_slot2`, … is maintained. It's value contains a colon-separated +pair of Base64 encoded data fields. The first field contains a salt value, the +second field the encrypted volume key. The latter is encrypted using AES256 in +counter mode, using a key derived from the password via PBKDF2-HMAC-SHA512 +together with the salt value. The construction is similar to what LUKS does for +`dm-crypt` encrypted volumes. Note that extended attributes are not encrypted +by `fscrypt` and hence are suitable for carry the key slots. Moreover, by using +extended attributes the slots are directly attached to the directory and an +independent sidecar key database is not required. + +## Storage Mechanism: `cifs` Home Directories + +In this storage mechanism the home directory is mounted from a CIFS server and +service at login, configured inside the user record. (Use `--storage=cifs` on +the `homectl` command line.) The local password of the user is used to log into +the CIFS service. The directory share needs to contain the user record in +`~/.identity` as well. Note that this means that the user record needs to be +registered locally before it can be mounted for the first time, since CIFS +domain and server information needs to be known *before* the mount. Note that +for all other storage mechanisms it is entirely sufficient if the directories +or storage artifacts are placed at the right locations — all information to +activate them can be derived automatically from their mere availability. + +## Storage Mechanism: `luks` Home Directories + +This is the most advanced and most secure storage mechanism and consists of a +Linux file system inside a LUKS2 volume inside a loopback file (or on removable +media). (Use `--storage=luks` on the `homectl` command line.) Specifically: + +* The image contains a GPT partition table. For now it should only contain a + single partition, and that partition must have the type UUID + `773f91ef-66d4-49b5-bd83-d683bf40ad16`. It's partition label must be the + user name. + +* This partition must contain a LUKS2 volume, whose label must be the user + name. The LUKS2 volume must contain a LUKS2 token field of type + `systemd-homed`. The JSON data of this token must have a `record` field, + containing a string with base64-encoded data. This data is the JSON user + record, in the same serialization as in `~/.identity`, though encrypted. The + JSON data of this token must also have an `iv` field, which contains a + base64-encoded binary initialization vector for the encryption. The + encryption used is the same as the LUKS2 volume itself uses, unlocked by the + same volume key, but based on its own IV. + +* Inside of this LUKS2 volume must be a Linux file system, one of `ext4`, + `btrfs` and `xfs`. The file system label must be the user name. + +* This file system should contain a single directory named after the user. This + directory will become the home directory of the user when activated. It + contains a second copy of the user record in the `~/.identity` file, like in + the other storage mechanisms. + +The image file should either reside in a directory `/home/` on the system, +named after the user, suffixed with `.home`. When activated the container home +directory is mounted to the same path, though with the `.home` suffix dropped — +unless a different mount point is defined in the user record. (e.g.: the +loopback file `/home/waldo.home` is mounted to `/home/waldo` while activated.) +When the image is stored on removable media (such as a USB stick) the image +file can be directly `dd`'ed onto it, the format is unchanged. The GPT envelope +should ensure the image is properly recognizable as a home directory both when +used in a loopback file and on a removable USB stick. (Note that when mounting +a home directory from an USB stick it too defaults to a directory in `/home/`, +named after the username, with no further suffix.) + +Rationale for the GPT partition table envelope: this way the image is nicely +discoverable and recognizable already by partition managers as a home +directory. Moreover, when copied onto a USB stick the GPT envelope makes sure +the stick is properly recognizable as a portable home directory +medium. (Moreover it allows to embed additional partitions later on, for +example for allowing a multi-purpose USB stick that contains both a home +directory and a generic storage volume.) + +Rationale for including the encrypted user record in the LUKS2 header: +Linux kernel file system implementations are generally not robust towards +maliciously formatted file systems; there's a good chance that file system +images can be used as attack vectors, exploiting the kernel. Thus it is +necessary to validate the home directory image *before* mounting it and +establishing a minimal level of trust. Since the user record data is +cryptographically signed and user records not signed with a recognized private +key are not accepted a minimal level of trust between the system and the home +directory image is established. + +Rationale for storing the home directory one level below to root directory of +the contained file system: this way special directories such as `lost+found/` +do not show up in the user's home directory. + +## Algorithm + +Regardless of the storage mechanism used, an activated home directory +necessarily involves a mount point to be established. In case of the +directory-based storage mechanisms (`directory`, `subvolume` and `fscrypt`) +this is a bind mount, in case of `cifs` this is a CIFS network mount, and in +case of the LUKS2 backend a regular block device mount of the file system +contained in the LUKS2 image. By requiring a mount for all cases (even for +those that already are a directory) a clear logic is defined to distinguish +active and inactive home directories, so that the directories become +inaccessible under their regular path the instant they are +deactivated. Moreover, the `nosuid`, `nodev` and `noexec` flags configured in +the user record are applied when the bind mount is established. + +During activation, the user records retained on the host, the user record +stored in the LUKS2 header (in case of the LUKS2 storage mechanism) and the +user record stored inside the home directory in `~/.identity` are +compared. Activation is only permitted if they match the same user and are +signed by a recognized key. When the three instances differ in `lastChangeUSec` +field, the newest record wins, and is propagated to the other two locations. + +During activation the file system checker (`fsck`) appropriate for the +selected file system is automatically invoked, ensuring the file system is in a +healthy state before it is mounted. + +If the UID assigned to a user does not match the owner of the home directory in +the file system, the home directory is automatically and recursively `chown()`ed +to the correct UID. + +Depending on the `luksDiscard` setting of the user record either the backing +loopback file is `fallocate()`ed during activation, or the mounted file system +is `FITRIM`ed after mounting, to ensure the setting is correctly enforced. + +When deactivating a home directory, the file system or block device is trimmed +or extended as configured in the `luksOfflineDiscard` setting of the user +record. diff --git a/docs/INITRD_INTERFACE.md b/docs/INITRD_INTERFACE.md new file mode 100644 index 0000000..e59bbcc --- /dev/null +++ b/docs/INITRD_INTERFACE.md @@ -0,0 +1,73 @@ +--- +title: Initrd Interface +category: Interfaces +layout: default +--- + + +# The initrd Interface of systemd + +The Linux initrd mechanism (short for "initial RAM disk") refers to a small +file system archive that is unpacked by the kernel and contains the first +userspace code that runs. It typically finds and transitions into the actual +root file system to use. systemd supports both initrd and initrd-less boots. If +an initrd is used it is a good idea to pass a few bits of runtime information +from the initrd to systemd in order to avoid duplicate work and to provide +performance data to the administrator. In this page we attempt to roughly +describe the interfaces that exist between the initrd and systemd. These +interfaces are currently used by dracut and the ArchLinux initrds. + +* The initrd should mount `/run/` as a tmpfs and pass it pre-mounted when + jumping into the main system when executing systemd. The mount options should + be `mode=755,nodev,nosuid,strictatime`. + +* It's highly recommended that the initrd also mounts `/usr/` (if split off) as + appropriate and passes it pre-mounted to the main system, to avoid the + problems described in [Booting without /usr is + Broken](http://freedesktop.org/wiki/Software/systemd/separate-usr-is-broken). + +* If the executable `/run/initramfs/shutdown` exists systemd will use it to + jump back into the initrd on shutdown. `/run/initramfs/` should be a usable + initrd environment to which systemd will pivot back and the `shutdown` + executable in it should be able to detach all complex storage that for + example was needed to mount the root file system. It's the job of the initrd + to set up this directory and executable in the right way so that this works + correctly. The shutdown binary is invoked with the shutdown verb as `argv[1]`, + optionally followed (in `argv[2]`, `argv[3]`, … systemd's original command + line options, for example `--log-level=` and similar. + +* Storage daemons run from the initrd should follow the guide on [systemd + and Storage Daemons for the Root File + System](https://systemd.io/ROOT_STORAGE_DAEMONS) to survive properly from the + boot initrd all the way to the point where systemd jumps back into the initrd + for shutdown. + +One last clarification: we use the term _initrd_ very generically here +describing any kind of early boot file system, regardless whether that might be +implemented as an actual ramdisk, ramfs or tmpfs. We recommend using _initrd_ +in this sense as a term that is unrelated to the actual backing technologies +used. + +Oh, and one last question before closing: instead of implementing these +features in your own distro's initrd, may I suggest just using Dracut instead? +It's all already implemented there! + +## Using systemd inside an initrd + +It is also possible and recommended to implement the initrd itself based on +systemd. Here are a few terse notes: + +* Provide `/etc/initrd-release` in the initrd image. The idea is that it follows + the same format as the usual `/etc/os-release` but describes the initial RAM + disk implementation rather than the OS. systemd uses the existence of this + file as a flag whether to run in initial RAM disk mode, or not. + +* When run in initrd mode, systemd and its components will read a couple of + additional command line arguments, which are generally prefixed with `rd.` + +* To transition into the main system image invoke `systemctl switch-root`. + +* The switch-root operation will result in a killing spree of all running + processes. Some processes might need to be excluded from that, see the guide + on [systemd and Storage Daemons for the Root File + System](https://systemd.io/ROOT_STORAGE_DAEMONS). diff --git a/docs/JOURNAL_FILE_FORMAT.md b/docs/JOURNAL_FILE_FORMAT.md new file mode 100644 index 0000000..3910669 --- /dev/null +++ b/docs/JOURNAL_FILE_FORMAT.md @@ -0,0 +1,694 @@ +--- +title: Journal File Format +category: Interfaces +layout: default +--- + +# Journal File Format + +_Note that this document describes the binary on-disk format of journals +only. For interfacing with web technologies there's the [Journal JSON +Format](http://www.freedesktop.org/wiki/Software/systemd/json). For transfer +of journal data across the network there's the [Journal Export +Format](http://www.freedesktop.org/wiki/Software/systemd/export)._ + +The systemd journal stores log data in a binary format with several features: + +* Fully indexed by all fields +* Can store binary data, up to 2^64-1 in size +* Seekable +* Primarily append-based, hence robust to corruption +* Support for in-line compression +* Support for in-line Forward Secure Sealing + +This document explains the basic structure of the file format on disk. We are +making this available primarily to allow review and provide documentation. Note +that the actual implementation in the [systemd +codebase](https://github.com/systemd/systemd/blob/master/src/journal/) is the +only ultimately authoritative description of the format, so if this document +and the code disagree, the code is right. That said we'll of course try hard to +keep this document up-to-date and accurate. + +Instead of implementing your own reader or writer for journal files we ask you +to use the [Journal's native C +API](http://www.freedesktop.org/software/systemd/man/sd-journal.html) to access +these files. It provides you with full access to the files, and will not +withhold any data. If you find a limitation, please ping us and we might add +some additional interfaces for you. + +If you need access to the raw journal data in serialized stream form without C +API our recommendation is to make use of the [Journal Export +Format](http://www.freedesktop.org/wiki/Software/systemd/export), which you can +get via "journalctl -o export" or via systemd-journal-gatewayd. The export +format is much simpler to parse, but complete and accurate. Due to its +stream-based nature it is not indexed. + +_Or, to put this in other words: this low-level document is probably not what +you want to use as base of your project. You want our [C +API](http://www.freedesktop.org/software/systemd/man/sd-journal.html) instead! +And if you really don't want the C API, then you want the [Journal Export +Format](http://www.freedesktop.org/wiki/Software/systemd/export) instead! This +document is primarily for your entertainment and education. Thank you!_ + +This document assumes you have a basic understanding of the journal concepts, +the properties of a journal entry and so on. If not, please go and read up, +then come back! This is a good opportunity to read about the [basic properties +of journal +entries](http://www.freedesktop.org/software/systemd/man/systemd.journal-fields.html), +in particular realize that they may include binary non-text data (though +usually don't), and the same field might have multiple values assigned within +the same entry. + +This document describes the current format of systemd 246. The documented +format is compatible with the format used in the first versions of the journal, +but received various compatible and incompatible additions since. + +If you are wondering why the journal file format has been created in the first +place instead of adopting an existing database implementation, please have a +look [at this +thread](https://lists.freedesktop.org/archives/systemd-devel/2012-October/007054.html). + + +## Basics + +* All offsets, sizes, time values, hashes (and most other numeric values) are 64bit unsigned integers in LE format. +* Offsets are always relative to the beginning of the file. +* The 64bit hash function siphash24 is used for newer journal files. For older files [Jenkins lookup3](https://en.wikipedia.org/wiki/Jenkins_hash_function) is used, more specifically `jenkins_hashlittle2()` with the first 32bit integer it returns as higher 32bit part of the 64bit value, and the second one uses as lower 32bit part. +* All structures are aligned to 64bit boundaries and padded to multiples of 64bit +* The format is designed to be read and written via memory mapping using multiple mapped windows. +* All time values are stored in usec since the respective epoch. +* Wall clock time values are relative to the Unix time epoch, i.e. January 1st, 1970. (`CLOCK_REALTIME`) +* Monotonic time values are always stored jointly with the kernel boot ID value (i.e. `/proc/sys/kernel/random/boot_id`) they belong to. They tend to be relative to the start of the boot, but aren't for containers. (`CLOCK_MONOTONIC`) +* Randomized, unique 128bit IDs are used in various locations. These are generally UUID v4 compatible, but this is not a requirement. + +## General Rules + +If any kind of corruption is noticed by a writer it should immediately rotate +the file and start a new one. No further writes should be attempted to the +original file, but it should be left around so that as little data as possible +is lost. + +If any kind of corruption is noticed by a reader it should try hard to handle +this gracefully, such as skipping over the corrupted data, but allowing access +to as much data around it as possible. + +A reader should verify all offsets and other data as it reads it. This includes +checking for alignment and range of offsets in the file, especially before +trying to read it via a memory map. + +A reader must interleave rotated and corrupted files as good as possible and +present them as single stream to the user. + +All fields marked as "reserved" must be initialized with 0 when writing and be +ignored on reading. They are currently not used but might be used later on. + + +## Structure + +The file format's data structures are declared in +[journal-def.h](https://github.com/systemd/systemd/blob/master/src/journal/journal-def.h). + +The file format begins with a header structure. After the header structure +object structures follow. Objects are appended to the end as time +progresses. Most data stored in these objects is not altered anymore after +having been written once, with the exception of records necessary for +indexing. When new data is appended to a file the writer first writes all new +objects to the end of the file, and then links them up at front after that's +done. Currently, seven different object types are known: + +```c +enum { + OBJECT_UNUSED, + OBJECT_DATA, + OBJECT_FIELD, + OBJECT_ENTRY, + OBJECT_DATA_HASH_TABLE, + OBJECT_FIELD_HASH_TABLE, + OBJECT_ENTRY_ARRAY, + OBJECT_TAG, + _OBJECT_TYPE_MAX +}; +``` + +* A **DATA** object, which encapsulates the contents of one field of an entry, i.e. a string such as `_SYSTEMD_UNIT=avahi-daemon.service`, or `MESSAGE=Foobar made a booboo.` but possibly including large or binary data, and always prefixed by the field name and "=". +* A **FIELD** object, which encapsulates a field name, i.e. a string such as `_SYSTEMD_UNIT` or `MESSAGE`, without any `=` or even value. +* An **ENTRY** object, which binds several **DATA** objects together into a log entry. +* A **DATA_HASH_TABLE** object, which encapsulates a hash table for finding existing **DATA** objects. +* A **FIELD_HASH_TABLE** object, which encapsulates a hash table for finding existing **FIELD** objects. +* An **ENTRY_ARRAY** object, which encapsulates a sorted array of offsets to entries, used for seeking by binary search. +* A **TAG** object, consisting of an FSS sealing tag for all data from the beginning of the file or the last tag written (whichever is later). + +## Header + +The Header struct defines, well, you guessed it, the file header: + +```c +_packed_ struct Header { + uint8_t signature[8]; /* "LPKSHHRH" */ + le32_t compatible_flags; + le32_t incompatible_flags; + uint8_t state; + uint8_t reserved[7]; + sd_id128_t file_id; + sd_id128_t machine_id; + sd_id128_t boot_id; /* last writer */ + sd_id128_t seqnum_id; + le64_t header_size; + le64_t arena_size; + le64_t data_hash_table_offset; + le64_t data_hash_table_size; + le64_t field_hash_table_offset; + le64_t field_hash_table_size; + le64_t tail_object_offset; + le64_t n_objects; + le64_t n_entries; + le64_t tail_entry_seqnum; + le64_t head_entry_seqnum; + le64_t entry_array_offset; + le64_t head_entry_realtime; + le64_t tail_entry_realtime; + le64_t tail_entry_monotonic; + /* Added in 187 */ + le64_t n_data; + le64_t n_fields; + /* Added in 189 */ + le64_t n_tags; + le64_t n_entry_arrays; + /* Added in 246 */ + le64_t data_hash_chain_depth; + le64_t field_hash_chain_depth; +}; +``` + +The first 8 bytes of Journal files must contain the ASCII characters `LPKSHHRH`. + +If a writer finds that the **machine_id** of a file to write to does not match +the machine it is running on it should immediately rotate the file and start a +new one. + +When journal file is first created the **file_id** is randomly and uniquely +initialized. + +When a writer opens a file it shall initialize the **boot_id** to the current +boot id of the system. + +The currently used part of the file is the **header_size** plus the +**arena_size** field of the header. If a writer needs to write to a file where +the actual file size on disk is smaller than the reported value it shall +immediately rotate the file and start a new one. If a writer is asked to write +to a file with a header that is shorter than its own definition of the struct +Header, it shall immediately rotate the file and start a new one. + +The **n_objects** field contains a counter for objects currently available in +this file. As objects are appended to the end of the file this counter is +increased. + +The first object in the file starts immediately after the header. The last +object in the file is at the offset **tail_object_offset**, which may be 0 if +no object is in the file yet. + +The **n_entries**, **n_data**, **n_fields**, **n_tags**, **n_entry_arrays** are +counters of the objects of the specific types. + +**tail_entry_seqnum** and **head_entry_seqnum** contain the sequential number +(see below) of the last or first entry in the file, respectively, or 0 if no +entry has been written yet. + +**tail_entry_realtime** and **head_entry_realtime** contain the wallclock +timestamp of the last or first entry in the file, respectively, or 0 if no +entry has been written yet. + +**tail_entry_monotonic** is the monotonic timestamp of the last entry in the +file, referring to monotonic time of the boot identified by **boot_id**. + +**data_hash_chain_depth** is a counter of the deepest chain in the data hash +table, minus one. This is updated whenever a chain is found that is longer than +the previous deepest chain found. Note that the counter is updated during hash +table lookups, as the chains are traversed. This counter is used to determine +when it is a good time to rotate the journal file, because hash collisions +became too frequent. + +Similar, **field_hash_chain_depth** is a counter of the deepest chain in the +field hash table, minus one. + + +## Extensibility + +The format is supposed to be extensible in order to enable future additions of +features. Readers should simply skip objects of unknown types as they read +them. If a compatible feature extension is made a new bit is registered in the +header's **compatible_flags** field. If a feature extension is used that makes +the format incompatible a new bit is registered in the header's +**incompatible_flags** field. Readers should check these two bit fields, if +they find a flag they don't understand in compatible_flags they should continue +to read the file, but if they find one in **incompatible_flags** they should +fail, asking for an update of the software. Writers should refuse writing if +there's an unknown bit flag in either of these fields. + +The file header may be extended as new features are added. The size of the file +header is stored in the header. All header fields up to **n_data** are known to +unconditionally exist in all revisions of the file format, all fields starting +with **n_data** needs to be explicitly checked for via a size check, since they +were additions after the initial release. + +Currently only five extensions flagged in the flags fields are known: + +```c +enum { + HEADER_INCOMPATIBLE_COMPRESSED_XZ = 1 << 0, + HEADER_INCOMPATIBLE_COMPRESSED_LZ4 = 1 << 1, + HEADER_INCOMPATIBLE_KEYED_HASH = 1 << 2, + HEADER_INCOMPATIBLE_COMPRESSED_ZSTD = 1 << 3, +}; + +enum { + HEADER_COMPATIBLE_SEALED = 1 << 0, +}; +``` + +HEADER_INCOMPATIBLE_COMPRESSED_XZ indicates that the file includes DATA objects +that are compressed using XZ. Similarly, HEADER_INCOMPATIBLE_COMPRESSED_LZ4 +indicates that the file includes DATA objects that are compressed with the LZ4 +algorithm. And HEADER_INCOMPATIBLE_COMPRESSED_ZSTD indicates that there are +objects compressed with ZSTD. + +HEADER_INCOMPATIBLE_KEYED_HASH indicates that instead of the unkeyed Jenkins +hash function the keyed siphash24 hash function is used for the two hash +tables, see below. + +HEADER_COMPATIBLE_SEALED indicates that the file includes TAG objects required +for Forward Secure Sealing. + + +## Dirty Detection + +```c +enum { + STATE_OFFLINE = 0, + STATE_ONLINE = 1, + STATE_ARCHIVED = 2, + _STATE_MAX +}; +``` + +If a file is opened for writing the **state** field should be set to +STATE_ONLINE. If a file is closed after writing the **state** field should be +set to STATE_OFFLINE. After a file has been rotated it should be set to +STATE_ARCHIVED. If a writer is asked to write to a file that is not in +STATE_OFFLINE it should immediately rotate the file and start a new one, +without changing the file. + +After and before the state field is changed `fdatasync()` should be executed on +the file to ensure the dirty state hits disk. + + +## Sequence Numbers + +All entries carry sequence numbers that are monotonically counted up for each +entry (starting at 1) and are unique among all files which carry the same +**seqnum_id** field. This field is randomly generated when the journal daemon +creates its first file. All files generated by the same journal daemon instance +should hence carry the same seqnum_id. This should guarantee a monotonic stream +of sequential numbers for easy interleaving even if entries are distributed +among several files, such as the system journal and many per-user journals. + + +## Concurrency + +The file format is designed to be usable in a simultaneous +single-writer/multiple-reader scenario. The synchronization model is very weak +in order to facilitate storage on the most basic of file systems (well, the +most basic ones that provide us with `mmap()` that is), and allow good +performance. No file locking is used. The only time where disk synchronization +via `fdatasync()` should be enforced is after and before changing the **state** +field in the file header (see below). It is recommended to execute a memory +barrier after appending and initializing new objects at the end of the file, +and before linking them up in the earlier objects. + +This weak synchronization model means that it is crucial that readers verify +the structural integrity of the file as they read it and handle invalid +structure gracefully. (Checking what you read is a pretty good idea out of +security considerations anyway.) This specifically includes checking offset +values, and that they point to valid objects, with valid sizes and of the type +and hash value expected. All code must be written with the fact in mind that a +file with inconsistent structure might just be inconsistent temporarily, and +might become consistent later on. Payload OTOH requires less scrutiny, as it +should only be linked up (and hence visible to readers) after it was +successfully written to memory (though not necessarily to disk). On non-local +file systems it is a good idea to verify the payload hashes when reading, in +order to avoid annoyances with `mmap()` inconsistencies. + +Clients intending to show a live view of the journal should use `inotify()` for +this to watch for files changes. Since file writes done via `mmap()` do not +result in `inotify()` writers shall truncate the file to its current size after +writing one or more entries, which results in inotify events being +generated. Note that this is not used as a transaction scheme (it doesn't +protect anything), but merely for triggering wakeups. + +Note that inotify will not work on network file systems if reader and writer +reside on different hosts. Readers which detect they are run on journal files +on a non-local file system should hence not rely on inotify for live views but +fall back to simple time based polling of the files (maybe recheck every 2s). + + +## Objects + +All objects carry a common header: + +```c +enum { + OBJECT_COMPRESSED_XZ = 1 << 0, + OBJECT_COMPRESSED_LZ4 = 1 << 1, + OBJECT_COMPRESSED_ZSTD = 1 << 2, +}; + +_packed_ struct ObjectHeader { + uint8_t type; + uint8_t flags; + uint8_t reserved[6]; + le64_t size; + uint8_t payload[]; +}; +``` + +The **type** field is one of the object types listed above. The **flags** field +currently knows three flags: OBJECT_COMPRESSED_XZ, OBJECT_COMPRESSED_LZ4 and +OBJECT_COMPRESSED_ZSTD. It is only valid for DATA objects and indicates that +the data payload is compressed with XZ/LZ4/ZSTD. If one of the +OBJECT_COMPRESSED_* flags is set for an object then the matching +HEADER_INCOMPATIBLE_COMPRESSED_XZ/HEADER_INCOMPATIBLE_COMPRESSED_LZ4/HEADER_INCOMPATIBLE_COMPRESSED_ZSTD +flag must be set for the file as well. At most one of these three bits may be +set. The **size** field encodes the size of the object including all its +headers and payload. + + +## Data Objects + +```c +_packed_ struct DataObject { + ObjectHeader object; + le64_t hash; + le64_t next_hash_offset; + le64_t next_field_offset; + le64_t entry_offset; /* the first array entry we store inline */ + le64_t entry_array_offset; + le64_t n_entries; + uint8_t payload[]; +}; +``` + +Data objects carry actual field data in the **payload[]** array, including a +field name, a `=` and the field data. Example: +`_SYSTEMD_UNIT=foobar.service`. The **hash** field is a hash value of the +payload. If the `HEADER_INCOMPATIBLE_KEYED_HASH` flag is set in the file header +this is the siphash24 hash value of the payload, keyed by the file ID as stored +in the **file_id** field of the file header. If the flag is not set it is the +non-keyed Jenkins hash of the payload instead. The keyed hash is preferred as +it makes the format more robust against attackers that want to trigger hash +collisions in the hash table. + +**next_hash_offset** is used to link up DATA objects in the DATA_HASH_TABLE if +a hash collision happens (in a singly linked list, with an offset of 0 +indicating the end). **next_field_offset** is used to link up data objects with +the same field name from the FIELD object of the field used. + +**entry_offset** is an offset to the first ENTRY object referring to this DATA +object. **entry_array_offset** is an offset to an ENTRY_ARRAY object with +offsets to other entries referencing this DATA object. Storing the offset to +the first ENTRY object in-line is an optimization given that many DATA objects +will be referenced from a single entry only (for example, `MESSAGE=` frequently +includes a practically unique string). **n_entries** is a counter of the total +number of ENTRY objects that reference this object, i.e. the sum of all +ENTRY_ARRAYS chained up from this object, plus 1. + +The **payload[]** field contains the field name and date unencoded, unless +OBJECT_COMPRESSED_XZ/OBJECT_COMPRESSED_LZ4/OBJECT_COMPRESSED_ZSTD is set in the +`ObjectHeader`, in which case the payload is compressed with the indicated +compression algorithm. + + +## Field Objects + +```c +_packed_ struct FieldObject { + ObjectHeader object; + le64_t hash; + le64_t next_hash_offset; + le64_t head_data_offset; + uint8_t payload[]; +}; +``` + +Field objects are used to enumerate all possible values a certain field name +can take in the entire journal file. + +The **payload[]** array contains the actual field name, without '=' or any +field value. Example: `_SYSTEMD_UNIT`. The **hash** field is a hash value of +the payload. As for the DATA objects, this too is either the `.file_id` keyed +siphash24 hash of the payload, or the non-keyed Jenkins hash. + +**next_hash_offset** is used to link up FIELD objects in the FIELD_HASH_TABLE +if a hash collision happens (in singly linked list, offset 0 indicating the +end). **head_data_offset** points to the first DATA object that shares this +field name. It is the head of a singly linked list using DATA's +**next_field_offset** offset. + + +## Entry Objects + +``` +_packed_ struct EntryItem { + le64_t object_offset; + le64_t hash; +}; + +_packed_ struct EntryObject { + ObjectHeader object; + le64_t seqnum; + le64_t realtime; + le64_t monotonic; + sd_id128_t boot_id; + le64_t xor_hash; + EntryItem items[]; +}; +``` + +An ENTRY object binds several DATA objects together into one log entry, and +includes other metadata such as various timestamps. + +The **seqnum** field contains the sequence number of the entry, **realtime** +the realtime timestamp, and **monotonic** the monotonic timestamp for the boot +identified by **boot_id**. + +The **xor_hash** field contains a binary XOR of the hashes of the payload of +all DATA objects referenced by this ENTRY. This value is usable to check the +contents of the entry, being independent of the order of the DATA objects in +the array. Note that even for files that have the +`HEADER_INCOMPATIBLE_KEYED_HASH` flag set (and thus siphash24 the otherwise +used hash function) the hash function used for this field, as singular +exception, is the Jenkins lookup3 hash function. The XOR hash value is used to +quickly compare the contents of two entries, and to define a well-defined order +between two entries that otherwise have the same sequence numbers and +timestamps. + +The **items[]** array contains references to all DATA objects of this entry, +plus their respective hashes (which are calculated the same way as in the DATA +objects, i.e. keyed by the file ID). + +In the file ENTRY objects are written ordered monotonically by sequence +number. For continuous parts of the file written during the same boot +(i.e. with the same boot_id) the monotonic timestamp is monotonic too. Modulo +wallclock time jumps (due to incorrect clocks being corrected) the realtime +timestamps are monotonic too. + + +## Hash Table Objects + +```c +_packed_ struct HashItem { + le64_t head_hash_offset; + le64_t tail_hash_offset; +}; + +_packed_ struct HashTableObject { + ObjectHeader object; + HashItem items[]; +}; +``` + +The structure of both DATA_HASH_TABLE and FIELD_HASH_TABLE objects are +identical. They implement a simple hash table, with each cell containing +offsets to the head and tail of the singly linked list of the DATA and FIELD +objects, respectively. DATA's and FIELD's next_hash_offset field are used to +chain up the objects. Empty cells have both offsets set to 0. + +Each file contains exactly one DATA_HASH_TABLE and one FIELD_HASH_TABLE +objects. Their payload is directly referred to by the file header in the +**data_hash_table_offset**, **data_hash_table_size**, +**field_hash_table_offset**, **field_hash_table_size** fields. These offsets do +_not_ point to the object headers but directly to the payloads. When a new +journal file is created the two hash table objects need to be created right +away as first two objects in the stream. + +If the hash table fill level is increasing over a certain fill level (Learning +from Java's Hashtable for example: > 75%), the writer should rotate the file +and create a new one. + +The DATA_HASH_TABLE should be sized taking into account to the maximum size the +file is expected to grow, as configured by the administrator or disk space +considerations. The FIELD_HASH_TABLE should be sized to a fixed size; the +number of fields should be pretty static as it depends only on developers' +creativity rather than runtime parameters. + + +## Entry Array Objects + + +```c +_packed_ struct EntryArrayObject { + ObjectHeader object; + le64_t next_entry_array_offset; + le64_t items[]; +}; +``` + +Entry Arrays are used to store a sorted array of offsets to entries. Entry +arrays are strictly sorted by offsets on disk, and hence by their timestamps +and sequence numbers (with some restrictions, see above). + +Entry Arrays are chained up. If one entry array is full another one is +allocated and the **next_entry_array_offset** field of the old one pointed to +it. An Entry Array with **next_entry_array_offset** set to 0 is the last in the +list. To optimize allocation and seeking, as entry arrays are appended to a +chain of entry arrays they should increase in size (double). + +Due to being monotonically ordered entry arrays may be searched with a binary +search (bisection). + +One chain of entry arrays links up all entries written to the journal. The +first entry array is referenced in the **entry_array_offset** field of the +header. + +Each DATA object also references an entry array chain listing all entries +referencing a specific DATA object. Since many DATA objects are only referenced +by a single ENTRY the first offset of the list is stored inside the DATA object +itself, an ENTRY_ARRAY object is only needed if it is referenced by more than +one ENTRY. + + +## Tag Object + +```c +#define TAG_LENGTH (256/8) + +_packed_ struct TagObject { + ObjectHeader object; + le64_t seqnum; + le64_t epoch; + uint8_t tag[TAG_LENGTH]; /* SHA-256 HMAC */ +}; +``` + +Tag objects are used to seal off the journal for alteration. In regular +intervals a tag object is appended to the file. The tag object consists of a +SHA-256 HMAC tag that is calculated from the objects stored in the file since +the last tag was written, or from the beginning if no tag was written yet. The +key for the HMAC is calculated via the externally maintained FSPRG logic for +the epoch that is written into **epoch**. The sequence number **seqnum** is +increased with each tag. When calculating the HMAC of objects header fields +that are volatile are excluded (skipped). More specifically all fields that +might validly be altered to maintain a consistent file structure (such as +offsets to objects added later for the purpose of linked lists and suchlike) +after an object has been written are not protected by the tag. This means a +verifier has to independently check these fields for consistency of +structure. For the fields excluded from the HMAC please consult the source code +directly. A verifier should read the file from the beginning to the end, always +calculating the HMAC for the objects it reads. Each time a tag object is +encountered the HMAC should be verified and restarted. The tag object sequence +numbers need to increase strictly monotonically. Tag objects themselves are +partially protected by the HMAC (i.e. seqnum and epoch is included, the tag +itself not). + + +## Algorithms + +### Reading + +Given an offset to an entry all data fields are easily found by following the +offsets in the data item array of the entry. + +Listing entries without filter is done by traversing the list of entry arrays +starting with the headers' **entry_array_offset** field. + +Seeking to an entry by timestamp or sequence number (without any matches) is +done via binary search in the entry arrays starting with the header's +**entry_array_offset** field. Since these arrays double in size as more are +added the time cost of seeking is O(log(n)*log(n)) if n is the number of +entries in the file. + +When seeking or listing with one field match applied the DATA object of the +match is first identified, and then its data entry array chain traversed. The +time cost is the same as for seeks/listings with no match. + +If multiple matches are applied, multiple chains of entry arrays should be +traversed in parallel. Since they all are strictly monotonically ordered by +offset of the entries, advancing in one can be directly applied to the others, +until an entry matching all matches is found. In the worst case seeking like +this is O(n) where n is the number of matching entries of the "loosest" match, +but in the common case should be much more efficient at least for the +well-known fields, where the set of possible field values tend to be closely +related. Checking whether an entry matches a number of matches is efficient +since the item array of the entry contains hashes of all data fields +referenced, and the number of data fields of an entry is generally small (< +30). + +When interleaving multiple journal files seeking tends to be a frequently used +operation, but in this case can be effectively suppressed by caching results +from previous entries. + +When listing all possible values a certain field can take it is sufficient to +look up the FIELD object and follow the chain of links to all DATA it includes. + +### Writing + +When an entry is appended to the journal, for each of its data fields the data +hash table should be checked. If the data field does not yet exist in the file, +it should be appended and added to the data hash table. When a data field's data +object is added, the field hash table should be checked for the field name of +the data field, and a field object be added if necessary. After all data fields +(and recursively all field names) of the new entry are appended and linked up +in the hashtables, the entry object should be appended and linked up too. + +At regular intervals a tag object should be written if sealing is enabled (see +above). Before the file is closed a tag should be written too, to seal it off. + +Before writing an object, time and disk space limits should be checked and +rotation triggered if necessary. + + +## Optimizing Disk IO + +_A few general ideas to keep in mind:_ + +The hash tables for looking up fields and data should be quickly in the memory +cache and not hurt performance. All entries and entry arrays are ordered +strictly by time on disk, and hence should expose an OK access pattern on +rotating media, when read sequentially (which should be the most common case, +given the nature of log data). + +The disk access patterns of the binary search for entries needed for seeking +are problematic on rotating disks. This should not be a major issue though, +since seeking should not be a frequent operation. + +When reading, collecting data fields for presenting entries to the user is +problematic on rotating disks. In order to optimize these patterns the item +array of entry objects should be sorted by disk offset before +writing. Effectively, frequently used data objects should be in the memory +cache quickly. Non-frequently used data objects are likely to be located +between the previous and current entry when reading and hence should expose an +OK access pattern. Problematic are data objects that are neither frequently nor +infrequently referenced, which will cost seek time. + +And that's all there is to it. + +Thanks for your interest! diff --git a/docs/PASSWORD_AGENTS.md b/docs/PASSWORD_AGENTS.md new file mode 100644 index 0000000..75b10da --- /dev/null +++ b/docs/PASSWORD_AGENTS.md @@ -0,0 +1,40 @@ +--- +title: Password Agents +category: Interfaces +layout: default +--- + +# Password Agents + +systemd 12 and newer support lightweight password agents which can be used to query the user for system-level passwords or passphrases. These are passphrases that are not related to a specific user, but to some kind of hardware or service. Right now this is used exclusively for encrypted hard-disk passphrases but later on this is likely to be used to query passphrases of SSL certificates at Apache startup time as well. The basic idea is that a system component requesting a password entry can simply drop a simple .ini-style file into `/run/systemd/ask-password` which multiple different agents may watch via `inotify()`, and query the user as necessary. The answer is then sent back to the querier via an `AF_UNIX`/`SOCK_DGRAM` socket. Multiple agents might be running at the same time in which case they all should query the user and the agent which answers first wins. Right now systemd ships with the following passphrase agents: + +* A Plymouth agent used for querying passwords during boot-up +* A console agent used in similar situations if Plymouth is not available +* A GNOME agent which can be run as part of the normal user session which pops up a notification message and icon which when clicked receives the passphrase from the user. This is useful and necessary in case an encrypted system hard-disk is plugged in when the machine is already up. +* A [`wall(1)`](http://man7.org/linux/man-pages/man1/wall.1.html) agent which sends wall messages as soon as a password shall be entered. +* A simple tty agent which is built into "`systemctl start`" (and similar commands) and asks passwords to the user during manual startup of a service +* A simple tty agent which can be run manually to respond to all queued passwords + +It is easy to write additional agents. The basic algorithm to follow looks like this: + +* Create an inotify watch on /run/systemd/ask-password, watch for `IN_CLOSE_WRITE|IN_MOVED_TO` +* Ignore all events on files in that directory that do not start with "`ask.`" +* As soon as a file named "`ask.xxxx`" shows up, read it. It's a simple `.ini` file that may be parsed with the usual parsers. The `xxxx` suffix is randomized. +* Make sure to ignore unknown `.ini` file keys in those files, so that we can easily extend the format later on. +* You'll find the question to ask the user in the `Message=` field in the `[Ask]` section. It is a single-line string in UTF-8, which might be internationalized (by the party that originally asks the question, not by the agent). +* You'll find an icon name (following the XDG icon naming spec) to show next to the message in the `Icon=` field in the `[Ask]` section +* You'll find the PID of the client asking the question in the `PID=` field in the `[Ask]` section (Before asking your question use `kill(PID, 0)` and ignore the file if this returns `ESRCH`; there's no need to show the data of this field but if you want to you may) +* `Echo=` specifies whether the input should be obscured. If this field is missing or is `Echo=0`, the input should not be shown. +* The socket to send the response to is configured via `Socket=` in the `[Ask]` section. It is a `AF_UNIX`/`SOCK_DGRAM` socket in the file system. +* Ignore files where the time specified in the `NotAfter=` field in the `[Ask]` section is in the past. The time is specified in usecs, and refers to the `CLOCK_MONOTONIC` clock. If `NotAfter=` is `0`, no such check should take place. +* Make sure to hide a password query dialog as soon as a) the `ask.xxxx` file is deleted, watch this with inotify. b) the `NotAfter=` time elapses, if it is set `!= 0`. +* Access to the socket is restricted to privileged users. To acquire the necessary privileges to send the answer back, consider using PolicyKit. In fact, the GNOME agent we ship does that, and you may simply piggyback on that, by executing "`/usr/bin/pkexec /lib/systemd/systemd-reply-password 1 /path/to/socket`" or "`/usr/bin/pkexec /lib/systemd/systemd-reply-password 0 /path/to/socket`" and writing the password to its standard input. Use '`1`' as argument if a password was entered by the user, or '`0`' if the user canceled the request. +* If you do not want to use PK ensure to acquire the necessary privileges in some other way and send a single datagram to the socket consisting of the password string either prefixed with "`+`" or with "`-`" depending on whether the password entry was successful or not. You may but don't have to include a final `NUL` byte in your message. + +Again, it is essential that you stop showing the password box/notification/status icon if the `ask.xxx` file is removed or when `NotAfter=` elapses (if it is set `!= 0`)! + +It may happen that multiple password entries are pending at the same time. Your agent needs to be able to deal with that. Depending on your environment you may either choose to show all outstanding passwords at the same time or instead only one and as soon as the user replied to that one go on to the next one. + +You may test this all with manually invoking the "`systemd-ask-password`" tool on the command line. Pass `--no-tty` to ensure the password is asked via the agent system. Note that only privileged users may use this tool (after all this is intended purely for system-level passwords). + +If you write a system level agent a smart way to activate it is using systemd `.path` units. This will ensure that systemd will watch the `/run/systemd/ask-password` directory and spawn the agent as soon as that directory becomes non-empty. In fact, the console, wall and Plymouth agents are started like this. If systemd is used to maintain user sessions as well you can use a similar scheme to automatically spawn your user password agent as well. (As of this moment we have not switched any DE over to use systemd for session management, however.) diff --git a/docs/PORTABILITY_AND_STABILITY.md b/docs/PORTABILITY_AND_STABILITY.md new file mode 100644 index 0000000..0caa5cc --- /dev/null +++ b/docs/PORTABILITY_AND_STABILITY.md @@ -0,0 +1,162 @@ +--- +title: Interface Portability and Stability +category: Interfaces +layout: default +--- + +# Interface Portability and Stability Promise + +systemd provides various interfaces developers and programs might rely on. Starting with version 26 (the first version released with Fedora 15) we promise to keep a number of them stable and compatible for the future. + +The stable interfaces are: + +* **The unit configuration file format**. Unit files written now will stay compatible with future versions of systemd. Extensions to the file format will happen in a way that existing files remain compatible. + +* **The command line interface** of `systemd`, `systemctl`, `loginctl`, `journalctl`, and all other command line utilities installed in `$PATH` and documented in a man page. We will make sure that scripts invoking these commands will continue to work with future versions of systemd. Note however that the output generated by these commands is generally not included in the promise, unless it is documented in the man page. Example: the output of `systemctl status` is not stable, but that of `systemctl show` is, because the former is intended to be human readable and the latter computer readable, and this is documented in the man page. + +* **The protocol spoken on the socket referred to by `$NOTIFY_SOCKET`**, as documented in [sd_notify(3)](https://www.freedesktop.org/software/systemd/man/sd_notify.html). + +* Some of the **"special" unit names** and their semantics. To be precise the ones that are necessary for normal services, and not those required only for early boot and late shutdown, with very few exceptions. To list them here: `basic.target`, `shutdown.target`, `sockets.target`, `network.target`, `getty.target`, `graphical.target`, `multi-user.target`, `rescue.target`, `emergency.target`, `poweroff.target`, `reboot.target`, `halt.target`, `runlevel[1-5].target`. + +* **The D-Bus interfaces of the main service daemon and other daemons**. We try to always preserve backwards compatibility, and intentional breakage is never introduced. Nevertheless, when we find bugs that mean that the existing interface was not useful, or when the implementation did something different than stated by the documentation and the implemented behaviour is not useful, we will fix the implementation and thus introduce a change in behaviour. But the API (parameter counts and types) is never changed, and existing attributes and methods will not be removed. + +* For a more comprehensive and authoritative list, consult the chart below. + +The following interfaces will not necessarily be kept stable for now, but we will eventually make a stability promise for these interfaces too. In the meantime we will however try to keep breakage of these interfaces at a minimum: + +* **The set of states of the various state machines used in systemd**, e.g. the high-level unit states inactive, active, deactivating, and so on, as well (and in particular) the low-level per-unit states. + +* **All "special" units that aren't listed above**. + +The following interfaces are considered private to systemd, and are not and will not be covered by any stability promise: + +* **Undocumented switches** to `systemd`, `systemctl` and otherwise. + +* **The internal protocols** used on the various sockets such as the sockets `/run/systemd/shutdown`, `/run/systemd/private`. + +One of the main goals of systemd is to unify basic Linux configurations and service behaviors across all distributions. Systemd project does not contain any distribution-specific parts. Distributions are expected to convert over time their individual configurations to the systemd format, or they will need to carry and maintain patches in their package if they still decide to stay different. + +What does this mean for you? When developing with systemd, don't use any of the latter interfaces, or we will tell your mom, and she won't love you anymore. You are welcome to use the other interfaces listed here, but if you use any of the second kind (i.e. those where we don't yet make a stability promise), then make sure to subscribe to our mailing list, where we will announce API changes, and be prepared to update your program eventually. + +Note that this is a promise, not an eternal guarantee. These are our intentions, but if in the future there are very good reasons to change or get rid of an interface we have listed above as stable, then we might take the liberty to do so, despite this promise. However, if we do this, then we'll do our best to provide a smooth and reasonably long transition phase. + + +## Interface Portability And Stability Chart + +systemd provides a number of APIs to applications. Below you'll find a table detailing which APIs are considered stable and how portable they are. + +This list is intended to be useful for distribution and OS developers who are interested in maintaining a certain level of compatibility with the new interfaces systemd introduced, without relying on systemd itself. + +In general it is our intention to cooperate through interfaces and not code with other distributions and OSes. That means that the interfaces where this applies are best reimplemented in a compatible fashion on those other operating systems. To make this easy we provide detailed interface documentation where necessary. That said, it's all Open Source, hence you have the option to a) fork our code and maintain portable versions of the parts you are interested in independently for your OS, or b) build systemd for your distro, but leave out all components except the ones you are interested in and run them without the core of systemd involved. We will try not to make this any more difficult than necessary. Patches to allow systemd code to be more portable will be accepted on case-by-case basis (essentially, patches to follow well-established standards instead of e.g. glibc or linux extensions have a very high chance of being accepted, while patches which make the code ugly or exist solely to work around bugs in other projects have a low chance of being accepted). + +Many of these interfaces are already being used by applications and 3rd party code. If you are interested in compatibility with these applications, please consider supporting these interfaces in your distribution, where possible. + + +## General Portability of systemd and its Components + +**Portability to OSes:** systemd is not portable to non-Linux systems. It makes use of a large number of Linux-specific interfaces, including many that are used by its very core. We do not consider it feasible to port systemd to other Unixes (let alone non-Unix operating systems) and will not accept patches for systemd core implementing any such portability (but hey, it's git, so it's as easy as it can get to maintain your own fork...). APIs that are supposed to be used as library code are exempted from this: it is important to us that these compile nicely on non-Linux and even non-Unix platforms, even if they might just become NOPs. + +**Portability to Architectures:** It is important to us that systemd is portable to little endian as well as big endian systems. We will make sure to provide portability with all important architectures and hardware Linux runs on and are happy to accept patches for this. + +**Portability to Distributions:** It is important to us that systemd is portable to all Linux distributions. However, the goal is to unify many of the needless differences between the distributions, and hence will not accept patches for certain distribution-specific work-arounds. Compatibility with the distribution's legacy should be maintained in the distribution's packaging, and not in the systemd source tree. + +**Compatibility with Specific Versions of Other packages:** We generally avoid adding compatibility kludges to systemd that work around bugs in certain versions of other software systemd interfaces with. We strongly encourage fixing bugs where they are, and if that's not systemd we rather not try to fix it there. (There are very few exceptions to this rule possible, and you need an exceptionally strong case for it). + + +## General Portability of systemd's APIs + +systemd's APIs are available everywhere where systemd is available. Some of the APIs we have defined are supposed to be generic enough to be implementable independently of systemd, thus allowing compatibility with systems systemd itself is not compatible with, i.e. other OSes, and distributions that are unwilling to fully adopt systemd. + +A number of systemd's APIs expose Linux or systemd-specific features that cannot sensibly be implemented elsewhere. Please consult the table below for information about which ones these are. + +Note that not all of these interfaces are our invention (but most), we just adopted them in systemd to make them more prominently implemented. For example, we adopted many Debian facilities in systemd to push it into the other distributions as well. + + +--- + + +And now, here's the list of (hopefully) all APIs that we have introduced with systemd: + +| API | Type | Covered by Interface Stability Promise | Fully documented | Known External Consumers | Reimplementable Independently | Known Other Implementations | systemd Implementation portable to other OSes or non-systemd distributions | +| --- | ---- | ----------------------------------------------------------------------------------------- | ---------------- | ------------------------ | ----------------------------- | --------------------------- | -------------------------------------------------------------------------- | +| [hostnamed](https://www.freedesktop.org/wiki/Software/systemd/hostnamed) | D-Bus | yes | yes | GNOME | yes | [Ubuntu](https://launchpad.net/ubuntu/+source/ubuntu-system-service), [Gentoo](http://www.gentoo.org/proj/en/desktop/gnome/openrc-settingsd.xml), [BSD](http://uglyman.kremlin.cc/gitweb/gitweb.cgi?p=systembsd.git;a=summary) | partially | +| [localed](https://www.freedesktop.org/wiki/Software/systemd/localed) | D-Bus | yes | yes | GNOME | yes | [Ubuntu](https://launchpad.net/ubuntu/+source/ubuntu-system-service), [Gentoo](http://www.gentoo.org/proj/en/desktop/gnome/openrc-settingsd.xml), [BSD](http://uglyman.kremlin.cc/gitweb/gitweb.cgi?p=systembsd.git;a=summary) | partially | +| [timedated](https://www.freedesktop.org/wiki/Software/systemd/timedated) | D-Bus | yes | yes | GNOME | yes | [Gentoo](http://www.gentoo.org/proj/en/desktop/gnome/openrc-settingsd.xml), [BSD](http://uglyman.kremlin.cc/gitweb/gitweb.cgi?p=systembsd.git;a=summary) | partially | +| [initrd interface](https://systemd.io/INITRD_INTERFACE/) | Environment, flag files | yes | yes | dracut, ArchLinux | yes | ArchLinux | no | +| [Container interface](https://systemd.io/CONTAINER_INTERFACE) | Environment, Mounts | yes | yes | libvirt/LXC | yes | - | no | +| [Boot Loader interface](https://systemd.io/BOOT_LOADER_INTERFACE) | EFI variables | yes | yes | gummiboot | yes | - | no | +| [Service bus API](https://www.freedesktop.org/wiki/Software/systemd/dbus) | D-Bus | yes | yes | system-config-services | no | - | no | +| [logind](https://www.freedesktop.org/wiki/Software/systemd/logind) | D-Bus | yes | yes | GNOME | no | - | no | +| [sd-login.h API](https://www.freedesktop.org/software/systemd/man/sd-login.html) | C Library | yes | yes | GNOME, polkit, ... | no | - | no | +| [sd-daemon.h API](https://www.freedesktop.org/software/systemd/man/sd-daemon.html) | C Library or Drop-in | yes | yes | numerous | yes | - | yes | +| [sd-id128.h API](https://www.freedesktop.org/software/systemd/man/sd-id128.html) | C Library | yes | yes | - | yes | - | no | +| [sd-journal.h API](https://www.freedesktop.org/software/systemd/man/sd-journal.html) | C Library | yes | yes | - | maybe | - | no | +| [$XDG_RUNTIME_DIR](https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html) | Environment | yes | yes | glib, GNOME | yes | - | no | +| [$LISTEN_FDS $LISTEN_PID FD Passing](https://www.freedesktop.org/software/systemd/man/sd_listen_fds.html) | Environment | yes | yes | numerous (via sd-daemon.h) | yes | - | no | +| [$NOTIFY_SOCKET Daemon Notifications](https://www.freedesktop.org/software/systemd/man/sd_notify.html) | Environment | yes | yes | a few, including udev | yes | - | no | +| [argv[0][0]='@' Logic](https://systemd.io/ROOT_STORAGE_DAEMONS/) | `/proc` marking | yes | yes | mdadm | yes | - | no | +| [Unit file format](https://www.freedesktop.org/software/systemd/man/systemd.unit.html) | File format | yes | yes | numerous | no | - | no | +| [Network](https://www.freedesktop.org/software/systemd/man/systemd.network.html) & [Netdev file format](https://www.freedesktop.org/software/systemd/man/systemd.netdev.html) | File format | yes | yes | no | no | - | no | +| [Link file format](https://www.freedesktop.org/software/systemd/man/systemd.link.html) | File format | yes | yes | no | no | - | no | +| [Journal File Format](https://systemd.io/JOURNAL_FILE_FORMAT) | File format | yes | yes | - | maybe | - | no | +| [Journal Export Format](https://www.freedesktop.org/wiki/Software/systemd/export) | File format | yes | yes | - | yes | - | no | +| [Cooperation in cgroup tree](https://www.freedesktop.org/wiki/Software/systemd/PaxControlGroups) | Treaty | yes | yes | libvirt | yes | libvirt | no | +| [Password Agents](https://systemd.io/PASSWORD_AGENTS/) | Socket+Files | yes | yes | - | yes | - | no | +| [udev multi-seat properties](https://www.freedesktop.org/software/systemd/man/sd-login.html) | udev Property | yes | yes | X11, gdm | no | - | no | +| udev session switch ACL properties | udev Property | no | no | - | no | - | no | +| [CLI of systemctl,...](https://www.freedesktop.org/software/systemd/man/systemctl.html) | CLI | yes | yes | numerous | no | - | no | +| [tmpfiles.d](https://www.freedesktop.org/software/systemd/man/tmpfiles.d.html) | File format | yes | yes | numerous | yes | ArchLinux | partially | +| [sysusers.d](https://www.freedesktop.org/software/systemd/man/sysusers.d.html) | File format | yes | yes | unknown | yes | | partially | +| [/etc/machine-id](https://www.freedesktop.org/software/systemd/man/machine-id.html) | File format | yes | yes | D-Bus | yes | - | no | +| [binfmt.d](https://www.freedesktop.org/software/systemd/man/binfmt.d.html) | File format | yes | yes | numerous | yes | - | partially | +| [/etc/hostname](https://www.freedesktop.org/software/systemd/man/hostname.html) | File format | yes | yes | numerous (it's a Debian thing) | yes | Debian, ArchLinux | no | +| [/etc/locale.conf](https://www.freedesktop.org/software/systemd/man/locale.conf.html) | File format | yes | yes | - | yes | ArchLinux | partially | +| [/etc/machine-info](https://www.freedesktop.org/software/systemd/man/machine-info.html) | File format | yes | yes | - | yes | - | partially | +| [modules-load.d](https://www.freedesktop.org/software/systemd/man/modules-load.d.html) | File format | yes | yes | numerous | yes | - | partially | +| [/usr/lib/os-release](https://www.freedesktop.org/software/systemd/man/os-release.html) | File format | yes | yes | some | yes | Fedora, OpenSUSE, ArchLinux, Angstrom, Frugalware, others... | no | +| [sysctl.d](https://www.freedesktop.org/software/systemd/man/sysctl.d.html) | File format | yes | yes | some (it's a Debian thing) | yes | procps/Debian, ArchLinux | partially | +| [/etc/timezone](https://www.freedesktop.org/software/systemd/man/timezone.html) | File format | yes | yes | numerous (it's a Debian thing) | yes | Debian | partially | +| [/etc/vconsole.conf](https://www.freedesktop.org/software/systemd/man/vconsole.conf.html) | File format | yes | yes | - | yes | ArchLinux | partially | +| `/run` | File hierarchy change | yes | yes | numerous | yes | OpenSUSE, Debian, ArchLinux | no | +| [Generators](https://www.freedesktop.org/software/systemd/man/systemd.generator.html) | Subprocess | yes | yes | - | no | - | no | +| [System Updates](https://www.freedesktop.org/software/systemd/man/systemd.offline-updates.html) | System Mode | yes | yes | - | no | - | no | +| [Presets](https://freedesktop.org/wiki/Software/systemd/Preset) | File format | yes | yes | - | no | - | no | +| Udev rules | File format | yes | yes | numerous | no | no | partially | + + +### Explanations + +Items for which "systemd implementation portable to other OSes" is "partially" means that it is possible to run the respective tools that are included in the systemd tarball outside of systemd. Note however that this is not officially supported, so you are more or less on your own if you do this. If you are opting for this solution simply build systemd as you normally would but drop all files except those which you are interested in. + +Of course, it is our intention to eventually document all interfaces we defined. If we haven't documented them for now, this is usually because we want the flexibility to still change things, or don't want 3rd party applications to make use of these interfaces already. That said, our sources are quite readable and open source, so feel free to spelunk around in the sources if you want to know more. + +If you decide to reimplement one of the APIs for which "Reimplementable independently" is "no", then we won't stop you, but you are on your own. + +This is not an attempt to comprehensively list all users of these APIs. We are just listing the most obvious/prominent ones which come to our mind. + +Of course, one last thing I can't make myself not ask you before we finish here, and before you start reimplementing these APIs in your distribution: are you sure it's time well spent if you work on reimplementing all this code instead of just spending it on adopting systemd on your distro as well? + +## Independent Operation of systemd Programs + +Some programs in the systemd suite are intended to operate independently of the +running init process (or even without an init process, for example when +creating system installation chroots). They can be safely called on systems with +a different init process or for example in package installation scriptlets. + +The following programs currently and in the future will support operation +without communicating with the `systemd` process: +`systemd-escape`, +`systemd-id128`, +`systemd-path`, +`systemd-tmpfiles`, +`systemd-sysctl`, +`systemd-sysusers`. + +Many other programs support operation without the system manager except when +the specific functionality requires such communication. For example +`journalctl` operates almost independently, but will query the boot id when +`--boot` option is used; it also requires `systemd-journald` (and thus +`systemd`) to be running for options like `--flush` and `--sync`. +`systemd-journal-remote`, `systemd-journal-upload`, `systemd-journal-gatewayd`, +`coredumpctl`, `busctl`, `systemctl --root` also fall into this category of +mostly-independent programs. diff --git a/docs/PORTABLE_SERVICES.md b/docs/PORTABLE_SERVICES.md new file mode 100644 index 0000000..8248275 --- /dev/null +++ b/docs/PORTABLE_SERVICES.md @@ -0,0 +1,287 @@ +--- +title: Portable Services Introduction +category: Concepts +layout: default +--- + +# Portable Services Introduction + +This systemd version includes a preview of the "portable service" +concept. "Portable Services" are supposed to be an incremental improvement over +traditional system services, making two specific facets of container management +available to system services more readily. Specifically: + +1. The bundling of applications, i.e. packing up multiple services, their + binaries and all their dependencies in a single image, and running them + directly from it. + +2. Stricter default security policies, i.e. sand-boxing of applications. + +The primary tool for interfacing with "portable services" is the new +"portablectl" program. It's currently shipped in /usr/lib/systemd/portablectl +(i.e. not in the `$PATH`), since it's not yet considered part of the officially +supported systemd interfaces — it's a preview still after all. + +Portable services don't bring anything inherently new to the table. All they do +is put together known concepts in a slightly nicer way to cover a specific set +of use-cases in a nicer way. + +## So, what *is* a "Portable Service"? + +A portable service is ultimately just an OS tree, either inside of a directory +tree, or inside a raw disk image containing a Linux file system. This tree is +called the "image". It can be "attached" or "detached" from the system. When +"attached" specific systemd units from the image are made available on the host +system, then behaving pretty much exactly like locally installed system +services. When "detached" these units are removed again from the host, leaving +no artifacts around (except maybe messages they might have logged). + +The OS tree/image can be created with any tool of your choice. For example, you +can use `dnf --installroot=` if you like, or `debootstrap`, the image format is +entirely generic, and doesn't have to carry any specific metadata beyond what +distribution images carry anyway. Or to say this differently: the image format +doesn't define any new metadata as unit files and OS tree directories or disk +images are already sufficient, and pretty universally available these days. One +particularly nice tool for creating suitable images is +[mkosi](https://github.com/systemd/mkosi), but many other existing tools will +do too. + +If you so will, "Portable Services" are a nicer way to manage chroot() +environments, with better security, tooling and behavior. + +## Where's the difference to a "Container"? + +"Container" is a very vague term, after all it is used for +systemd-nspawn/LXC-type OS containers, for Docker/rkt-like micro service +containers, and even certain 'lightweight' VM runtimes. + +The "portable service" concept ultimately will not provide a fully isolated +environment to the payload, like containers mostly intend to. Instead they are +from the beginning more alike regular system services, can be controlled with +the same tools, are exposed the same way in all infrastructure and so on. Their +main difference is that the use a different root directory than the rest of the +system. Hence, the intention is not to run code in a different, isolated world +from the host — like most containers would do it —, but to run it in the same +world, but with stricter access controls on what the service can see and do. + +As one point of differentiation: as programs run as "portable services" are +pretty much regular system services, they won't run as PID 1 (like Docker would +do it), but as normal process. A corollary of that is that they aren't supposed +to manage anything in their own environment (such as the network) as the +execution environment is mostly shared with the rest of the system. + +The primary focus use-case of "portable services" is to extend the host system +with encapsulated extensions, but provide almost full integration with the rest +of the system, though possibly restricted by effective security knobs. This +focus includes system extensions otherwise sometimes called "super-privileged +containers". + +Note that portable services are only available for system services, not for +user services. i.e. the functionality cannot be used for the stuff +bubblewrap/flatpak is focusing on. + +## Mode of Operation + +If you have portable service image, maybe in a raw disk image called +`foobar_0.7.23.raw`, then attaching the services to the host is as easy as: + +``` +# /usr/lib/systemd/portablectl attach foobar_0.7.23.raw +``` + +This command does the following: + +1. It dissects the image, checks and validates the `/etc/os-release` + (or `/usr/lib/os-release`, see below) data of the image, and looks for + all included unit files. + +2. It copies out all unit files with a suffix of `.service`, `.socket`, + `.target`, `.timer` and `.path`. whose name begins with the image's name + (with the .raw removed), truncated at the first underscore (if there is + one). This prefix name generated from the image name must be followed by a + ".", "-" or "@" character in the unit name. Or in other words, given the + image name of `foobar_0.7.23.raw` all unit files matching + `foobar-*.{service|socket|target|timer|path}`, + `foobar@.{service|socket|target|timer|path}` as well as + `foobar.*.{service|socket|target|timer|path}` and + `foobar.{service|socket|target|timer|path}` are copied out. These unit files + are placed in `/etc/systemd/system.attached/` (which is part of the normal + unit file search path of PID 1, and thus loaded exactly like regular unit + files). Within the images the unit files are looked for at the usual + locations, i.e. in `/usr/lib/systemd/system/` and `/etc/systemd/system/` and + so on, relative to the image's root. + +3. For each such unit file a drop-in file is created. Let's say + `foobar-waldo.service` was one of the unit files copied to + `/etc/systemd/system.attached/`, then a drop-in file + `/etc/systemd/system.attached/foobar-waldo.service.d/20-portable.conf` is + created, containing a few lines of additional configuration: + + ``` + [Service] + RootImage=/path/to/foobar.raw + Environment=PORTABLE=foobar + LogExtraFields=PORTABLE=foobar + ``` + +4. For each such unit a "profile" drop-in is linked in. This "profile" drop-in + generally contains security options that lock down the service. By default + the `default` profile is used, which provides a medium level of + security. There's also `trusted` which runs the service at the highest + privileges, i.e. host's root and everything. The `strict` profile comes with + the toughest security restrictions. Finally, `nonetwork` is like `default` + but without network access. Users may define their own profiles too (or + modify the existing ones) + +And that's already it. + +Note that the images need to stay around (and the same location) as long as the +portable service is attached. If an image is moved, the `RootImage=` line +written to the unit drop-in would point to an non-existing place, and break the +logic. + +The `portablectl detach` command executes the reverse operation: it looks for +the drop-ins and the unit files associated with the image, and removes them +again. + +Note that `portable attach` won't enable or start any of the units it copies +out. This still has to take place in a second, separate step. (That said We +might add options to do this automatically later on.). + +## Requirements on Images + +Note that portable services don't introduce any new image format, but most OS +images should just work the way they are. Specifically, the following +requirements are made for an image that can be attached/detached with +`portablectl`. + +1. It must contain an executable that shall be invoked, along with all its + dependencies. If binary code, the code needs to be compiled for an + architecture compatible with the host. + +2. The image must either be a plain sub-directory (or btrfs subvolume) + containing the binaries and its dependencies in a classic Linux OS tree, or + must be a raw disk image either containing only one, naked file system, or + an image with a partition table understood by the Linux kernel with only a + single partition defined, or alternatively, a GPT partition table with a set + of properly marked partitions following the [Discoverable Partitions + Specification](https://systemd.io/DISCOVERABLE_PARTITIONS). + +3. The image must at least contain one matching unit file, with the right name + prefix and suffix (see above). The unit file is searched in the usual paths, + i.e. primarily /etc/systemd/system/ and /usr/lib/systemd/system/ within the + image. (The implementation will check a couple of other paths too, but it's + recommended to use these two paths.) + +4. The image must contain an os-release file, either in `/etc/os-release` or + `/usr/lib/os-release`. The file should follow the standard format. + +5. The image must contain the files `/etc/resolv.conf` and `/etc/machine-id` + (empty files are ok), they will be bind mounted from the host at runtime. + +6. The image must contain directories `/proc/`, `/sys/`, `/dev/`, `/run/`, + `/tmp/`, `/var/tmp/` that can be mounted over with the corresponding version + from the host. + +7. The OS might require other files or directories to be in place. For example, + if the image is built based on glibc, the dynamic loader needs to be + available in `/lib/ld-linux.so.2` or `/lib64/ld-linux-x86-64.so.2` (or + similar, depending on architecture), and if the distribution implements a + merged `/usr/` tree, this means `/lib` and/or `/lib64` need to be symlinks + to their respective counterparts below `/usr/`. For details see your + distribution's documentation. + +Note that images created by tools such as `debootstrap`, `dnf --installroot=` +or `mkosi` generally qualify for all of the above in one way or another. If you +wonder what the most minimal image would be that complies with the requirements +above, it could consist of this: + +``` +/usr/bin/minimald # a statically compiled binary +/usr/lib/systemd/system/minimal-test.service # the unit file for the service, with ExecStart=/usr/bin/minimald +/usr/lib/os-release # an os-release file explaining what this is +/etc/resolv.conf # empty file to mount over with host's version +/etc/machine-id # ditto +/proc/ # empty directory to use as mount point for host's API fs +/sys/ # ditto +/dev/ # ditto +/run/ # ditto +/tmp/ # ditto +/var/tmp/ # ditto +``` + +And that's it. + +Note that qualifying images do not have to contain an init system of their +own. If they do, it's fine, it will be ignored by the portable service logic, +but they generally don't have to, and it might make sense to avoid any, to keep +images minimal. + +If the image is writable, and some of the files or directories that are +overmounted from the host do not exist yet they are automatically created. On +read-only, immutable images (e.g. squashfs images) all files and directories to +over-mount must exist already. + +Note that as no new image format or metadata is defined, it's very +straight-forward to define images than can be made use of it a number of +different ways. For example, by using `mkosi -b` you can trivially build a +single, unified image that: + +1. Can be attached as portable service, to run any container services natively + on the host. + +2. Can be run as OS container, using `systemd-nspawn`, by booting the image + with `systemd-nspawn -i -b`. + +3. Can be booted directly as VM image, using a generic VM executor such as + `virtualbox`/`qemu`/`kvm` + +4. Can be booted directly on bare-metal systems. + +Of course, to facilitate 2, 3 and 4 you need to include an init system in the +image. To facility 3 and 4 you also need to include a boot loader in the +image. As mentioned `mkosi -b` takes care of all of that for you, but any other +image generator should work too. + +## Execution Environment + +Note that the code in portable service images is run exactly like regular +services. Hence there's no new execution environment to consider. Oh, unlike +Docker would do it, as these are regular system services they aren't run as PID +1 either, but with regular PID values. + +## Access to host resources + +If services shipped with this mechanism shall be able to access host resources +(such as files or AF_UNIX sockets for IPC), use the normal `BindPaths=` and +`BindReadOnlyPaths=` settings in unit files to mount them in. In fact the +`default` profile mentioned above makes use of this to ensure +`/etc/resolv.conf`, the D-Bus system bus socket or write access to the logging +subsystem are available to the service. + +## Instantiation + +Sometimes it makes sense to instantiate the same set of services multiple +times. The portable service concept does not introduce a new logic for this. It +is recommended to use the regular unit templating of systemd for this, i.e. to +include template units such as `foobar@.service`, so that instantiation is as +simple as: + +``` +# /usr/lib/systemd/portablectl attach foobar_0.7.23.raw +# systemctl enable --now foobar@instancea.service +# systemctl enable --now foobar@instanceb.service +… +``` + +The benefit of this approach is that templating works exactly the same for +units shipped with the OS itself as for attached portable services. + +## Immutable images with local data + +It's a good idea to keep portable service images read-only during normal +operation. In fact all but the `trusted` profile will default to this kind of +behaviour, by setting the `ProtectSystem=strict` option. In this case writable +service data may be placed on the host file system. Use `StateDirectory=` in +the unit files to enable such behaviour and add a local data directory to the +services copied onto the host. diff --git a/docs/PREDICTABLE_INTERFACE_NAMES.md b/docs/PREDICTABLE_INTERFACE_NAMES.md new file mode 100644 index 0000000..07529e7 --- /dev/null +++ b/docs/PREDICTABLE_INTERFACE_NAMES.md @@ -0,0 +1,69 @@ +--- +title: Predictable Network Interface Names +category: Concepts +layout: default +--- + +# Predictable Network Interface Names + +Starting with v197 systemd/udev will automatically assign predictable, stable network interface names for all local Ethernet, WLAN and WWAN interfaces. This is a departure from the traditional interface naming scheme (`eth0`, `eth1`, `wlan0`, ...), but should fix real problems. + +## Why? + +The classic naming scheme for network interfaces applied by the kernel is to simply assign names beginning with `eth0`, `eth1`, ... to all interfaces as they are probed by the drivers. As the driver probing is generally not predictable for modern technology this means that as soon as multiple network interfaces are available the assignment of the names `eth0`, `eth1` and so on is generally not fixed anymore and it might very well happen that `eth0` on one boot ends up being `eth1` on the next. This can have serious security implications, for example in firewall rules which are coded for certain naming schemes, and which are hence very sensitive to unpredictable changing names. + +To fix this problem multiple solutions have been proposed and implemented. For a longer time udev shipped support for assigning permanent `ethX` names to certain interfaces based on their MAC addresses. This turned out to have a multitude of problems, among them: this required a writable root directory which is generally not available; the statelessness of the system is lost as booting an OS image on a system will result in changed configuration of the image; on many systems MAC addresses are not actually fixed, such as on a lot of embedded hardware and particularly on all kinds of virtualization solutions. The biggest of all however is that the userspace components trying to assign the interface name raced against the kernel assigning new names from the same `ethX` namespace, a race condition with all kinds of weird effects, among them that assignment of names sometimes failed. As a result support for this has been removed from systemd/udev a while back. + +Another solution that has been implemented is `biosdevname` which tries to find fixed slot topology information in certain firmware interfaces and uses them to assign fixed names to interfaces which incorporate their physical location on the mainboard. In a way this naming scheme is similar to what is already done natively in udev for various device nodes via `/dev/*/by-path/` symlinks. In many cases, biosdevname departs from the low-level kernel device identification schemes that udev generally uses for these symlinks, and instead invents its own enumeration schemes. + +Finally, many distributions support renaming interfaces to user-chosen names (think: `internet0`, `dmz0`, ...) keyed off their MAC addresses or physical locations as part of their networking scripts. This is a very good choice but does have the problem that it implies that the user is willing and capable of choosing and assigning these names. + +We believe it is a good default choice to generalize the scheme pioneered by `biosdevname`. Assigning fixed names based on firmware/topology/location information has the big advantage that the names are fully automatic, fully predictable, that they stay fixed even if hardware is added or removed (i.e. no reenumeration takes place) and that broken hardware can be replaced seamlessly. That said, they admittedly are sometimes harder to read than the `eth0` or `wlan0` everybody is used to. Example: `enp5s0` + + +## What precisely has changed in v197? + +With systemd 197 we have added native support for a number of different naming policies into systemd/udevd proper and made a scheme similar to biosdevname's (but generally more powerful, and closer to kernel-internal device identification schemes) the default. The following different naming schemes for network interfaces are now supported by udev natively: + +1. Names incorporating Firmware/BIOS provided index numbers for on-board devices (example: `eno1`) +1. Names incorporating Firmware/BIOS provided PCI Express hotplug slot index numbers (example: `ens1`) +1. Names incorporating physical/geographical location of the connector of the hardware (example: `enp2s0`) +1. Names incorporating the interfaces's MAC address (example: `enx78e7d1ea46da`) +1. Classic, unpredictable kernel-native ethX naming (example: `eth0`) + +By default, systemd v197 will now name interfaces following policy 1) if that information from the firmware is applicable and available, falling back to 2) if that information from the firmware is applicable and available, falling back to 3) if applicable, falling back to 5) in all other cases. Policy 4) is not used by default, but is available if the user chooses so. + +This combined policy is only applied as last resort. That means, if the system has biosdevname installed, it will take precedence. If the user has added udev rules which change the name of the kernel devices these will take precedence too. Also, any distribution specific naming schemes generally take precedence. + + +## Come again, what good does this do? + +With this new scheme you now get: + +* Stable interface names across reboots +* Stable interface names even when hardware is added or removed, i.e. no re-enumeration takes place (to the level the firmware permits this) +* Stable interface names when kernels or drivers are updated/changed +* Stable interface names even if you have to replace broken ethernet cards by new ones +* The names are automatically determined without user configuration, they just work +* The interface names are fully predictable, i.e. just by looking at lspci you can figure out what the interface is going to be called +* Fully stateless operation, changing the hardware configuration will not result in changes in `/etc` +* Compatibility with read-only root +* The network interface naming now follows more closely the scheme used for aliasing block device nodes and other device nodes in `/dev` via symlinks +* Applicability to both x86 and non-x86 machines +* The same on all distributions that adopted systemd/udev +* It's easy to opt out of the scheme (see below) + +Does this have any drawbacks? Yes, it does. Previously it was practically guaranteed that hosts equipped with a single ethernet card only had a single `eth0` interface. With this new scheme in place, an administrator now has to check first what the local interface name is before he can invoke commands on it where previously he had a good chance that `eth0` was the right name. + + +## I don't like this, how do I disable this? + +You basically have three options: + +1. You disable the assignment of fixed names, so that the unpredictable kernel names are used again. For this, simply mask udev's .link file for the default policy: `ln -s /dev/null /etc/systemd/network/99-default.link` +1. You create your own manual naming scheme, for example by naming your interfaces `internet0`, `dmz0` or `lan0`. For that create your own `.link` files in `/etc/systemd/network/`, that choose an explicit name or a better naming scheme for one, some, or all of your interfaces. See [systemd.link(5)](http://www.freedesktop.org/software/systemd/man/systemd.link.html) for more information. +1. You pass the `net.ifnames=0` on the kernel command line + +## How does the new naming scheme look like, precisely? + +That's documented in detail the [systemd.net-naming-scheme(7)](https://www.freedesktop.org/software/systemd/man/systemd.net-naming-scheme.html) man page. Please refer to this in case you are wondering how to decode the new interface names. diff --git a/docs/RANDOM_SEEDS.md b/docs/RANDOM_SEEDS.md new file mode 100644 index 0000000..da3fe40 --- /dev/null +++ b/docs/RANDOM_SEEDS.md @@ -0,0 +1,435 @@ +--- +title: Random Seeds +category: Concepts +layout: default +--- + +# Random Seeds + +systemd can help in a number of ways with providing reliable, high quality +random numbers from early boot on. + +## Linux Kernel Entropy Pool + +Today's computer systems require random number generators for numerous +cryptographic and other purposes. On Linux systems, the kernel's entropy pool +is typically used as high-quality source of random numbers. The kernel's +entropy pool combines various entropy inputs together, mixes them and provides +an API to userspace as well as to internal kernel subsystems to retrieve +it. This entropy pool needs to be initialized with a minimal level of entropy +before it can provide high quality, cryptographic random numbers to +applications. Until the entropy pool is fully initialized application requests +for high-quality random numbers cannot be fulfilled. + +The Linux kernel provides three relevant userspace APIs to request random data +from the kernel's entropy pool: + +* The [`getrandom()`](http://man7.org/linux/man-pages/man2/getrandom.2.html) + system call with its `flags` parameter set to 0. If invoked the calling + program will synchronously block until the random pool is fully initialized + and the requested bytes can be provided. + +* The `getrandom()` system call with its `flags` parameter set to + `GRND_NONBLOCK`. If invoked the request for random bytes will fail if the + pool is not initialized yet. + +* Reading from the + [`/dev/urandom`](http://man7.org/linux/man-pages/man4/urandom.4.html) + pseudo-device will always return random bytes immediately, even if the pool + is not initialized. The provided random bytes will be of low quality in this + case however. Moreover the kernel will log about all programs using this + interface in this state, and which thus potentially rely on an uninitialized + entropy pool. + +(Strictly speaking there are more APIs, for example `/dev/random`, but these +should not be used by almost any application and hence aren't mentioned here.) + +Note that the time it takes to initialize the random pool may differ between +systems. If local hardware random number generators are available, +initialization is likely quick, but particularly in embedded and virtualized +environments available entropy is small and thus random pool initialization +might take a long time (up to tens of minutes!). + +Modern hardware tends to come with a number of hardware random number +generators (hwrng), that may be used to relatively quickly fill up the entropy +pool. Specifically: + +* All recent Intel and AMD CPUs provide the CPU opcode + [RDRAND](https://en.wikipedia.org/wiki/RdRand) to acquire random bytes. Linux + includes random bytes generated this way in its entropy pool, but didn't use + to credit entropy for it (i.e. data from this source wasn't considered good + enough to consider the entropy pool properly filled even though it was + used). This has changed recently however, and most big distributions have + turned on the `CONFIG_RANDOM_TRUST_CPU=y` kernel compile time option. This + means systems with CPUs supporting this opcode will be able to very quickly + reach the "pool filled" state. + +* The TPM security chip that is available on all modern desktop systems has a + hwrng. It is also fed into the entropy pool, but generally not credited + entropy. You may use `rng_core.default_quality=1000` on the kernel command + line to change that, but note that this is a global setting affect all + hwrngs. (Yeah, that's weird.) + +* Many Intel and AMD chipsets have hwrng chips. Their Linux drivers usually + don't credit entropy. (But there's `rng_core.default_quality=1000`, see + above.) + +* Various embedded boards have hwrng chips. Some drivers automatically credit + entropy, others do not. Some WiFi chips appear to have hwrng sources too, and + they usually do not credit entropy for them. + +* `virtio-rng` is used in virtualized environments and retrieves random data + from the VM host. It credits full entropy. + +* The EFI firmware typically provides a RNG API. When transitioning from UEFI + to kernel mode Linux will query some random data through it, and feed it into + the pool, but not credit entropy to it. What kind of random source is behind + the EFI RNG API is often not entirely clear, but it hopefully is some kind of + hardware source. + +If neither of these are available (in fact, even if they are), Linux generates +entropy from various non-hwrng sources in various subsystems, all of which +ultimately are rooted in IRQ noise, a very "slow" source of entropy, in +particular in virtualized environments. + +## `systemd`'s Use of Random Numbers + +systemd is responsible for bringing up the OS. It generally runs as the first +userspace process the kernel invokes. Because of that it runs at a time where +the entropy pool is typically not yet initialized, and thus requests to acquire +random bytes will either be delayed, will fail or result in a noisy kernel log +message (see above). + +Various other components run during early boot that require random bytes. For +example, initial RAM disks nowadays communicate with encrypted networks or +access encrypted storage which might need random numbers. systemd itself +requires random numbers as well, including for the following uses: + +* systemd assigns 'invocation' UUIDs to all services it invokes that uniquely + identify each invocation. This is useful retain a global handle on a specific + service invocation and relate it to other data. For example, log data + collected by the journal usually includes the invocation UUID and thus the + runtime context the service manager maintains can be neatly matched up with + the log data a specific service invocation generated. systemd also + initializes `/etc/machine-id` with a randomized UUID. (systemd also makes use + of the randomized "boot id" the kernel exposes in + `/proc/sys/kernel/random/boot_id`). These UUIDs are exclusively Type 4 UUIDs, + i.e. randomly generated ones. + +* systemd maintains various hash tables internally. In order to harden them + against [collision + attacks](https://www.cs.auckland.ac.nz/~mcw/Teaching/refs/misc/denial-of-service.pdf) + they are seeded with random numbers. + +* At various places systemd needs random bytes for temporary file name + generation, UID allocation randomization, and similar. + +* systemd-resolved and systemd-networkd use random number generators to harden + the protocols they implement against packet forgery. + +* systemd-udevd and systemd-nspawn can generate randomized MAC addresses for + network devices. + +Note that these cases generally do not require a cryptographic-grade random +number generator, as most of these utilize random numbers to minimize risk of +collision and not to generate secret key material. However, they usually do +require "medium-grade" random data. For example: systemd's hash-maps are +reseeded if they grow beyond certain thresholds (and thus collisions are more +likely). This means they are generally fine with low-quality (even constant) +random numbers initially as long as they get better with time, so that +collision attacks are eventually thwarted as better, non-guessable seeds are +acquired. + +## Keeping `systemd'`s Demand on the Kernel Entropy Pool Minimal + +Since most of systemd's own use of random numbers do not require +cryptographic-grade RNGs, it tries to avoid reading entropy from the kernel +entropy pool if possible. If it succeeds this has the benefit that there's no +need to delay the early boot process until entropy is available, and noisy +kernel log messages about early reading from `/dev/urandom` are avoided +too. Specifically: + +1. When generating [Type 4 + UUIDs](https://en.wikipedia.org/wiki/Universally_unique_identifier#Version_4_\(random\)), + systemd tries to use Intel's and AMD's RDRAND CPU opcode directly, if + available. While some doubt the quality and trustworthiness of the entropy + provided by these opcodes, they should be good enough for generating UUIDs, + if not key material (though, as mentioned, today's big distributions opted + to trust it for that too, now, see above — but we are not going to make that + decision for you, and for anything key material related will only use the + kernel's entropy pool). If RDRAND is not available or doesn't work, it will + use synchronous `getrandom()` as fallback, and `/dev/urandom` on old kernels + where that system call doesn't exist yet. This means on non-Intel/AMD + systems UUID generation will block on kernel entropy initialization. + +2. For seeding hash tables, and all the other similar purposes systemd first + tries RDRAND, and if that's not available will try to use asynchronous + `getrandom()` (if the kernel doesn't support this system call, + `/dev/urandom` is used). This may fail too in case the pool is not + initialized yet, in which case it will fall back to glibc's internal rand() + calls, i.e. weak pseudo-random numbers. This should make sure we use good + random bytes if we can, but neither delay boot nor trigger noisy kernel log + messages during early boot for these use-cases. + +## `systemd`'s Support for Filling the Kernel Entropy Pool + +systemd has various provisions to ensure the kernel entropy is filled during +boot, in order to ensure the entropy pool is filled up quickly. + +1. When systemd's PID 1 detects it runs in a virtualized environment providing + the `virtio-rng` interface it will load the necessary kernel modules to make + use of it during earliest boot, if possible — much earlier than regular + kernel module loading done by `systemd-udevd.service`. This should ensure + that in VM environments the entropy pool is quickly filled, even before + systemd invokes the first service process — as long as the VM environment + provides virtualized RNG hardware (and VM environments really should!). + +2. The + [`systemd-random-seed.service`](https://www.freedesktop.org/software/systemd/man/systemd-random-seed.service.html) + system service will load a random seed from `/var/lib/systemd/random-seed` + into the kernel entropy pool. By default it does not credit entropy for it + though, since the seed is — more often than not — not reset when 'golden' + master images of an OS are created, and thus replicated into every + installation. If OS image builders carefully reset the random seed file + before generating the image it should be safe to credit entropy, which can + be enabled by setting the `$SYSTEMD_RANDOM_SEED_CREDIT` environment variable + for the service to `1` (or even `force`, see man page). Note however, that + this service typically runs relatively late during early boot: long after + the initial RAM disk (`initrd`) completed, and after the `/var/` file system + became writable. This is usually too late for many applications, it is hence + not advised to rely exclusively on this functionality to seed the kernel's + entropy pool. Also note that this service synchronously waits until the + kernel's entropy pool is initialized before completing start-up. It may thus + be used by other services as synchronization point to order against, if they + require an initialized entropy pool to operate correctly. + +3. The + [`systemd-boot`](https://www.freedesktop.org/software/systemd/man/systemd-boot.html) + EFI boot loader included in systemd is able to maintain and provide a random + seed stored in the EFI System Partition (ESP) to the booted OS, which allows + booting up with a fully initialized entropy pool from earliest boot + on. During installation of the boot loader (or when invoking [`bootctl + random-seed`](https://www.freedesktop.org/software/systemd/man/bootctl.html#random-seed)) + a seed file with an initial seed is placed in a file `/loader/random-seed` + in the ESP. In addition, an identically sized randomized EFI variable called + the 'system token' is set, which is written to the machine's firmware NVRAM. + During boot, when `systemd-boot` finds both the random seed file and the + system token they are combined and hashed with SHA256 (in counter mode, to + generate sufficient data), to generate a new random seed file to store in + the ESP as well as a random seed to pass to the OS kernel. The new random + seed file for the ESP is then written to the ESP, ensuring this is completed + before the OS is invoked. Very early during initialization PID 1 will read + the random seed provided in the EFI variable and credit it fully to the + kernel's entropy pool. + + This mechanism is able to safely provide an initialized entropy pool already + in the `initrd` and guarantees that different seeds are passed from the boot + loader to the OS on every boot (in a way that does not allow regeneration of + an old seed file from a new seed file). Moreover, when an OS image is + replicated between multiple images and the random seed is not reset, this + will still result in different random seeds being passed to the OS, as the + per-machine 'system token' is specific to the physical host, and not + included in OS disk images. If the 'system token' is properly initialized + and kept sufficiently secret it should not be possible to regenerate the + entropy pool of different machines, even if this seed is the only source of + entropy. + + Note that the writes to the ESP needed to maintain the random seed should be + minimal. The size of the random seed file is directly derived from the Linux + kernel's entropy pool size, which defaults to 512 bytes. This means updating + the random seed in the ESP should be doable safely with a single sector + write (since hard-disk sectors typically happen to be 512 bytes long, too), + which should be safe even with FAT file system drivers built into + low-quality EFI firmwares. + + As a special restriction: in virtualized environments PID 1 will refrain + from using this mechanism, for safety reasons. This is because on VM + environments the EFI variable space and the disk space is generally not + maintained physically separate (for example, `qemu` in EFI mode stores the + variables in the ESP itself). The robustness towards sloppy OS image + generation is the main purpose of maintaining the 'system token' however, + and if the EFI variable storage is not kept physically separate from the OS + image there's no point in it. That said, OS builders that know that they are + not going to replicate the built image on multiple systems may opt to turn + off the 'system token' concept by setting `random-seed-mode always` in the + ESP's + [`/loader/loader.conf`](https://www.freedesktop.org/software/systemd/man/loader.conf.html) + file. If done, `systemd-boot` will use the random seed file even if no + system token is found in EFI variables. + +4. A kernel command line option `systemd.random_seed=` may be used to pass in a + base64 encoded seed to initialize the kernel's entropy pool from during + early service manager initialization. This option is only safe in testing + environments, as the random seed passed this way is accessible to + unprivileged programs via `/proc/cmdline`. Using this option outside of + testing environments is a security problem since cryptographic key material + derived from the entropy pool initialized with a seed accessible to + unprivileged programs should not be considered secret. + +With the four mechanisms described above it should be possible to provide +early-boot entropy in most cases. Specifically: + +1. On EFI systems, `systemd-boot`'s random seed logic should make sure good + entropy is available during earliest boot — as long as `systemd-boot` is + used as boot loader, and outside of virtualized environments. + +2. On virtualized systems, the early `virtio-rng` hookup should ensure entropy + is available early on — as long as the VM environment provides virtualized + RNG devices, which they really should all do in 2019. Complain to your + hosting provider if they don't. For VMs used in testing environments, + `systemd.random_seed=` may be used as an alternative to a virtualized RNG. + +3. On Intel/AMD systems systemd's own reliance on the kernel entropy pool is + minimal (as RDRAND is used on those for UUID generation). This only works if + the CPU has RDRAND of course, which most physical CPUs do (but I hear many + virtualized CPUs do not. Pity.) + +4. In all other cases, `systemd-random-seed.service` will help a bit, but — as + mentioned — is too late to help with early boot. + +This primarily leaves two kind of systems in the cold: + +1. Some embedded systems. Many embedded chipsets have hwrng functionality these + days. Consider using them while crediting + entropy. (i.e. `rng_core.default_quality=1000` on the kernel command line is + your friend). Or accept that the system might take a bit longer to + boot. Alternatively, consider implementing a solution similar to + systemd-boot's random seed concept in your platform's boot loader. + +2. Virtualized environments that lack both virtio-rng and RDRAND, outside of + test environments. Tough luck. Talk to your hosting provider, and ask them + to fix this. + +3. Also note: if you deploy an image without any random seed and/or without + installing any 'system token' in an EFI variable, as described above, this + means that on the first boot no seed can be passed to the OS + either. However, as the boot completes (with entropy acquired elsewhere), + systemd will automatically install both a random seed in the GPT and a + 'system token' in the EFI variable space, so that any future boots will have + entropy from earliest boot on — all provided `systemd-boot` is used. + +## Frequently Asked Questions + +1. *Why don't you just use getrandom()? That's all you need!* + + Did you read any of the above? getrandom() is hooked to the kernel entropy + pool, and during early boot it's not going to be filled yet, very likely. We + do use it in many cases, but not in all. Please read the above again! + +2. *Why don't you use + [getentropy()](http://man7.org/linux/man-pages/man3/getentropy.3.html)? That's + all you need!* + + Same story. That call is just a different name for `getrandom()` with + `flags` set to zero, and some additional limitations, and thus it also needs + the kernel's entropy pool to be initialized, which is the whole problem we + are trying to address here. + +3. *Why don't you generate your UUIDs with + [`uuidd`](http://man7.org/linux/man-pages/man8/uuidd.8.html)? That's all you + need!* + + First of all, that's a system service, i.e. something that runs as "payload" + of systemd, long after systemd is already up and hence can't provide us + UUIDs during earliest boot yet. Don't forget: to assign the invocation UUID + for the `uuidd.service` start we already need a UUID that the service is + supposed to provide us. More importantly though, `uuidd` needs state/a random + seed/a MAC address/host ID to operate, all of which are not available during + early boot. + +4. *Why don't you generate your UUIDs with `/proc/sys/kernel/random/uuid`? + That's all you need!* + + This is just a different, more limited interface to `/dev/urandom`. It gains + us nothing. + +5. *Why don't you use [`rngd`](https://github.com/nhorman/rng-tools), + [`haveged`](http://www.issihosts.com/haveged/), + [`egd`](http://egd.sourceforge.net/)? That's all you need!* + + Like `uuidd` above these are system services, hence come too late for our + use-case. In addition much of what `rngd` provides appears to be equivalent + to `CONFIG_RANDOM_TRUST_CPU=y` or `rng_core.default_quality=1000`, except + being more complex and involving userspace. These services partly measure + system behavior (such as scheduling effects) which the kernel either + already feeds into its pool anyway (and thus shouldn't be fed into it a + second time, crediting entropy for it a second time) or is at least + something the kernel could much better do on its own. Hence, if what these + daemons do is still desirable today, this would be much better implemented + in kernel (which would be very welcome of course, but wouldn't really help + us here in our specific problem, see above). + +6. *Why don't you use [`arc4random()`](https://man.openbsd.org/arc4random.3)? + That's all you need!* + + This doesn't solve the issue, since it requires a nonce to start from, and + it gets that from `getrandom()`, and thus we have to wait for random pool + initialization the same way as calling `getrandom()` + directly. `arc4random()` is nothing more than optimization, in fact it + implements similar algorithms that the kernel entropy pool implements + anyway, hence besides being able to provide random bytes with higher + throughput there's little it gets us over just using `getrandom()`. Also, + it's not supported by glibc. And as long as that's the case we are not keen + on using it, as we'd have to maintain that on our own, and we don't want to + maintain our own cryptographic primitives if we don't have to. Since + systemd's uses are not performance relevant (besides the pool initialization + delay, which this doesn't solve), there's hence little benefit for us to + call these functions. That said, if glibc learns these APIs one day, we'll + certainly make use of them where appropriate. + +7. *This is boring: NetBSD had [boot loader entropy seed + support](https://netbsd.gw.com/cgi-bin/man-cgi?boot+8) since ages!* + + Yes, NetBSD has that, and the above is inspired by that (note though: this + article is about a lot more than that). NetBSD's support is not really safe, + since it neither updates the random seed before using it, nor has any + safeguards against replicating the same disk image with its random seed on + multiple machines (which the 'system token' mentioned above is supposed to + address). This means reuse of the same random seed by the boot loader is + much more likely. + +8. *Why does PID 1 upload the boot loader provided random seed into kernel + instead of kernel doing that on its own?* + + That's a good question. Ideally the kernel would do that on its own, and we + wouldn't have to involve userspace in this. + +9. *What about non-EFI?* + + The boot loader random seed logic described above uses EFI variables to pass + the seed from the boot loader to the OS. Other systems might have similar + functionality though, and it shouldn't be too hard to implement something + similar for them. Ideally, we'd have an official way to pass such a seed as + part of the `struct boot_params` from the boot loader to the kernel, but + this is currently not available. + +10. *I use a different boot loader than `systemd-boot`, I'd like to use boot + loader random seeds too!* + + Well, consider just switching to `systemd-boot`, it's worth it. See + [systemd-boot(7)](https://www.freedesktop.org/software/systemd/man/systemd-boot.html) + for an introduction why. That said, any boot loader can re-implement the + logic described above, and can pass a random seed that systemd as PID 1 + will then upload into the kernel's entropy pool. For details see the [Boot + Loader Interface](https://systemd.io/BOOT_LOADER_INTERFACE) documentation. + +11. *Why not pass the boot loader random seed via kernel command line instead + of as EFI variable?* + + The kernel command line is accessible to unprivileged processes via + `/proc/cmdline`. It's not desirable if unprivileged processes can use this + information to possibly gain too much information about the current state + of the kernel's entropy pool. + + That said, we actually do implement this with the `systemd.random_seed=` + kernel command line option. Don't use this outside of testing environments, + however, for the aforementioned reasons. + +12. *Why doesn't `systemd-boot` rewrite the 'system token' too each time + when updating the random seed file stored in the ESP?* + + The system token is stored as persistent EFI variable, i.e. in some form of + NVRAM. These memory chips tend be of low quality in many machines, and + hence we shouldn't write them too often. Writing them once during + installation should generally be OK, but rewriting them on every single + boot would probably wear the chip out too much, and we shouldn't risk that. diff --git a/docs/RELEASE.md b/docs/RELEASE.md new file mode 100644 index 0000000..a93c0c6 --- /dev/null +++ b/docs/RELEASE.md @@ -0,0 +1,22 @@ +--- +title: Steps to a Successful Release +category: Contributing +layout: default +--- + +# Steps to a Successful Release + +1. Add all items to NEWS +2. Update the contributors list in NEWS (`ninja -C build git-contrib`) +3. Update the time and place in NEWS +4. [RC1] Update version and library numbers in `meson.build` +5. Check dbus docs with `ninja -C build man/update-dbus-docs` +6. Tag the release: `version=vXXX-rcY && git tag -s "${version}" -m "systemd ${version}"` +7. Do `ninja -C build` +8. Make sure that the version string and package string match: `build/systemctl --version` +9. Upload the documentation: `ninja -C build doc-sync` +10. [FINAL] Close the github milestone and open a new one (https://github.com/systemd/systemd/milestones) +11. "Draft" a new release on github (https://github.com/systemd/systemd/releases/new), mark "This is a pre-release" if appropriate. +12. Check that announcement to systemd-devel, with a copy&paste from NEWS, was sent. This should happen automatically. +13. Update IRC topic (`/msg chanserv TOPIC #systemd Version NNN released`) +14. [FINAL] Push commits to stable, create an empty -stable branch: `git push systemd-stable origin/master:master origin/master:refs/heads/${version}-stable`, and change the default branch to latest release (https://github.com/systemd/systemd-stable/settings/branches). diff --git a/docs/RESOLVED-VPNS.md b/docs/RESOLVED-VPNS.md new file mode 100644 index 0000000..1010bed --- /dev/null +++ b/docs/RESOLVED-VPNS.md @@ -0,0 +1,267 @@ +--- +title: systemd-resolved and VPNs +category: Networking +layout: default +--- + +# `systemd-resolved.service` and VPNs + +`systemd-resolved.service` supports routing lookups for specific domains to specific +interfaces. This is useful for hooking up VPN software with systemd-resolved +and making sure the exact right lookups end up on the VPN and on the other +interfaces. + +For a verbose explanation of `systemd-resolved.service`'s domain routing logic, +see its [man +page](https://www.freedesktop.org/software/systemd/man/systemd-resolved.service.html). This +document is supposed to provide examples to use the concepts for the specific +purpose of managing VPN DNS configuration. + +Let's first define two distinct VPN use-cases: + +1. *Corporate* VPNs, i.e. VPNs that open access to a specific set of additional + hosts. Only specific domains should be resolved via the VPN's DNS servers, + and everything that is not related to the company's domain names should go + to regular, non-VPN DNS instead. + +2. *Privacy* VPNs, i.e. VPNs that should be used for basically all DNS traffic, + once they are up. If this type of VPN is used, any regular, non-VPN DNS + servers should not get any traffic anymore. + +Then, let's briefly introduce three DNS routing concepts that software managing +a network interface may configure. + +1. Search domains: these are traditional DNS configuration parameters and are + used to suffix non-qualified domain names (i.e. single-label ones), to turn + them into fully qualified domain names. Traditionally (before + `systemd-resolved.service`), search domain names are attached to a system's + IP configuration as a whole, in `systemd-resolved.service` they are + associated to individual interfaces instead, since they are typically + acquired through some network associated concept, such as a DHCP, IPv6RA or + PPP lease. Most importantly though: in `systemd-resolved.service` they are + not just used to suffix single-label domain names, but also for routing + domain name lookups: if a network interface has a search domain `foo.com` + configured on it, then any lookups for names ending in `.foo.com` (or for + `foo.com` itself) are preferably routed to the DNS servers configured on the + same network interface. + +2. Routing domains: these are very similar to search domains, but are purely + about DNS domain name lookup routing — they are not used for qualifying + single-label domain names. When it comes to routing assigning a routing + domain to a network interface is identical to assigning a search domain to + it. + + Why the need to have both concepts, i.e. search *and* routing domains? + Mostly because in many cases the qualifying of single-label names is not + desirable (since security-sensitive), but needs to be supported for specific + use-cases. Routing domains are a concept `systemd-resolved.service` + introduced, while search domains are traditionally available and are part of + DHCP/IPv6RA/PPP leases and thus universally supported. In many cases routing + domains are probably the more appropriate concept, but not easily available, + since not part of DHCP/IPv6RA/PPP. + + Routing domains for `systemd-resolved.service` are usually presented along + with search domains in mostly the same way, but prefixed with `~` to + differentiate them. i.e. `~foo.com` is a configured routing domain, while + `foo.com` would be a configured search domain. + + One routing domain is particular interesting: `~.` — the catch-all routing + domain. (The *dot* domain `.` is how DNS denotes the "root" domain, i.e. the + parent domain of all domains, but itself.) When used on an interface any DNS + traffic is preferably routed to its DNS servers. (A search domain – i.e. `.` + instead of `~.` — would have the same effect, but given that it's mostly + pointless to suffix an unqualified domain with `.`, we generally declare it + as a routing domain, not a search domain). + + Routing domains also have particular relevance when it comes to the reverse + lookup DNS domains `.in-addr.arpa` and `.ip6.arpa`. An interface that has + these (or sub-domains thereof) defined as routing domains, will be preferably + used for doing reverse IP to domain name lookups. e.g. declaring + `~168.192.in-addr.arpa` on an interface means that all lookups to find the + domain names for IPv4 addresses 192.168.x.y are preferable routed to it. + +3. The `default-route` boolean. This is a simple boolean value that may be set + on an interface. If true (the default), any DNS lookups for which no + matching routing or search domains are defined are routed to interfaces + marked like this. If false then the DNS servers on this interface are not + considered for routing lookups to except for the ones listed in the + search/routing domain list. An interface that has no search/routing domain + associated and also has this boolean off is not considered for *any* + lookups. + +One more thing to mention: in `systemd-resolved.service` if lookups match the +search/routing domains of multiple interfaces at once, then they are sent to +all of them in parallel, and the first positive reply used. If all lookups fail +the last negative reply is used. This means the DNS zones on the relevant +interfaces are "merged": domains existing on one but not the other will "just +work" and vice versa. + +And one more note: the domain routing logic implemented is a tiny bit more +complex that what described above: if there two interfaces have search domains +that are suffix of each other, and a name is looked up that matches both, the +interface with the longer match will win and get the lookup routed to is DNS +servers. Only if the match has the same length, then both will be used in +parallel. Example: one interface has `~foo.example.com` as routing domain, and +another one `example.com` has search domain. A lookup for +`waldo.foo.example.com` is the exclusively routed to the first interface's DNS +server, since it matches by three suffix labels instead of just two. The fact +that the matching length is taken into consideration for the routing decision +is particularly relevant if you have one interface with the `~.` routing domain +and another one with `~corp.company.example` — both suffixes match a lookup for +`foo.corp.company.example`, but the latter interface wins, since the match is +for four labels, while the other is for zero labels. + +# Putting it Together + +Let's discuss how the three DNS routing concepts above are best used for a +reasonably complex scenario consisting of: + +1. One VPN interface of the *corporate* kind, maybe called `company0`. It makes + available a bunch of servers, all in the domain `corp.company.example`. + +2. One VPN interface of the *privacy* kind, maybe called `privacy0`. When it is + up all DNS traffic shall preferably routed to its DNS servers. + +3. One regular WiFi interface, maybe called `wifi0`. It has a regular DNS + server on it. + +Here's how to best configure this for `systemd-resolved.service`: + +1. `company0` should get a routing domain `~corp.company.example` + configured. (A search domain `corp.company.example` would work too, if + qualifying of single-label names is desired or the VPN lease information + does not provide for the concept of routing domains, but does support search + domains.) This interface should also set `default-route` to false, to ensure + that really only the DNS lookups for the company's servers are routed there + and nothing else. Finally, it might make sense to also configure a routing + domain `~2.0.192.in-addr.arpa` on the interface, ensuring that all IPv4 + addresses from the 192.0.2.x range are preferably resolved via the DNS + server on this interface (assuming that that's the IPv4 address range the + company uses internally). + +2. `privacy0` should get a routing domain `~.` configured. The setting of + `default-route` for this interface is then irrelevant. This means: once the + interface is up, all DNS traffic is preferably routed there. + +3. `wifi0` should not get any special settings, except possibly whatever the + local WiFi router considers suitable as search domain, for example + `fritz.box`. The default `true` setting for `default-route` is good too. + +With this configuration if only `wifi0` is up, all DNS traffic goes to its DNS +server, since there are no other interfaces with better matching DNS +configuration. If `privacy0` is then upped, all DNS traffic will exclusively go +to this interface now — with the exception of names below the `fritz.box` +domain, which will continue to go directly to `wifi0`, as the search domain +there says so. Now, if `company0` is also upped, it will receive DNS traffic +for the company's internal domain and internal IP subnet range, but nothing +else. If `privacy0` is then downed again, `wifi0` will get the regular DNS +traffic again, and `company0` will still get the company's internal domain and +IP subnet traffic and nothing else. Everything hence works as intended. + +# How to Implement this in Your VPN Software + +Most likely you want to expose a boolean in some way that declares whether a +specific VPN is of the *corporate* or the *privacy* kind: + +1. If managing a *corporate* VPN, you configure any search domains the user or + the VPN contact point provided. And you set `default-route` to false. If you + have IP subnet information for the VPN, it might make sense to insert + `~….in-addr.arpa` and `~….ip6.arpa` reverse lookup routing domains for it. + +2. If managing a *privacy* VPN, you include `~.` in the routing domains, the + value for `default-route` is actually irrelevant, but I'd set it to true. No + need to configure any reverse lookup routing domains for it. + +(If you also manage regular WiFi/Ethernet devices, just configure them as +traditional, i.e. with any search domains as acquired, do not set `~.` though, +and do not disable `default-route`.) + +# The APIs + +Now we determined how we want to configure things, but how do you actually get +the configuration to `systemd-resolved.service`? There are three relevant +interfaces: + +1. Ideally, you use D-Bus and talk to [`systemd-resolved.service`'s D-Bus + API](https://www.freedesktop.org/software/systemd/man/org.freedesktop.resolve1.html) + directly. Use `SetLinkDomains()` to set the per-interface search and routing + domains on the interfaces you manage, and `SetLinkDefaultRoute()` to manage + the `default-route` boolean, all on the `org.freedesktop.resolve1.Manager` + interface of the `/org/freedesktop/resolve1` object. + +2. If that's not in the cards, you may shell out to + [`resolvectl`](https://www.freedesktop.org/software/systemd/man/resolvectl.html), + which is a thin wrapper around the D-Bus interface mentioned above. Use + `resolvectl domain <iface> …` to set the search/routing domains and + `resolvectl default-route <iface> …` to set the `default-route` boolean. + + Example use from a shell callout of your VPN software for a *corporate* VPN: + + resolvectl domain corporate0 '~corp-company.example' '~2.0.192.in-addr.arpa' + resolvectl default-route corporate0 false + resolvectl dns corporate0 192.0.2.1 + + Example use from a shell callout of your VPN software for a *privacy* VPN: + + resolvectl domain privacy0 '~.' + resolvectl default-route privacy0 true + resolvectl dns privacy0 8.8.8.8 + +3. If you don't want to use any `systemd-resolved` commands, you may use the + `resolvconf` wrapper we provide. `resolvectl` is actually a multi-call + binary and may be symlinked to `resolvconf`, and when invoked like that + behaves in a way that is largely compatible with FreeBSD's and + Ubuntu's/Debian's + [`resolvconf(8)`](https://manpages.ubuntu.com/manpages/trusty/man8/resolvconf.8.html) + tool. When the `-x` switch is specified, the `~.` routing domain is + automatically appended to the domain list configured, as appropriate for a + *privacy* VPN. Note that the `resolvconf` interface only covers *privacy* + VPNs and regular network interfaces (such as WiFi or Ethernet) well. The + *corporate* kind of VPN is not well covered, since the interface cannot + propagate the `default-route` boolean, nor can be used to configure the + `~….in-addr.arpa` or `~.ip6.arpa` routing domains. + +# Ordering + +When configuring per-interface DNS configuration settings it is wise to +configure everything *before* actually upping the interface. Once the interface +is up `systemd-resolved.service` might start using it, and hence it's important +to have everything configured properly (this is particularly relevant when +LLMNR or MulticastDNS is enabled, since that works without any explicitly +configured DNS configuration). It is also wise to configure search/routing +domains and the `default-route` boolean *before* configuring the DNS servers, +as the former without the latter has no effect, but the latter without the +former will result in DNS traffic possibly being generated, in a non-desirable +way given that the routing information is not set yet. + +# Downgrading Search Domains to Routing Domains + +Many VPN implementations provide a way how VPN servers can inform VPN clients +about search domains to use. In some cases it might make sense to install those +as routing domains instead of search domains. Unqualified domain names usually +imply a context of locality: the same unqualified name typically is expected to +resolve to one system in one local network, and to another one in a different +network. Search domains thus generally come with security implications: they +might cause that unqualified domains are resolved in a different (possibly +remote) context, contradicting user expectations. Thus it might be wise to +downgrade *search domains* provided by VPN servers to *routing domains*, so +that local unqualified name resolution remains untouched and strictly maintains +its local focus — in particular in the aforementioned less trusted *corporate* +VPN scenario. + +To illustrate this further, here's an example for an attack scenario using +search domains: a user assumes the printer system they daily contact under the +unqualified name "printer" is the network printer in their basement (with the +fully qualified domain name "printer.home"). Sometimes the user joins the +corporate VPN of their employer, which comes with a search domain +"foocorp.example", so that the user's confidential documents (maybe a job +application to a competing company) might end up being printed on +"printer.foocorp.example" instead of "printer.home". If the local VPN software +had downgraded the VPN's search domain to a routing domain "~foocorp.example", +this mismapping would not have happened. + +When connecting to untrusted WiFi networks it might be wise to go one step +further even: suppress installation of search/routing domains by the network +entirely, to ensure that the local DNS information is only used for name +resolution of qualified names and only when no better DNS configuration is +available. diff --git a/docs/ROOT_STORAGE_DAEMONS.md b/docs/ROOT_STORAGE_DAEMONS.md new file mode 100644 index 0000000..e18ac45 --- /dev/null +++ b/docs/ROOT_STORAGE_DAEMONS.md @@ -0,0 +1,193 @@ +--- +title: Storage Daemons for the Root File System +category: Interfaces +layout: default +--- + +# systemd and Storage Daemons for the Root File System + +a.k.a. _Pax Cellae pro Radix Arbor_ + +(or something like that, my Latin is a bit rusty) + +A number of complex storage technologies on Linux (e.g. RAID, volume +management, networked storage) require user space services to run while the +storage is active and mountable. This requirement becomes tricky as soon as the +root file system of the Linux operating system is stored on such storage +technology. Previously no clear path to make this work was available. This text +tries to clear up the resulting confusion, and what is now supported and what +is not. + +## A Bit of Background + +When complex storage technologies are used as backing for the root file system +this needs to be set up by the initial RAM file system (initrd), i.e. on Fedora +by Dracut. In newer systemd versions tear-down of the root file system backing +is also done by the initrd: after terminating all remaining running processes +and unmounting all file systems it can (which means excluding the root fs) +systemd will jump back into the initrd code allowing it to unmount the final +file systems (and its storage backing) that could not be unmounted as long as +the OS was still running from the main root file system. The initrd' job is to +detach/unmount the root fs, i.e. inverting the exact commands it used to set +them up in the first place. This is not only cleaner, but also allows for the +first time arbitrary complex stacks of storage technology. + +Previous attempts to handle root file system setups with complex storage as +backing usually tried to maintain the root storage with program code stored on +the root storage itself, thus creating a number of dependency loops. Safely +detaching such a root file system becomes messy, since the program code on the +storage needs to stay around longer than the storage, which is technically +contradicting. + + +## What's new? + +As a result, we hereby clarify that we do not support storage technology setups +where the storage daemons are being run from the storage it maintains +itself. In other words: a storage daemon backing the root file system cannot be +stored on the root file system itself. + +What we do support instead is that these storage daemons are started from the +initrd, stay running all the time during normal operation and are terminated +only after we returned control back to the initrd and by the initrd. As such, +storage daemons involved with maintaining the root file system storage +conceptually are more like kernel threads than like normal system services: +from the perspective of the init system (i.e. systemd) these services have been +started before systemd got initialized and stay around until after systemd is +already gone. These daemons can only be updated by updating the initrd and +rebooting, a takeover from initrd-supplied services to replacements from the +root file system is not supported. + + +## What does this mean? + +Near the end of system shutdown, systemd executes a small tool called +systemd-shutdown, replacing its own process. This tool (which runs as PID 1, as +it entirely replaces the systemd init process) then iterates through the +mounted file systems and running processes (as well as a couple of other +resources) and tries to unmount/read-only mount/detach/kill them. It continues +to do this in a tight loop as long as this results in any effect. From this +killing spree a couple of processes are automatically excluded: PID 1 itself of +course, as well as all kernel threads. After the killing/unmounting spree +control is passed back to the initrd, whose job is then to unmount/detach +whatever might be remaining. + +The same killing spree logic (but not the unmount/detach/read-only logic) is +applied during the transition from the initrd to the main system (i.e. the +"`switch_root`" operation), so that no processes from the initrd survive to the +main system. + +To implement the supported logic proposed above (i.e. where storage daemons +needed for the root fs which are started by the initrd stay around during +normal operation and are only killed after control is passed back to the +initrd) we need to exclude these daemons from the shutdown/switch_root killing +spree. To accomplish this the following logic is available starting with +systemd 38: + +Processes (run by the root user) whose first character of the zeroth command +line argument is `@` are excluded from the killing spree, much the same way as +kernel threads are excluded too. Thus, a daemon which wants to take advantage +of this logic needs to place the following at the top of its `main()` function: + +```c +... +argv[0][0] = '@'; +... +``` + +And that's already it. Note that this functionality is only to be used by +programs running from the initrd, and **not** for programs running from the +root file system itself. Programs which use this functionality and are running +from the root file system are considered buggy since they effectively prohibit +clean unmounting/detaching of the root file system and its backing storage. + +_Again: if your code is being run from the root file system, then this logic +suggested above is **NOT** for you. Sorry. Talk to us, we can probably help you +to find a different solution to your problem._ + +The recommended way to distinguish between run-from-initrd and run-from-rootfs +for a daemon is to check for `/etc/initrd-release` (which exists on all modern +initrd implementations, see the [initrd +Interface](https://systemd.io/INITRD_INTERFACE/) for details) which when exists +results in `argv[0][0]` being set to `@`, and otherwise doesn't. Something like +this: + +```c +#include <unistd.h> + +int main(int argc, char *argv[]) { + ... + if (access("/etc/initrd-release", F_OK) >= 0) + argv[0][0] = '@'; + ... + } +``` + +Why `@`? Why `argv[0][0]`? First of all, a technique like this is not without +precedent: traditionally Unix login shells set `argv[0][0]` to `-` to clarify +they are login shells. This logic is also very easy to implement. We have been +looking for other ways to mark processes for exclusion from the killing spree, +but could not find any that was equally simple to implement and quick to read +when traversing through `/proc/`. Also, as a side effect replacing the first +character of `argv[0]` with `@` also visually invalidates the path normally +stored in `argv[0]` (which usually starts with `/`) thus helping the +administrator to understand that your daemon is actually not originating from +the actual root file system, but from a path in a completely different +namespace (i.e. the initrd namespace). Other than that we just think that `@` +is a cool character which looks pretty in the ps output... 😎 + +Note that your code should only modify `argv[0][0]` and leave the comm name +(i.e. `/proc/self/comm`) of your process untouched. + +## To which technologies does this apply? + +These recommendations apply to those storage daemons which need to stay around +until after the storage they maintain is unmounted. If your storage daemon is +fine with being shut down before its storage device is unmounted you may ignore +the recommendations above. + +This all applies to storage technology only, not to daemons with any other +(non-storage related) purposes. + +## What else to keep in mind? + +If your daemon implements the logic pointed out above it should work nicely +from initrd environments. In many cases it might be necessary to additionally +support storage daemons to be started from within the actual OS, for example +when complex storage setups are used for auxiliary file systems, i.e. not the +root file system, or created by the administrator during runtime. Here are a +few additional notes for supporting these setups: + +* If your storage daemon is run from the main OS (i.e. not the initrd) it will + also be terminated when the OS shuts down (i.e. before we pass control back + to the initrd). Your daemon needs to handle this properly. + +* It is not acceptable to spawn off background processes transparently from + user commands or udev rules. Whenever a process is forked off on Unix it + inherits a multitude of process attributes (ranging from the obvious to the + not-so-obvious such as security contexts or audit trails) from its parent + process. It is practically impossible to fully detach a service from the + process context of the spawning process. In particular, systemd tracks which + processes belong to a service or login sessions very closely, and by spawning + off your storage daemon from udev or an administrator command you thus make + it part of its service/login. Effectively this means that whenever udev is + shut down, your storage daemon is killed too, resp. whenever the login + session goes away your storage might be terminated as well. (Also note that + recent udev versions will automatically kill all long running background + processes forked off udev rules now.) So, in summary: double-forking off + processes from user commands or udev rules is **NOT** OK! + +* To automatically spawn storage daemons from udev rules or administrator + commands, the recommended technology is socket-based activation as + implemented by systemd. Transparently for your client code connecting to the + socket of your storage daemon will result in the storage to be started. For + that it is simply necessary to inform systemd about the socket you'd like it + to listen on on behalf of your daemon and minimally modify the daemon to + receive the listening socket for its services from systemd instead of + creating it on its own. Such modifications can be minimal, and are easily + written in a way that does not negatively impact usability on non-systemd + systems. For more information on making use of socket activation in your + program consult this blog story: [Socket + Activation](http://0pointer.de/blog/projects/socket-activation.html) + +* Consider having a look at the [initrd Interface of systemd](https://systemd.io/INITRD_INTERFACE/). diff --git a/docs/SECURITY.md b/docs/SECURITY.md new file mode 100644 index 0000000..bd2915b --- /dev/null +++ b/docs/SECURITY.md @@ -0,0 +1,13 @@ +--- +title: Reporting of Security Vulnerabilities +category: Contributing +layout: default +--- + +# Reporting of Security Vulnerabilities + +If you discover a security vulnerability, we'd appreciate a non-public disclosure. systemd developers can be contacted privately on the **[systemd-security@redhat.com](mailto:systemd-security@redhat.com) mailing list**. The disclosure will be coordinated with distributions. + +(The [issue tracker](https://github.com/systemd/systemd/issues) and [systemd-devel mailing list](https://lists.freedesktop.org/mailman/listinfo/systemd-devel) are fully public.) + +Subscription to the systemd-security mailing list is open to **regular systemd contributors and people working in the security teams of various distributions**. Those conditions should be backed by publicly accessible information (ideally, a track of posts and commits from the mail address in question). If you fall into one of those categories and wish to be subscribed, submit a **[subscription request](https://www.redhat.com/mailman/listinfo/systemd-security)**. diff --git a/docs/TEMPORARY_DIRECTORIES.md b/docs/TEMPORARY_DIRECTORIES.md new file mode 100644 index 0000000..c0f945c --- /dev/null +++ b/docs/TEMPORARY_DIRECTORIES.md @@ -0,0 +1,222 @@ +--- +title: Using /tmp/ and /var/tmp/ Safely +category: Interfaces +layout: default +--- + +# Using `/tmp/` and `/var/tmp/` Safely + +`/tmp/` and `/var/tmp/` are two world-writable directories Linux systems +provide for temporary files. The former is typically on `tmpfs` and thus +backed by RAM/swap, and flushed out on each reboot. The latter is typically a +proper, persistent file system, and thus backed by physical storage. This +means: + +1. `/tmp/` should be used for smaller, size-bounded files only; `/var/tmp/` + should be used for everything else. + +2. Data that shall survive a boot cycle shouldn't be placed in `/tmp/`. + +If the `$TMPDIR` environment variable is set, use that path, and neither use +`/tmp/` nor `/var/tmp/` directly. + +See +[file-hierarchy(7)](https://www.freedesktop.org/software/systemd/man/file-hierarchy.html) +for details about these two (and most other) directories of a Linux system. + +## Common Namespace + +Note that `/tmp/` and `/var/tmp/` each define a common namespace shared by all +local software. This means guessable file or directory names below either +directory directly translate into a 🚨 Denial-of-Service (DoS) 🚨 vulnerability +or worse: if some software creates a file or directory `/tmp/foo` then any +other software that wants to create the same file or directory `/tmp/foo` +either will fail (as the file already exists) or might be tricked into using +untrusted files. Hence: do not use guessable names in `/tmp/` or `/var/tmp/` — +if you do you open yourself up to a local DoS exploit or worse. (You can get +away with using guessable names, if you pre-create subdirectories below `/tmp/` +for them, like X11 does with `/tmp/.X11-unix/` through `tmpfiles.d/` +drop-ins. However this is not recommended, as it is fully safe only if these +directories are pre-created during early boot, and thus problematic if package +installation during runtime is permitted.) + +To protect yourself against these kinds of attacks Linux provides a couple of +APIs that help you avoiding guessable names. Specifically: + +1. Use [`mkstemp()`](http://man7.org/linux/man-pages/man3/mkstemp.3.html) + (POSIX), `mkostemp()` (glibc), + [`mkdtemp()`](http://man7.org/linux/man-pages/man3/mkdtemp.3.html) (POSIX), + [`tmpfile()`](http://man7.org/linux/man-pages/man3/tmpfile.3.html) (C89) + +2. Use [`open()`](http://man7.org/linux/man-pages/man2/open.2.html) with + `O_TMPFILE` (Linux) + +3. [`memfd_create()`](http://man7.org/linux/man-pages/man2/memfd_create.2.html) + (Linux; this doesn't bother with `/tmp/` or `/var/tmp/` at all, but uses the + same RAM/swap backing as `tmpfs` uses, hence is very similar to `/tmp/` + semantics.) + +For system services systemd provides the `PrivateTmp=` boolean setting. If +turned on for a service (👍 which is highly recommended), `/tmp/` and +`/var/tmp/` are replaced by private sub-directories, implemented through Linux +file system namespacing and bind mounts. This means from the service's point of +view `/tmp/` and `/var/tmp/` look and behave like they normally do, but in +reality they are private sub-directories of the host's real `/tmp/` and +`/var/tmp/`, and thus not system-wide locations anymore, but service-specific +ones. This reduces the surface for local DoS attacks substantially. While it is +recommended to turn this option on, it's highly recommended for applications +not to rely on this solely to avoid DoS vulnerabilities, because this option is +not available in environments where file system namespaces are prohibited, for +example in certain container environments. This option is hence an extra line +of defense, but should not be used as an excuse to rely on guessable names in +`/tmp/` and `/var/tmp/`. When this option is used, the per-service temporary +directories are removed whenever the service shuts down, hence the lifecycle of +temporary files stored in it is substantially different from the case where +this option is not used. Also note that some applications use `/tmp/` and +`/var/tmp/` for sharing files and directories. If this option is turned on this +is not possible anymore as after all each service gets its own instances of +both directories. + +## Automatic Clean-Up + +By default, `systemd-tmpfiles` will apply a concept of ⚠️ "ageing" to all files +and directories stored in `/tmp/` and `/var/tmp/`. This means that files that +have neither been changed nor read within a specific time frame are +automatically removed in regular intervals. (This concept is not new to +`systemd-tmpfiles` btw, it's inherited from previous subsystems such as +`tmpwatch`.) By default files in `/tmp/` are cleaned up after 10 days, and +those in `/var/tmp` after 30 days. + +This automatic clean-up is important to ensure disk usage of these temporary +directories doesn't grow without bounds, even when programs abort unexpectedly +or otherwise don't clean up the temporary files/directories they create. On the +other hand it creates problems for long-running software that does not expect +temporary files it operates on to be suddenly removed. There are a couple of +strategies to avoid these issues: + +1. Make sure to always keep a file descriptor to the temporary files you + operate on open, and only access the files through them. This way it doesn't + matter whether the files have been unlinked from the file system: as long as + you have the file descriptor open you can still access the file for both + reading and writing. When operating this way it is recommended to delete the + files right after creating them to ensure that on unexpected program + termination the files or directories are implicitly released by the kernel. + +2. 🥇 Use `memfd_create()` or `O_TMPFILE`. This is an extension of the + suggestion above: files created this way are never linked under a filename + in the file system. This means they are not subject to ageing (as they come + unlinked out of the box), and there's no time window where a directory entry + for the file exists in the file system, and thus behaviour is fully robust + towards unexpected program termination as there are never files on disk that + need to be explicitly deleted. + +3. 🥇 Operate below a sub-directory of `/tmp/` and `/var/tmp/` you created, and + take a BSD file lock ([`flock(dir_fd, + LOCK_SH)`](http://man7.org/linux/man-pages/man2/flock.2.html)) on that + sub-directory. This is particularly interesting when operating on more than + a single file, or on file nodes that are not plain regular files, for + example when extracting a tarball to a temporary directory. The ageing + algorithm will skip all directories (and everything below them) that are + locked through a BSD file lock. As BSD file locks are automatically released + when the file descriptor they are taken on is closed, and all file + descriptors opened by a process are implicitly closed when it exits, this is + a robust mechanism that ensures all temporary files are subject to ageing + when the program that owns them dies, but not while it is still running. Use + this when decompressing tarballs that contain files with old + modification/access times, as extracted files are otherwise immediately + candidates for deletion by the ageing algorithm. The + [`flock`](http://man7.org/linux/man-pages/man1/flock.1.html) tool of the + `util-linux` packages makes this concept available to shell scripts. Note + that `systemd-tmpfiles` only checks for BSD file locks on directories, locks + on other types of file nodes (including regular files) are not considered. + +4. Keep the access time of all temporary files created current. In regular + intervals, use `utimensat()` or a related call to update the access time + ("atime") of all files that shall be kept around. Since the ageing algorithm + looks at the access time of files when deciding whether to delete them, it's + sufficient to update their access times in sufficiently frequent intervals to + ensure the files are not deleted. Since most applications (and tools such as + `ls`) primarily care for the modification time (rather than the access time) + using the access time for this purpose should be acceptable. + +5. Set the "sticky" bit on regular files. The ageing logic skips deletion of + all regular files that have the sticky bit (`chmod +t`) set. This is + honoured for regular files only however, and has no effect on directories as + the sticky bit has a different meaning for them. + +6. Don't use `/tmp/` or `/var/tmp/`, but use your own sub-directory under + `/run/` or `$XDG_RUNTIME_DIRECTORY` (the former if privileged, the latter if + unprivileged), or `/var/lib/` and `~/.config/` (similar, but with + persistency and suitable for larger data). The two temporary directories + `/tmp/` and `/var/tmp/` come with the implicit clean-up semantics described + above. When this is not desired, it's possible to create private per-package + runtime or state directories, and place all temporary files there. However, + do note that this means opting out of any kind of automatic clean-up, and it + is hence particularly essential that the program cleans up generated files + in these directories when they are no longer needed, in particular when the + program dies unexpectedly. Note: this strategy is only really suitable for + packages that operate in a "system wide singleton" fashion with "long" + persistence of its data or state, i.e. as opposed to programs that run in + multiple parallel or short-living instances. This is because a private + directory under `/run` (and the other mentioned directories) is itself + system and package specific singleton with greater longevity. + +5. Exclude your temporary files from clean-ups via a `tmpfiles.d/` drop-in + (which includes drop-ins in the runtime-only directory + `/run/tmpfiles.d/`). The `x`/`X` line types may be used to exclude files + matching the specified globbing patterns from the ageing logic. If this is + used, automatic clean-up is not done for matching files and directory, and + much like with the previous option it's hence essential that the program + generating these temporary files carefully removes the temporary files it + creates again, and in particular so if it dies unexpectedly. + +🥇 The semantics of options 2 (in case you only deal with temporary files, not +directories) and 3 (in case you deal with both) in the list above are in most +cases the most preferable. It is thus recommended to stick to these two +options. + +While the ageing logic is very useful as a safety concept to ensure unused +files and directories are eventually removed a well written program avoids even +creating files that need such a clean-up. In particular: + +1. Use `memfd_create()` or `O_TMPFILE` when creating temporary files. + +2. `unlink()` temporary files right after creating them. This is very similar + to `O_TMPFILE` behaviour: consider deleting temporary files right after + creating them, while keeping open a file descriptor to them. Unlike + `O_TMPFILE` this method also works on older Linux systems and other OSes + that do not implement `O_TMPFILE`. + +## Disk Quota + +Generally, files allocated from `/tmp/` and `/var/tmp/` are allocated from a +pool shared by all local users. Moreover the space available in `/tmp/` is +generally more restricted than `/var/tmp/`. This means, that in particular in +`/tmp/` space should be considered scarce, and programs need to be prepared +that no space is available. Essential programs might require a fallback logic +using a different location for storing temporary files hence. Non-essential +programs at least need to be prepared for `ENOSPC` errors and generate useful, +actionable error messages. + +Some setups employ per-user quota on `/var/tmp/` and possibly `/tmp/`, to make +`ENOSPC` situations less likely, and harder to trigger from unprivileged +users. However, in the general case no such per-user quota is implemented +though, in particular not when `tmpfs` is used as backing file system, because +— even today — `tmpfs` still provides no native quota support in the kernel. + +## Early Boot Considerations + +Both `/tmp/` and `/var/tmp/` are not necessarily available during early boot, +or — if they are available early — are not writable. This means software that +is intended to run during early boot (i.e. before `basic.target` — or more +specifically `local-fs.target` — is up) should not attempt to make use of +either. Interfaces such as `memfd_create()` or files below a package-specific +directory in `/run/` are much better options in this case. (Note that some +packages instead use `/dev/shm/` for temporary files during early boot; this is +not advisable however, as it offers no benefits over a private directory in +`/run/` as both are backed by the same concept: `tmpfs`. The directory +`/dev/shm/` exists to back POSIX shared memory (see +[`shm_open()`](http://man7.org/linux/man-pages/man3/shm_open.3.html) and +related calls), and not as a place for temporary files. `/dev/shm` is +problematic as it is world-writable and there's no automatic clean-up logic in +place.) diff --git a/docs/TESTING_WITH_SANITIZERS.md b/docs/TESTING_WITH_SANITIZERS.md new file mode 100644 index 0000000..2622682 --- /dev/null +++ b/docs/TESTING_WITH_SANITIZERS.md @@ -0,0 +1,90 @@ +--- +title: Testing systemd Using Sanitizers +category: Contributing +layout: default +--- + +# Testing systemd Using Sanitizers + +To catch the *nastier* kind of bugs, you can run your code with [Address Sanitizer](https://clang.llvm.org/docs/AddressSanitizer.html) +and [Undefined Behavior Sanitizer](https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html). +This is mostly done automagically by various CI systems for each PR, but you may +want to do it locally as well. The process slightly varies depending on the +compiler you want to use and which part of the test suite you want to run. + +## gcc +gcc compiles in sanitizer libraries dynamically by default, so you need to get +the shared libraries first - on Fedora these are shipped as a separate packages +(`libasan` for Address Sanitizer and `libubsan` for Undefined Behavior Sanitizer). + +The compilation itself is then a matter of simply adding `-Db_sanitize=address,undefined` +to `meson`. That's it - following executions of `meson test` and integrations tests +under `test/` subdirectory will run with sanitizers enabled. However, to get +truly useful results, you should tweak the runtime configuration of respective +sanitizers; e.g. in systemd we set the following environment variables: + +```bash +ASAN_OPTIONS=strict_string_checks=1:detect_stack_use_after_return=1:check_initialization_order=1:strict_init_order=1 +UBSAN_OPTIONS=print_stacktrace=1:print_summary=1:halt_on_error=1 +``` +## clang +In case of clang things are somewhat different - the sanitizer libraries are +compiled in statically by default. This is not an issue if you plan to run +only the unit tests, but for integration tests you'll need to convince clang +to use the dynamic versions of sanitizer libraries. + +First of all, pass `-shared-libsan` to both `clang` and `clang++`: + +```bash +CFLAGS=-shared-libasan +CXXFLAGS=-shared-libasan +``` + +The `CXXFLAGS` are necessary for `src/libsystemd/sd-bus/test-bus-vtable-cc.c`. Compilation +is then the same as in case of gcc, simply add `-Db_sanitize=address,undefined` +to the `meson` call and use the same environment variables for runtime configuration. + +```bash +ASAN_OPTIONS=strict_string_checks=1:detect_stack_use_after_return=1:check_initialization_order=1:strict_init_order=1 +UBSAN_OPTIONS=print_stacktrace=1:print_summary=1:halt_on_error=1 +``` + +After this, you'll probably notice that all compiled binaries complain about +missing `libclang_rt.asan*` library. To fix this, you have to install clang's +runtime libraries, usually shipped in the `compiler-rt` package. As these libraries +are installed in a non-standard location (non-standard for `ldconfig`), you'll +need to manually direct binaries to the respective runtime libraries. + +``` +# Optionally locate the respective runtime DSO +$ ldd build/systemd | grep libclang_rt.asan + libclang_rt.asan-x86_64.so => not found + libclang_rt.asan-x86_64.so => not found +$ find /usr/lib* /usr/local/lib* -type f -name libclang_rt.asan-x86_64.so 2>/dev/null +/usr/lib64/clang/7.0.1/lib/libclang_rt.asan-x86_64.so + +# Set the LD_LIBRARY_PATH accordingly +export LD_LIBRARY_PATH=/usr/lib64/clang/7.0.1/lib/ + +# If the path is correct, the "not found" message should change to an actual path +$ ldd build/systemd | grep libclang_rt.asan + libclang_rt.asan-x86_64.so => /usr/lib64/clang/7.0.1/lib/libclang_rt.asan-x86_64.so (0x00007fa9752fc000) +``` + +This should help binaries to correctly find necessary sanitizer DSOs. + +Also, to make the reports useful, `llvm-symbolizer` tool is required (usually +part of the `llvm` package). + +## Background notes +The reason why you need to force dynamic linking in case of `clang` is that some +applications make use of `libsystemd`, which is compiled with sanitizers as well. +However, if a *standard* (uninstrumented) application loads an instrumented library, +it will immediately fail due to unresolved symbols. To fix/workaround this, you +need to pre-load the ASan DSO using `LD_PRELOAD=/path/to/asan/dso`, which will +make things work as expected in most cases. This will, obviously, not work with +statically linked sanitizer libraries. + +These shenanigans are performed automatically when running the integration test +suite (i.e. `test/TEST-??-*`) and are located in `test/test-functions` (mainly, +but not only, in the `create_asan_wrapper` function). diff --git a/docs/TRANSIENT-SETTINGS.md b/docs/TRANSIENT-SETTINGS.md new file mode 100644 index 0000000..50b9a42 --- /dev/null +++ b/docs/TRANSIENT-SETTINGS.md @@ -0,0 +1,498 @@ +--- +title: What Settings Are Currently Available For Transient Units? +category: Interfaces +layout: default +--- + +# What Settings Are Currently Available For Transient Units? + +Our intention is to make all settings that are available as unit file settings +also available for transient units, through the D-Bus API. At the moment, +device, swap, and target units are not supported at all as transient units, but +others are pretty well supported. + +The lists below contain all settings currently available in unit files. The +ones currently available in transient units are prefixed with `✓`. + +## Generic Unit Settings + +Most generic unit settings are available for transient units. + +``` +✓ Description= +✓ Documentation= +✓ SourcePath= +✓ Requires= +✓ Requisite= +✓ Wants= +✓ BindsTo= +✓ Conflicts= +✓ Before= +✓ After= +✓ OnFailure= +✓ PropagatesReloadTo= +✓ ReloadPropagatedFrom= +✓ PartOf= +✓ JoinsNamespaceOf= +✓ RequiresMountsFor= +✓ StopWhenUnneeded= +✓ RefuseManualStart= +✓ RefuseManualStop= +✓ AllowIsolate= +✓ DefaultDependencies= +✓ OnFailureJobMode= +✓ IgnoreOnIsolate= +✓ JobTimeoutSec= +✓ JobRunningTimeoutSec= +✓ JobTimeoutAction= +✓ JobTimeoutRebootArgument= +✓ StartLimitIntervalSec= +✓ StartLimitBurst= +✓ StartLimitAction= +✓ FailureAction= +✓ SuccessAction= +✓ FailureActionExitStatus= +✓ SuccessActionExitStatus= +✓ RebootArgument= +✓ ConditionPathExists= +✓ ConditionPathExistsGlob= +✓ ConditionPathIsDirectory= +✓ ConditionPathIsSymbolicLink= +✓ ConditionPathIsMountPoint= +✓ ConditionPathIsReadWrite= +✓ ConditionDirectoryNotEmpty= +✓ ConditionFileNotEmpty= +✓ ConditionFileIsExecutable= +✓ ConditionNeedsUpdate= +✓ ConditionFirstBoot= +✓ ConditionKernelCommandLine= +✓ ConditionKernelVersion= +✓ ConditionArchitecture= +✓ ConditionVirtualization= +✓ ConditionSecurity= +✓ ConditionCapability= +✓ ConditionHost= +✓ ConditionACPower= +✓ ConditionUser= +✓ ConditionGroup= +✓ ConditionControlGroupController= +✓ AssertPathExists= +✓ AssertPathExistsGlob= +✓ AssertPathIsDirectory= +✓ AssertPathIsSymbolicLink= +✓ AssertPathIsMountPoint= +✓ AssertPathIsReadWrite= +✓ AssertDirectoryNotEmpty= +✓ AssertFileNotEmpty= +✓ AssertFileIsExecutable= +✓ AssertNeedsUpdate= +✓ AssertFirstBoot= +✓ AssertKernelCommandLine= +✓ AssertKernelVersion= +✓ AssertArchitecture= +✓ AssertVirtualization= +✓ AssertSecurity= +✓ AssertCapability= +✓ AssertHost= +✓ AssertACPower= +✓ AssertUser= +✓ AssertGroup= +✓ AssertControlGroupController= +✓ CollectMode= +``` + +## Execution-Related Settings + +All execution-related settings are available for transient units. + +``` +✓ WorkingDirectory= +✓ RootDirectory= +✓ RootImage= +✓ User= +✓ Group= +✓ SupplementaryGroups= +✓ Nice= +✓ OOMScoreAdjust= +✓ CoredumpFilter= +✓ IOSchedulingClass= +✓ IOSchedulingPriority= +✓ CPUSchedulingPolicy= +✓ CPUSchedulingPriority= +✓ CPUSchedulingResetOnFork= +✓ CPUAffinity= +✓ UMask= +✓ Environment= +✓ EnvironmentFile= +✓ PassEnvironment= +✓ UnsetEnvironment= +✓ DynamicUser= +✓ RemoveIPC= +✓ StandardInput= +✓ StandardOutput= +✓ StandardError= +✓ StandardInputText= +✓ StandardInputData= +✓ TTYPath= +✓ TTYReset= +✓ TTYVHangup= +✓ TTYVTDisallocate= +✓ SyslogIdentifier= +✓ SyslogFacility= +✓ SyslogLevel= +✓ SyslogLevelPrefix= +✓ LogLevelMax= +✓ LogExtraFields= +✓ LogRateLimitIntervalSec= +✓ LogRateLimitBurst= +✓ SecureBits= +✓ CapabilityBoundingSet= +✓ AmbientCapabilities= +✓ TimerSlackNSec= +✓ NoNewPrivileges= +✓ KeyringMode= +✓ ProtectProc= +✓ ProcSubset= +✓ SystemCallFilter= +✓ SystemCallArchitectures= +✓ SystemCallErrorNumber= +✓ SystemCallLog= +✓ MemoryDenyWriteExecute= +✓ RestrictNamespaces= +✓ RestrictRealtime= +✓ RestrictSUIDSGID= +✓ RestrictAddressFamilies= +✓ RootHash= +✓ RootHashSignature= +✓ RootVerity= +✓ LockPersonality= +✓ LimitCPU= +✓ LimitFSIZE= +✓ LimitDATA= +✓ LimitSTACK= +✓ LimitCORE= +✓ LimitRSS= +✓ LimitNOFILE= +✓ LimitAS= +✓ LimitNPROC= +✓ LimitMEMLOCK= +✓ LimitLOCKS= +✓ LimitSIGPENDING= +✓ LimitMSGQUEUE= +✓ LimitNICE= +✓ LimitRTPRIO= +✓ LimitRTTIME= +✓ ReadWritePaths= +✓ ReadOnlyPaths= +✓ InaccessiblePaths= +✓ BindPaths= +✓ BindReadOnlyPaths= +✓ TemporaryFileSystem= +✓ PrivateTmp= +✓ PrivateDevices= +✓ PrivateMounts= +✓ ProtectKernelTunables= +✓ ProtectKernelModules= +✓ ProtectKernelLogs= +✓ ProtectControlGroups= +✓ PrivateNetwork= +✓ PrivateUsers= +✓ ProtectSystem= +✓ ProtectHome= +✓ ProtectClock= +✓ MountFlags= +✓ MountAPIVFS= +✓ Personality= +✓ RuntimeDirectoryPreserve= +✓ RuntimeDirectoryMode= +✓ RuntimeDirectory= +✓ StateDirectoryMode= +✓ StateDirectory= +✓ CacheDirectoryMode= +✓ CacheDirectory= +✓ LogsDirectoryMode= +✓ LogsDirectory= +✓ ConfigurationDirectoryMode= +✓ ConfigurationDirectory= +✓ PAMName= +✓ IgnoreSIGPIPE= +✓ UtmpIdentifier= +✓ UtmpMode= +✓ SELinuxContext= +✓ SmackProcessLabel= +✓ AppArmorProfile= +✓ Slice= +``` + +## Resource Control Settings + +All cgroup/resource control settings are available for transient units + +``` +✓ CPUAccounting= +✓ CPUWeight= +✓ StartupCPUWeight= +✓ CPUShares= +✓ StartupCPUShares= +✓ CPUQuota= +✓ CPUQuotaPeriodSec= +✓ AllowedCPUs= +✓ AllowedMemoryNodes= +✓ MemoryAccounting= +✓ DefaultMemoryMin= +✓ MemoryMin= +✓ DefaultMemoryLow= +✓ MemoryLow= +✓ MemoryHigh= +✓ MemoryMax= +✓ MemorySwapMax= +✓ MemoryLimit= +✓ DeviceAllow= +✓ DevicePolicy= +✓ IOAccounting= +✓ IOWeight= +✓ StartupIOWeight= +✓ IODeviceWeight= +✓ IOReadBandwidthMax= +✓ IOWriteBandwidthMax= +✓ IOReadIOPSMax= +✓ IOWriteIOPSMax= +✓ BlockIOAccounting= +✓ BlockIOWeight= +✓ StartupBlockIOWeight= +✓ BlockIODeviceWeight= +✓ BlockIOReadBandwidth= +✓ BlockIOWriteBandwidth= +✓ TasksAccounting= +✓ TasksMax= +✓ Delegate= +✓ DisableControllers= +✓ IPAccounting= +✓ IPAddressAllow= +✓ IPAddressDeny= +✓ ManagedOOMSwap= +✓ ManagedOOMMemoryPressure= +✓ ManagedOOMMemoryPressureLimitPercent= +``` + +## Process Killing Settings + +All process killing settings are available for transient units: + +``` +✓ SendSIGKILL= +✓ SendSIGHUP= +✓ KillMode= +✓ KillSignal= +✓ RestartKillSignal= +✓ FinalKillSignal= +✓ WatchdogSignal= +``` + +## Service Unit Settings + +Most service unit settings are available for transient units. + +``` +✓ BusName= +✓ ExecCondition= +✓ ExecReload= +✓ ExecStart= +✓ ExecStartPost= +✓ ExecStartPre= +✓ ExecStop= +✓ ExecStopPost= +✓ FileDescriptorStoreMax= +✓ GuessMainPID= +✓ NonBlocking= +✓ NotifyAccess= +✓ OOMPolicy= +✓ PIDFile= +✓ RemainAfterExit= +✓ Restart= +✓ RestartForceExitStatus= +✓ RestartPreventExitStatus= +✓ RestartSec= +✓ RootDirectoryStartOnly= +✓ RuntimeMaxSec= + Sockets= +✓ SuccessExitStatus= +✓ TimeoutAbortSec= +✓ TimeoutSec= +✓ TimeoutStartFailureMode= +✓ TimeoutStartSec= +✓ TimeoutStopFailureMode= +✓ TimeoutStopSec= +✓ Type= +✓ USBFunctionDescriptors= +✓ USBFunctionStrings= +✓ WatchdogSec= +``` + +## Mount Unit Settings + +All mount unit settings are available to transient units: + +``` +✓ What= +✓ Where= +✓ Options= +✓ Type= +✓ TimeoutSec= +✓ DirectoryMode= +✓ SloppyOptions= +✓ LazyUnmount= +✓ ForceUnmount= +✓ ReadWriteOnly= +``` + +## Automount Unit Settings + +All automount unit setting is available to transient units: + +``` +✓ Where= +✓ DirectoryMode= +✓ TimeoutIdleSec= +``` + +## Timer Unit Settings + +Most timer unit settings are available to transient units. + +``` +✓ OnActiveSec= +✓ OnBootSec= +✓ OnCalendar= +✓ OnClockChange= +✓ OnStartupSec= +✓ OnTimezoneChange= +✓ OnUnitActiveSec= +✓ OnUnitInactiveSec= +✓ Persistent= +✓ WakeSystem= +✓ RemainAfterElapse= +✓ AccuracySec= +✓ RandomizedDelaySec= +✓ FixedRandomDelay= + Unit= +``` + +## Slice Unit Settings + +Slice units are fully supported as transient units, but they have no settings +of their own beyond the generic unit and resource control settings. + +## Scope Unit Settings + +Scope units are fully supported as transient units (in fact they only exist as +such). + +``` +✓ RuntimeMaxSec= +✓ TimeoutStopSec= +``` + +## Socket Unit Settings + +Most socket unit settings are available to transient units. + +``` +✓ ListenStream= +✓ ListenDatagram= +✓ ListenSequentialPacket= +✓ ListenFIFO= +✓ ListenNetlink= +✓ ListenSpecial= +✓ ListenMessageQueue= +✓ ListenUSBFunction= +✓ SocketProtocol= +✓ BindIPv6Only= +✓ Backlog= +✓ BindToDevice= +✓ ExecStartPre= +✓ ExecStartPost= +✓ ExecStopPre= +✓ ExecStopPost= +✓ TimeoutSec= +✓ SocketUser= +✓ SocketGroup= +✓ SocketMode= +✓ DirectoryMode= +✓ Accept= +✓ FlushPending= +✓ Writable= +✓ MaxConnections= +✓ MaxConnectionsPerSource= +✓ KeepAlive= +✓ KeepAliveTimeSec= +✓ KeepAliveIntervalSec= +✓ KeepAliveProbes= +✓ DeferAcceptSec= +✓ NoDelay= +✓ Priority= +✓ ReceiveBuffer= +✓ SendBuffer= +✓ IPTOS= +✓ IPTTL= +✓ Mark= +✓ PipeSize= +✓ FreeBind= +✓ Transparent= +✓ Broadcast= +✓ PassCredentials= +✓ PassSecurity= +✓ PassPacketInfo= +✓ TCPCongestion= +✓ ReusePort= +✓ MessageQueueMaxMessages= +✓ MessageQueueMessageSize= +✓ RemoveOnStop= +✓ Symlinks= +✓ FileDescriptorName= + Service= +✓ TriggerLimitIntervalSec= +✓ TriggerLimitBurst= +✓ SmackLabel= +✓ SmackLabelIPIn= +✓ SmackLabelIPOut= +✓ SELinuxContextFromNet= +``` + +## Swap Unit Settings + +Swap units are currently not available at all as transient units: + +``` + What= + Priority= + Options= + TimeoutSec= +``` + +## Path Unit Settings + +Most path unit settings are available to transient units. + +``` +✓ PathExists= +✓ PathExistsGlob= +✓ PathChanged= +✓ PathModified= +✓ DirectoryNotEmpty= + Unit= +✓ MakeDirectory= +✓ DirectoryMode= +``` + +## Install Section + +The `[Install]` section is currently not available at all for transient units, and it probably doesn't even make sense. + +``` + Alias= + WantedBy= + RequiredBy= + Also= + DefaultInstance= +``` diff --git a/docs/TRANSLATORS.md b/docs/TRANSLATORS.md new file mode 100644 index 0000000..a120bca --- /dev/null +++ b/docs/TRANSLATORS.md @@ -0,0 +1,80 @@ +--- +title: Notes for Translators +category: Contributing +layout: default +--- + +# Notes for Translators + +systemd depends on the `gettext` package for multilingual support. + +You'll find the i18n files in the `po/` directory. + +The build system (meson/ninja) can be used to generate a template (`*.pot`), +which can be used to create new translations. + +It can also merge the template into the existing translations (`*.po`), to pick +up new strings in need of translation. + +Finally, it is able to compile the translations (to `*.gmo` files), so that +they can be used by systemd software. (This step is also useful to confirm the +syntax of the `*.po` files is correct.) + +## Creating a New Translation + +To create a translation to a language not yet available, start by creating the +initial template: + +``` +$ ninja -C build/ systemd-pot +``` + +This will generate file `po/systemd.pot` in the source tree. + +Then simply copy it to a new <code><i>${lang_code}</i>.po</code> file, where +<code><i>${lang_code}</i></code> is the two-letter code for a language +(possibly followed by a two-letter uppercase country code), according to the +ISO 639 standard. + +In short: + +<pre> +$ cp po/systemd.pot po/<i>${lang_code}</i>.po +</pre> + +Then edit the new <code>po/<i>${lang_code}</i>.po</code> file (for example, +using the `poedit` GUI editor.) + +## Updating an Existing Translation + +Start by updating the `*.po` files from the latest template: + +``` +$ ninja -C build/ systemd-update-po +``` + +This will touch all the `*.po` files, so you'll want to pay attention when +creating a git commit from this change, to only include the one translation +you're actually updating. + +Edit the `*.po` file, looking for empty translations and translations marked as +"fuzzy" (which means the merger found a similar message that needs to be +reviewed as it's expected not to match exactly.) + +You can use any text editor to update the `*.po` files, but a good choice is +the `poedit` editor, a graphical application specifically designed for this +purpose. + +Once you're done, create a git commit for the update of the `po/*.po` file you +touched. Remember to undo the changes to the other `*.po` files (for instance, +using `git checkout -- po/` after you commit the changes you do want to keep.) + +## Recompiling Translations + +You can recompile the `*.po` files using the following command: + +``` +$ ninja -C build/ systemd-gmo +``` + +The resulting files will be saved in the `build/po/` directory. diff --git a/docs/UIDS-GIDS.md b/docs/UIDS-GIDS.md new file mode 100644 index 0000000..e289a9b --- /dev/null +++ b/docs/UIDS-GIDS.md @@ -0,0 +1,302 @@ +--- +title: Users, Groups, UIDs and GIDs on systemd Systems +category: Users, Groups and Home Directories +layout: default +--- + +# Users, Groups, UIDs and GIDs on systemd Systems + +Here's a summary of the requirements `systemd` (and Linux) make on UID/GID +assignments and their ranges. + +Note that while in theory UIDs and GIDs are orthogonal concepts they really +aren't IRL. With that in mind, when we discuss UIDs below it should be assumed +that whatever we say about UIDs applies to GIDs in mostly the same way, and all +the special assignments and ranges for UIDs always have mostly the same +validity for GIDs too. + +## Special Linux UIDs + +In theory, the range of the C type `uid_t` is 32bit wide on Linux, +i.e. 0…4294967295. However, four UIDs are special on Linux: + +1. 0 → The `root` super-user + +2. 65534 → The `nobody` UID, also called the "overflow" UID or similar. It's + where various subsystems map unmappable users to, for example file systems + only supporting 16bit UIDs, NFS or user namespacing. (The latter can be + changed with a sysctl during runtime, but that's not supported on + `systemd`. If you do change it you void your warranty.) Because Fedora is a + bit confused the `nobody` user is called `nfsnobody` there (and they have a + different `nobody` user at UID 99). I hope this will be corrected eventually + though. (Also, some distributions call the `nobody` group `nogroup`. I wish + they didn't.) + +3. 4294967295, aka "32bit `(uid_t) -1`" → This UID is not a valid user ID, as + `setresuid()`, `chown()` and friends treat -1 as a special request to not + change the UID of the process/file. This UID is hence not available for + assignment to users in the user database. + +4. 65535, aka "16bit `(uid_t) -1`" → Before Linux kernel 2.4 `uid_t` used to be + 16bit, and programs compiled for that would hence assume that `(uid_t) -1` + is 65535. This UID is hence not usable either. + +The `nss-systemd` glibc NSS module will synthesize user database records for +the UIDs 0 and 65534 if the system user database doesn't list them. This means +that any system where this module is enabled works to some minimal level +without `/etc/passwd`. + +## Special Distribution UID ranges + +Distributions generally split the available UID range in two: + +1. 1…999 → System users. These are users that do not map to actual "human" + users, but are used as security identities for system daemons, to implement + privilege separation and run system daemons with minimal privileges. + +2. 1000…65533 and 65536…4294967294 → Everything else, i.e. regular (human) users. + +Note that most distributions allow changing the boundary between system and +regular users, even during runtime as user configuration. Moreover, some older +systems placed the boundary at 499/500, or even 99/100. In `systemd`, the +boundary is configurable only during compilation time, as this should be a +decision for distribution builders, not for users. Moreover, we strongly +discourage downstreams to change the boundary from the upstream default of +999/1000. + +Also note that programs such as `adduser` tend to allocate from a subset of the +available regular user range only, usually 1000..60000. And it's also usually +user-configurable, too. + +Note that systemd requires that system users and groups are resolvable without +networking available — a requirement that is not made for regular users. This +means regular users may be stored in remote LDAP or NIS databases, but system +users may not (except when there's a consistent local cache kept, that is +available during earliest boot, including in the initial RAM disk). + +## Special `systemd` GIDs + +`systemd` defines no special UIDs beyond what Linux already defines (see +above). However, it does define some special group/GID assignments, which are +primarily used for `systemd-udevd`'s device management. The precise list of the +currently defined groups is found in this `sysusers.d` snippet: +[basic.conf](https://raw.githubusercontent.com/systemd/systemd/master/sysusers.d/basic.conf.in) + +It's strongly recommended that downstream distributions include these groups in +their default group databases. + +Note that the actual GID numbers assigned to these groups do not have to be +constant beyond a specific system. There's one exception however: the `tty` +group must have the GID 5. That's because it must be encoded in the `devpts` +mount parameters during earliest boot, at a time where NSS lookups are not +possible. (Note that the actual GID can be changed during `systemd` build time, +but downstreams are strongly advised against doing that.) + +## Special `systemd` UID ranges + +`systemd` defines a number of special UID ranges: + +1. 60001…60513 → UIDs for home directories managed by + [`systemd-homed.service(8)`](https://www.freedesktop.org/software/systemd/man/systemd-homed.service.html). UIDs + from this range are automatically assigned to any home directory discovered, + and persisted locally on first login. On different systems the same user + might get different UIDs assigned in case of conflict, though it is + attempted to make UID assignments stable, by deriving them from a hash of + the user name. + +2. 61184…65519 → UIDs for dynamic users are allocated from this range (see the + `DynamicUser=` documentation in + [`systemd.exec(5)`](https://www.freedesktop.org/software/systemd/man/systemd.exec.html)). This + range has been chosen so that it is below the 16bit boundary (i.e. below + 65535), in order to provide compatibility with container environments that + assign a 64K range of UIDs to containers using user namespacing. This range + is above the 60000 boundary, so that its allocations are unlikely to be + affected by `adduser` allocations (see above). And we leave some room + upwards for other purposes. (And if you wonder why precisely these numbers: + if you write them in hexadecimal, they might make more sense: 0xEF00 and + 0xFFEF). The `nss-systemd` module will synthesize user records implicitly + for all currently allocated dynamic users from this range. Thus, NSS-based + user record resolving works correctly without those users being in + `/etc/passwd`. + +3. 524288…1879048191 → UID range for `systemd-nspawn`'s automatic allocation of + per-container UID ranges. When the `--private-users=pick` switch is used (or + `-U`) then it will automatically find a so far unused 16bit subrange of this + range and assign it to the container. The range is picked so that the upper + 16bit of the 32bit UIDs are constant for all users of the container, while + the lower 16bit directly encode the 65536 UIDs assigned to the + container. This mode of allocation means that the upper 16bit of any UID + assigned to a container are kind of a "container ID", while the lower 16bit + directly expose the container's own UID numbers. If you wonder why precisely + these numbers, consider them in hexadecimal: 0x00080000…0x6FFFFFFF. This + range is above the 16bit boundary. Moreover it's below the 31bit boundary, + as some broken code (specifically: the kernel's `devpts` file system) + erroneously considers UIDs signed integers, and hence can't deal with values + above 2^31. The `systemd-machined.service` service will synthesize user + database records for all UIDs assigned to a running container from this + range. + +Note for both allocation ranges: when an UID allocation takes place NSS is +checked for collisions first, and a different UID is picked if an entry is +found. Thus, the user database is used as synchronization mechanism to ensure +exclusive ownership of UIDs and UID ranges. To ensure compatibility with other +subsystems allocating from the same ranges it is hence essential that they +ensure that whatever they pick shows up in the user/group databases, either by +providing an NSS module, or by adding entries directly to `/etc/passwd` and +`/etc/group`. For performance reasons, do note that `systemd-nspawn` will only +do an NSS check for the first UID of the range it allocates, not all 65536 of +them. Also note that while the allocation logic is operating, the glibc +`lckpwdf()` user database lock is taken, in order to make this logic race-free. + +## Figuring out the system's UID boundaries + +The most important boundaries of the local system may be queried with +`pkg-config`: + +``` +$ pkg-config --variable=systemuidmax systemd +999 +$ pkg-config --variable=dynamicuidmin systemd +61184 +$ pkg-config --variable=dynamicuidmax systemd +65519 +$ pkg-config --variable=containeruidbasemin systemd +524288 +$ pkg-config --variable=containeruidbasemax systemd +1878982656 +``` + +(Note that the latter encodes the maximum UID *base* `systemd-nspawn` might +pick — given that 64K UIDs are assigned to each container according to this +allocation logic, the maximum UID used for this range is hence +1878982656+65535=1879048191.) + +Systemd has compile-time default for these boundaries. Using those defaults is +recommended. It will nevertheless query `/etc/login.defs` at runtime, when +compiled with `-Dcompat-mutable-uid-boundaries=true` and that file is present. +Support for this is considered only a compatibility feature and should not be +used except when upgrading systems which were creating with different defaults. + +## Considerations for container managers + +If you hack on a container manager, and wonder how and how many UIDs best to +assign to your containers, here are a few recommendations: + +1. Definitely, don't assign less than 65536 UIDs/GIDs. After all the `nobody` +user has magic properties, and hence should be available in your container, and +given that it's assigned the UID 65534, you should really cover the full 16bit +range in your container. Note that systemd will — as mentioned — synthesize +user records for the `nobody` user, and assumes its availability in various +other parts of its codebase, too, hence assigning fewer users means you lose +compatibility with running systemd code inside your container. And most likely +other packages make similar restrictions. + +2. While it's fine to assign more than 65536 UIDs/GIDs to a container, there's +most likely not much value in doing so, as Linux distributions won't use the +higher ranges by default (as mentioned neither `adduser` nor `systemd`'s +dynamic user concept allocate from above the 16bit range). Unless you actively +care for nested containers, it's hence probably a good idea to allocate exactly +65536 UIDs per container, and neither less nor more. A pretty side-effect is +that by doing so, you expose the same number of UIDs per container as Linux 2.2 +supported for the whole system, back in the days. + +3. Consider allocating UID ranges for containers so that the first UID you +assign has the lower 16bits all set to zero. That way, the upper 16bits become +a container ID of some kind, while the lower 16bits directly encode the +internal container UID. This is the way `systemd-nspawn` allocates UID ranges +(see above). Following this allocation logic ensures best compatibility with +`systemd-nspawn` and all other container managers following the scheme, as it +is sufficient then to check NSS for the first UID you pick regarding conflicts, +as that's what they do, too. Moreover, it makes `chown()`ing container file +system trees nicely robust to interruptions: as the external UID encodes the +internal UID in a fixed way, it's very easy to adjust the container's base UID +without the need to know the original base UID: to change the container base, +just mask away the upper 16bit, and insert the upper 16bit of the new container +base instead. Here are the easy conversions to derive the internal UID, the +external UID, and the container base UID from each other: + + ``` + INTERNAL_UID = EXTERNAL_UID & 0x0000FFFF + CONTAINER_BASE_UID = EXTERNAL_UID & 0xFFFF0000 + EXTERNAL_UID = INTERNAL_UID | CONTAINER_BASE_UID + ``` + +4. When picking a UID range for containers, make sure to check NSS first, with +a simple `getpwuid()` call: if there's already a user record for the first UID +you want to pick, then it's already in use: pick a different one. Wrap that +call in a `lckpwdf()` + `ulckpwdf()` pair, to make allocation +race-free. Provide an NSS module that makes all UIDs you end up taking show up +in the user database, and make sure that the NSS module returns up-to-date +information before you release the lock, so that other system components can +safely use the NSS user database as allocation check, too. Note that if you +follow this scheme no changes to `/etc/passwd` need to be made, thus minimizing +the artifacts the container manager persistently leaves in the system. + +## Summary + +| UID/GID | Purpose | Defined By | Listed in | +|-----------------------|-----------------------|---------------|-------------------------------| +| 0 | `root` user | Linux | `/etc/passwd` + `nss-systemd` | +| 1…4 | System users | Distributions | `/etc/passwd` | +| 5 | `tty` group | `systemd` | `/etc/passwd` | +| 6…999 | System users | Distributions | `/etc/passwd` | +| 1000…60000 | Regular users | Distributions | `/etc/passwd` + LDAP/NIS/… | +| 60001…60513 | Human Users (homed) | `systemd` | `nss-systemd` | +| 60514…61183 | Unused | | | +| 61184…65519 | Dynamic service users | `systemd` | `nss-systemd` | +| 65520…65533 | Unused | | | +| 65534 | `nobody` user | Linux | `/etc/passwd` + `nss-systemd` | +| 65535 | 16bit `(uid_t) -1` | Linux | | +| 65536…524287 | Unused | | | +| 524288…1879048191 | Container UID ranges | `systemd` | `nss-systemd` | +| 1879048192…2147483647 | Unused | | | +| 2147483648…4294967294 | HIC SVNT LEONES | | | +| 4294967295 | 32bit `(uid_t) -1` | Linux | | + +Note that "Unused" in the table above doesn't meant that these ranges are +really unused. It just means that these ranges have no well-established +pre-defined purposes between Linux, generic low-level distributions and +`systemd`. There might very well be other packages that allocate from these +ranges. + +Note that the range 2147483648…4294967294 (i.e. 2^31…2^32-2) should be handled +with care. Various programs (including kernel file systems, see `devpts`) have +trouble with UIDs outside of the signed 32bit range, i.e any UIDs equal to or +above 2147483648. It is thus strongly recommended to stay away from this range +in order to avoid complications. This range should be considered reserved for +future, special purposes. + +## Notes on resolvability of user and group names + +User names, UIDs, group names and GIDs don't have to be resolvable using NSS +(i.e. getpwuid() and getpwnam() and friends) all the time. However, systemd +makes the following requirements: + +System users generally have to be resolvable during early boot already. This +means they should not be provided by any networked service (as those usually +become available during late boot only), except if a local cache is kept that +makes them available during early boot too (i.e. before networking is +up). Specifically, system users need to be resolvable at least before +`systemd-udevd.service` and `systemd-tmpfiles.service` are started, as both +need to resolve system users — but note that there might be more services +requiring full resolvability of system users than just these two. + +Regular users do not need to be resolvable during early boot, it is sufficient +if they become resolvable during late boot. Specifically, regular users need to +be resolvable at the point in time the `nss-user-lookup.target` unit is +reached. This target unit is generally used as synchronization point between +providers of the user database and consumers of it. Services that require that +the user database is fully available (for example, the login service +`systemd-logind.service`) are ordered *after* it, while services that provide +parts of the user database (for example an LDAP user database client) are +ordered *before* it. Note that `nss-user-lookup.target` is a *passive* unit: in +order to minimize synchronization points on systems that don't need it the unit +is pulled into the initial transaction only if there's at least one service +that really needs it, and that means only if there's a service providing the +local user database somehow through IPC or suchlike. Or in other words: if you +hack on some networked user database project, then make sure you order your +service `Before=nss-user-lookup.target` and that you pull it in with +`Wants=nss-user-lookup.target`. However, if you hack on some project that needs +the user database to be up in full, then order your service +`After=nss-user-lookup.target`, but do *not* pull it in via a `Wants=` +dependency. diff --git a/docs/USERDB_AND_DESKTOPS.md b/docs/USERDB_AND_DESKTOPS.md new file mode 100644 index 0000000..a19f746 --- /dev/null +++ b/docs/USERDB_AND_DESKTOPS.md @@ -0,0 +1,169 @@ +--- +title: systemd-homed and JSON User/Group Record Support in Desktop Environments +category: Users, Groups and Home Directories +layout: default +--- + +# `systemd-homed` and JSON User/Group Record Support in Desktop Environments + +Starting with version 245, systemd supports a new subsystem +[`systemd-homed.service`](https://www.freedesktop.org/software/systemd/man/systemd-homed.service.html) +for managing regular ("human") users and their home directories. Along with it +a new concept `userdb` got merged that brings rich, extensible JSON user/group +records, extending the classic UNIX/glibc NSS `struct passwd`/`struct group` +structures. Both additions are added in a fully backwards compatible way, +accessible through `getpwnam()`/`getgrnam()`/… (i.e. libc NSS) and PAM as +usual, meaning that for basic support no changes in the upper layers of the +stack (in particular desktop environments, such as GNOME or KDE) have to be +made. However, for better support a number of changes to desktop environments +are recommended. A few areas where that applies are discussed below. + +Before reading on, please read up on the basic concepts, specifically: + +* [Home Directories](https://systemd.io/HOME_DIRECTORY) +* [JSON User Records](https://systemd.io/USER_RECORD) +* [JSON Group Records](https://systemd.io/GROUP_RECORD) +* [User/Group Record Lookup API via Varlink](https://systemd.io/USER_GROUP_API) + +## Support for Suspending Home Directory Access during System Suspend + +One key feature of `systemd-homed` managed encrypted home directories is the +ability that access to them can be suspended automatically during system sleep, +removing any cryptographic key material from memory while doing so. This is +important in a world where most laptop users seldom shut down their computers +but most of the time just suspend them instead. Previously, the encryption keys +for the home directories remained in memory during system suspend, so that +sufficiently equipped attackers could read them from there and gain full access +to the device. By removing the key material from memory before suspend, and +re-requesting it on resume this attack vector can be closed down effectively. + +Supporting this mechanism requires support in the desktop environment, since +the encryption keys (i.e. the user's login password) need to be reacquired on +system resume, from a lock screen or similar. This lock screen must run in +system context, and cannot run in the user's own context, since otherwise it +might end up accessing the home directory of the user even though access to it +is temporarily suspended and thus will hang if attempted. + +It is suggested that desktop environments that implement lock screens run them +from system context, for example by switching back to the display manager, and +only revert back to the session after re-authentication via this system lock +screen (re-authentication in this case refers to passing the user's login +credentials to the usual PAM authentication hooks). Or in other words, when +going into system suspend it is recommended that GNOME Shell switches back to +the GNOME Display Manager login screen which now should double as screen lock, +and only switches back to the shell's UI after the user re-authenticated there. + +Note that this change in behavior is a good idea in any case, and does not +create any dependencies on `systemd-homed` or systemd-specific APIs. It's +simply a change of behavior regarding use of existing APIs, not a suggested +hook-up to a any new API. + +A display manager which supports this kind of out-of-context screen lock +operation needs to inform systemd-homed about this so that systemd-homed knows +that it is safe to suspend the user's home directory on suspend. This is done +via the `suspend=` argument to the +[`pam_systemd_home`](https://www.freedesktop.org/software/systemd/man/pam_systemd_home.html) +PAM module. A display manager should hence change its PAM stack configuration +to set this parameter to on. `systemd-homed` will not suspend home directories +if there's at least one active session of the user that does not support +suspending, as communicated via this parameter. + +## User Management UIs + +The rich user/group records `userdb` and `systemd-homed` support carry various +fields of relevance to UIs that manage the local user database or parts +thereof. In particular, most of the metadata `accounts-daemon` (also see below) +supports is directly available in these JSON records. Hence it makes sense for +any user management UI to expose them directly. + +`systemd-homed` exposes APIs to add, remove and make changes to local users via +D-Bus, with full [polkit](https://www.freedesktop.org/software/polkit/docs/latest/) +hook-up. On the command line this is exposed via the +`homectl` command. A graphical UI that exposes similar functionality would be +very useful, exposing the various new account settings, and in particular +providing a stream-lined UI for enrolling new-style authentication tokens such +as PKCS#11/YubiKey-style devices. (Ideally, if the user plugs in an +uninitialized YubiKey during operation it might be nice if the Desktop would +automatically ask if a key pair shall be written to it and the local account be +bound to it, `systemd-homed` provides enough YubiKey/PKCS#11 support to make +this a reality today; except that it will not take care of token +initialization). + +A strong point of `systemd-homed` is per-user resource management. In +particular disk space assignments are something that most likely should be +exposed in a user management UI. Various metadata fields are supplied allowing +exposure of disk space assignment "slider" UI. Note however that the file system +back-ends of `systemd-homed.service` have different feature sets. Specifically, +only btrfs has online file system shrinking support, ext4 only offline file +system shrinking support, and xfs no shrinking support at all (all three file +systems support online file system growing however). This means if the LUKS +back-end is used, disk space assignment cannot be instant for logged in users, +unless btrfs is used. + +Note that only `systemd-homed` provides an API for modifying/creating/deleting +users. The generic `userdb` subsystem (which might have other back-ends, besides +`systemd-homed`, for example LDAP or Windows) exclusively provides a read-only +interface. (This is unlikely to change, as the other back-ends might have very +different concepts of adding or modifying users, i.e. might not even have any +local concept for that at all). This means any user management UI that intends +to change (and not just view) user accounts should talk directly to +`systemd-homed` to make use of its features; there's no abstraction available +to support other back-ends under the same API. + +Unfortunately there's currently no documentation for the `systemd-homed` D-Bus +API. Consider using the `homectl` sources as guidelines for implementing a user +management UI. The JSON user/records are well documented however, see above, +and the D-Bus API provides limited introspection. + +## Relationship to `accounts-daemon` + +For a long time `accounts-daemon` has been included in Linux distributions +providing richer user accounts. The functionality of this daemon overlaps in +many areas with the functionality of `systemd-homed` or `userdb`, but there are +systematic differences, which means that `systemd-homed` cannot replace +`accounts-daemon` fully. Most importantly: `accounts-daemon` provides +"side-car" metadata for *any* type of user account, while `systemd-homed` only +provides additional metadata for the users it defines itself. In other words: +`accounts-daemon` will augment foreign accounts; `systemd-homed` cannot be used +to augment users defined elsewhere, for example in LDAP or as classic +`/etc/passwd` records. + +This probably means that for the time being, a user management UI (or other UI) +that wants to support rich user records with compatibility with the status quo +ante should probably talk to both `systemd-homed` and `accounts-daemon` at the +same time, and ignore `accounts-daemon`'s records if `systemd-homed` defines +them. While I (Lennart) personally believe in the long run `systemd-homed` is +the way to go for rich user records, any UI that wants to manage and support +rich records for classic records has to support `accounts-daemon` in parallel +for the time being. + +In the short term, it might make sense to also expose the `userdb` provided +records via `accounts-daemon`, so that clients of the latter can consume them +without changes. However, I think in the long run `accounts-daemon` should +probably be removed from the general stack, hence this sounds like a temporary +solution only. + +In case you wonder, there's no automatic mechanism for converting existing +users registered in `/etc/passwd` or LDAP to users managed by +`systemd-homed`. There's documentation for doing this manually though, see +[Converting Existing Users to systemd-homed managed +Users](https://systemd.io/CONVERTING_TO_HOMED). + +## Future Additions + +JSON user/group records are extensible, hence we can easily add any additional +fields desktop environments require. For example pattern-based authentication +is likely very useful on touch-based devices, and the user records should hence +learn them natively. Fields for other authentication mechanisms, such as +fingerprint authentication should be provided as well, eventually. + +It is planned to extend the `userdb` Varlink API to support look-ups by partial +user name and real name (GECOS) data, so that log-in screens can optionally +implement simple complete-as-you-type login screens. + +It is planned to extend the `systemd-homed` D-Bus API to instantly inform clients +about hardware associated with a specific user being plugged in, to which login +screens can listen in order to initiate authentication. Specifically, any +YubiKey-like security token plugged in that is associated with a local user +record should initiate authentication for that user, making typing in of the +username unnecessary. diff --git a/docs/USER_GROUP_API.md b/docs/USER_GROUP_API.md new file mode 100644 index 0000000..ca88b3a --- /dev/null +++ b/docs/USER_GROUP_API.md @@ -0,0 +1,279 @@ +--- +title: User/Group Record Lookup API via Varlink +category: Users, Groups and Home Directories +layout: default +--- + +# User/Group Record Lookup API via Varlink + +JSON User/Group Records (as described in the [JSON User +Records](https://systemd.io/USER_RECORD) and [JSON Group +Records](https://systemd.io/GROUP_RECORD) documents) that are defined on the +local system may be queried with a [Varlink](https://varlink.org/) API. This +API takes both the role of what +[`getpwnam(3)`](http://man7.org/linux/man-pages/man3/getpwnam.3.html) and +related calls are for `struct passwd`, as well as the interfaces modules +implementing the [glibc Name Service Switch +(NSS)](https://www.gnu.org/software/libc/manual/html_node/Name-Service-Switch.html) +expose. Or in other words, it both allows applications to efficiently query +user/group records from local services, and allows local subsystems to provide +user/group records efficiently to local applications. + +This simple API only exposes only three method calls, and requires only a small +subset of the Varlink functionality. + +## Why Varlink? + +The API described in this document is based on a simple subset of the +mechanisms described by [Varlink](https://varlink.org/). The choice of +preferring Varlink over D-Bus and other IPCs in this context was made for three +reasons: + +1. User/Group record resolution should work during early boot and late shutdown + without special handling. This is very hard to do with D-Bus, as the broker + service for D-Bus generally runs as regular system daemon and is hence only + available at the latest boot stage. + +2. The JSON user/group records are native JSON data, hence picking an IPC + system that natively operates with JSON data is natural and clean. + +3. IPC systems such as D-Bus do not provide flow control and are thus unusable + for streaming data. They are useful to pass around short control messages, + but as soon as potentially many and large objects shall be transferred, + D-Bus is not suitable, as any such streaming of messages would be considered + flooding in D-Bus' logic, and thus possibly result in termination of + communication. Since the APIs defined in this document need to support + enumerating potentially large numbers of users and groups, D-Bus is simply + not an appropriate option. + +## Concepts + +Each subsystem that needs to define users and groups on the local system is +supposed to implement this API, and offer its interfaces on a Varlink +`AF_UNIX`/`SOCK_STREAM` file system socket bound into the +`/run/systemd/userdb/` directory. When a client wants to look up a user or +group record, it contacts all sockets bound in this directory in parallel, and +enqueues the same query to each. The first positive reply is then returned to +the application, or if all fail the last seen error is returned +instead. (Alternatively a special Varlink service is available, +`io.systemd.Multiplexer` which acts as frontend and will do the parallel +queries on behalf of the client, drastically simplifying client +development. This service is not available during earliest boot and final +shutdown phases.) + +Unlike with glibc NSS there's no order or programmatic expression language +defined in which queries are issued to the various services. Instead, all +queries are always enqueued in parallel to all defined services, in order to +make look-ups efficient, and the simple rule of "first successful lookup wins" +is unconditionally followed for user and group look-ups (though not for +membership lookups, see below). + +This simple scheme only works safely as long as every service providing +user/group records carefully makes sure not to answer with conflicting +records. This API does not define any mechanisms for dealing with user/group +name/ID collisions during look-up nor during record registration. It assumes +the various subsystems that want to offer user and group records to the rest of +the system have made sufficiently sure in advance that their definitions do not +collide with those of other services. Clients are not expected to merge +multiple definitions for the same user or group, and will also not be able to +detect conflicts and suppress such conflicting records. + +It is recommended to name the sockets in the directory in reverse domain name +notation, but this is neither required nor enforced. + +## Well-Known Services + +Any subsystem that wants to provide user/group records can do so, simply by +binding a socket in the aforementioned directory. By default two +services are listening there, that have special relevance: + +1. `io.systemd.NameServiceSwitch` → This service makes the classic UNIX/glibc + NSS user/group records available as JSON User/Group records. Any such + records are automatically converted as needed, and possibly augmented with + information from the shadow databases. + +2. `io.systemd.Multiplexer` → This service multiplexes client queries to all + other running services. It's supposed to simplify client development: in + order to look up or enumerate user/group records it's sufficient to talk to + one service instead of all of them in parallel. Note that it is not available + during earliest boot and final shutdown phases, hence for programs running + in that context it is preferable to implement the parallel lookup + themselves. + +Both these services are implemented by the same daemon +`systemd-userdbd.service`. + +Note that these services currently implement a subset of Varlink only. For +example, introspection is not available, and the resolver logic is not used. + +## Other Services + +The `systemd` project provides three other services implementing this +interface. Specifically: + +1. `io.systemd.DynamicUser` → This service is implemented by the service + manager itself, and provides records for the users and groups synthesized + via `DynamicUser=` in unit files. + +2. `io.systemd.Home` → This service is implemented by `systemd-homed.service` + and provides records for the users and groups defined by the home + directories it manages. + +3. `io.systemd.Machine` → This service is implemented by + `systemd-machined.service` and provides records for the users and groups used + by local containers that use user namespacing. + +Other projects are invited to implement these services too. For example it +would make sense for LDAP/ActiveDirectory projects to implement these +interfaces, which would provide them a way to do per-user resource management +enforced by systemd and defined directly in LDAP directories. + +## Compatibility with NSS + +Two-way compatibility with classic UNIX/glibc NSS user/group records is +provided. When using the Varlink API, lookups into databases provided only via +NSS (and not natively via Varlink) are handled by the +`io.systemd.NameServiceSwitch` service (see above). When using the NSS API +(i.e. `getpwnam()` and friends) the `nss-systemd` module will automatically +synthesize NSS records for users/groups natively defined via a Varlink +API. Special care is taken to avoid recursion between these two compatibility +mechanisms. + +Subsystems that shall provide user/group records to the system may choose +between offering them via an NSS module or via a this Varlink API, either way +all records are accessible via both APIs, due to the bidirectional +forwarding. It is also possible to provide the same records via both APIs +directly, but in that case the compatibility logic must be turned off. There +are mechanisms in place for this, please contact the systemd project for +details, as these are currently not documented. + +## Caching of User Records + +This API defines no concepts for caching records. If caching is desired it +should be implemented in the subsystems that provide the user records, not in +the clients consuming them. + +## Method Calls + +``` +interface io.systemd.UserDatabase + +method GetUserRecord( + uid : ?int, + userName : ?string, + service : string +) -> ( + record : object, + incomplete : bool +) + +method GetGroupRecord( + gid : ?int, + groupName : ?string, + service : string +) -> ( + record : object, + incomplete : bool +) + +method GetMemberships( + userName : ?string, + groupName : ?string, + service : string +) -> ( + userName : string, + groupName : string +) + +error NoRecordFound() +error BadService() +error ServiceNotAvailable() +error ConflictingRecordFound() +error EnumerationNotSupported() +``` + +The `GetUserRecord` method looks up or enumerates a user record. If the `uid` +parameter is set it specifies the numeric UNIX UID to search for. If the +`userName` parameter is set it specifies the name of the user to search +for. Typically, only one of the two parameters are set, depending whether a +look-up by UID or by name is desired. However, clients may also specify both +parameters, in which case a record matching both will be returned, and if only +one exists that matches one of the two parameters but not the other an error of +`ConflictingRecordFound` is returned. If neither of the two parameters are set +the whole user database is enumerated. In this case the method call needs to be +made with `more` set, so that multiple method call replies may be generated as +effect, each carrying one user record. + +The `service` parameter is mandatory and should be set to the service name +being talked to (i.e. to the same name as the `AF_UNIX` socket path, with the +`/run/systemd/userdb/` prefix removed). This is useful to allow implementation +of multiple services on the same socket (which is used by +`systemd-userdbd.service`). + +The method call returns one or more user records, depending which type of query is +used (see above). The record is returned in the `record` field. The +`incomplete` field indicates whether the record is complete. Services providing +user record lookup should only pass the `privileged` section of user records to +clients that either match the user the record is about or to sufficiently +privileged clients, for all others the section must be removed so that no +sensitive data is leaked this way. The `incomplete` parameter should indicate +whether the record has been modified like this or not (i.e. it is `true` if a +`privileged` section existed in the user record and was removed, and `false` if +no `privileged` section existed or one existed but hasn't been removed). + +If no user record matching the specified UID or name is known the error +`NoRecordFound` is returned (this is also returned if neither UID nor name are +specified, and hence enumeration requested but the subsystem currently has no +users defined). + +If a method call with an incorrectly set `service` field is received +(i.e. either not set at all, or not to the service's own name) a `BadService` +error is generated. Finally, `ServiceNotAvailable` should be returned when the +backing subsystem is not operational for some reason and hence no information +about existence or non-existence of a record can be returned nor any user +record at all. (The `service` field is defined in order to allow implementation +of daemons that provide multiple distinct user/group services over the same +`AF_UNIX` socket: in order to correctly determine which service a client wants +to talk to the client needs to provide the name in each request.) + +The `GetGroupRecord` method call works analogously but for groups. + +The `GetMemberships` method call may be used to inquire about group +memberships. The `userName` and `groupName` arguments take what the name +suggests. If one of the two is specified all matching memberships are returned, +if neither is specified all known memberships of any user and any group are +returned. The return value is a pair of user name and group name, where the +user is a member of the group. If both arguments are specified the specified +membership will be tested for, but no others, and the pair is returned if it is +defined. Unless both arguments are specified the method call needs to be made +with `more` set, so that multiple replies can be returned (since typically +there are are multiple members per group and also multiple groups a user is +member of). As with `GetUserRecord` and `GetGroupRecord` the `service` +parameter needs to contain the name of the service being talked to, in order to +allow implementation of multiple service within the same IPC socket. In case no +matching membership is known `NoRecordFound` is returned. The other two errors +are also generated in the same cases as for `GetUserRecord` and +`GetGroupRecord`. + +Unlike with `GetUserRecord` and `GetGroupRecord` the lists of memberships +returned by services are always combined. Thus unlike the other two calls a +membership lookup query has to wait for the last simultaneous query to complete +before the complete list is acquired. + +Note that only the `GetMemberships` call is authoritative about memberships of +users in groups. i.e. it should not be considered sufficient to check the +`memberOf` field of user records and the `members` field of group records to +acquire the full list of memberships. The full list can only bet determined by +`GetMemberships`, and as mentioned requires merging of these lists of all local +services. Result of this is that it can be one service that defines a user A, +and another service that defines a group B, and a third service that declares +that A is a member of B. + +Looking up explicit users/groups by their name or UID/GID, or querying +user/group memberships must be supported by all services implementing these +interfaces. However, supporting enumeration (i.e. user/group lookups that may +result in more than one reply, because neither UID/GID nor name is specified) +is optional. Services which are asked for enumeration may return the +`EnumerationNotSupported` error in this case. + +And that's really all there is to it. diff --git a/docs/USER_NAMES.md b/docs/USER_NAMES.md new file mode 100644 index 0000000..ec07b19 --- /dev/null +++ b/docs/USER_NAMES.md @@ -0,0 +1,169 @@ +--- +title: User/Group Name Syntax +category: Users, Groups and Home Directories +layout: default +--- + +# User/Group Name Syntax + +The precise set of allowed user and group names on Linux systems is weakly +defined. Depending on the distribution a different set of requirements and +restrictions on the syntax of user/group names are enforced — on some +distributions the accepted syntax is even configurable by the administrator. In +the interest of interoperability systemd enforces different rules when +processing users/group defined by other subsystems and when defining users/groups +itself, following the principle of "Be conservative in what you send, be +liberal in what you accept". Also in the interest of interoperability systemd +will enforce the same rules everywhere and not make them configurable or +distribution dependent. The precise rules are described below. + +Generally, the same rules apply for user as for group names. + +## Other Systems + +* On POSIX the set of [valid user + names](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_437) + is defined as [lower and upper case ASCII letters, digits, period, + underscore, and + hyphen](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_282), + with the restriction that hyphen is not allowed as first character of the + user name. Interestingly no size limit is declared, i.e. in neither + direction, meaning that strictly speaking according to POSIX both the empty + string is a valid user name as well as a string of gigabytes in length. + +* Debian/Ubuntu based systems enforce the regular expression + `^[a-z][-a-z0-9]*$`, i.e. only lower case ASCII letters, digits and + hyphens. As first character only lowercase ASCII letters are allowed. This + regular expression is configurable by the administrator at runtime + though. This rule enforces a minimum length of one character but no maximum + length. + +* Upstream shadow-utils enforces the regular expression + `^[a-z_][a-z0-9_-]*[$]$`, i.e. is similar to the Debian/Ubuntu rule, but + allows underscores and hyphens, but the latter not as first character. Also, + an optional trailing dollar character is permitted. + +* Fedora/Red Hat based systems enforce the regular expression of + `^[a-zA-Z0-9_.][a-zA-Z0-9_.-]{0,30}[a-zA-Z0-9_.$-]?$`, i.e. a size limit of + 32 characters, with upper and lower case letters, digits, underscores, + hyphens and periods. No hyphen as first character though, and the last + character may be a dollar character. On top of that, `.` and `..` are not + allowed as user/group names. + +* sssd is known to generate user names with embedded `@` and white-space + characters, as well as non-ASCII (i.e. UTF-8) user/group names. + +* winbindd is known to generate user/group names with embedded `\` and + white-space characters, as well as non-ASCII (i.e. UTF-8) user/group names. + +Other operating systems enforce different rules; in this documentation we'll +focus on Linux systems only however, hence those are out of scope. That said, +software like Samba is frequently deployed on Linux for providing compatibility +with Windows systems; on such systems it might be wise to stick to user/group +names also valid according to Windows rules. + +## Rules systemd enforces + +Distilled from the above, below are the rules systemd enforces on user/group +names. An additional, common rule between both modes listed below is that empty +strings are not valid user/group names. + +Philosophically, the strict mode described below enforces an allow list of +what's allowed and prohibits everything else, while the relaxed mode described +below implements a deny list of what's not allowed and permits everything else. + +### Strict mode + +Strict user/group name syntax is enforced whenever a systemd component is used +to register a user or group in the system, for example a system user/group +using +[`systemd-sysusers.service`](https://www.freedesktop.org/software/systemd/man/systemd-sysusers.html) +or a regular user with +[`systemd-homed.service`](https://www.freedesktop.org/software/systemd/man/systemd-homed.html). + +In strict mode, only uppercase and lowercase characters are allowed, as well as +digits, underscores and hyphens. The first character may not be a digit or +hyphen. A size limit is enforced: the minimum of `sysconf(_SC_LOGIN_NAME_MAX)` +(typically 256 on Linux; rationale: this is how POSIX suggests to detect the +limit), `UT_NAMESIZE-1` (typically 31 on Linux; rationale: names longer than +this cannot correctly appear in `utmp`/`wtmp` and create ambiguity with login +accounting) and `FILENAME_MAX` (4096 on Linux; rationale: user names typically +appear in directory names, i.e. the home directory), thus MIN(256, 31, 4096) = +31. + +Note that these rules are both more strict and more relaxed than all of the +rules enforced by other systems listed above. A user/group name conforming to +systemd's strict rules will not necessarily pass a test by the rules enforced +by these other subsystems. + +Written as regular expression the above is: `^[a-zA-Z_][a-zA-Z0-9_-]{0,30}$` + +### Relaxed mode + +Relaxed user/group name syntax is enforced whenever a systemd component accepts +and makes use of user/group names registered by other (non-systemd) +components of the system, for example in +[`systemd-logind.service`](https://www.freedesktop.org/software/systemd/man/systemd-logind.html). + +Relaxed syntax is also enforced by the `User=` setting in service unit files, +i.e. for system services used for running services. Since these users may be +registered by a variety of tools relaxed mode is used, but since the primary +purpose of these users is to run a system service and thus a job for systemd a +warning is shown if the specified user name does not qualify by the strict +rules above. + +* No embedded NUL bytes (rationale: handling in C must be possible and + straight-forward) + +* No names consisting fully of digits (rationale: avoid confusion with numeric + UID/GID specifications) + +* Similar, no names consisting of an initial hyphen and otherwise entirely made + up of digits (rationale: avoid confusion with negative, numeric UID/GID + specifications, e.g. `-1`) + +* No strings that do not qualify as valid UTF-8 (rationale: we want to be able + to embed these strings in JSON, with permits only valid UTF-8 in its strings; + user names using other character sets, such as JIS/Shift-JIS will cause + validation errors) + +* No control characters (i.e. characters in ASCII range 1…31; rationale: they + tend to have special meaning when output on a terminal in other contexts, + moreover the newline character — as a specific control character — is used as + record separator in `/etc/passwd`, and hence it's crucial to avoid + ambiguities here) + +* No colon characters (rationale: it is used as field separator in `/etc/passwd`) + +* The two strings `.` and `..` are not permitted, as these have special meaning + in file system paths, and user names are frequently included in file system + paths, in particular for the purpose of home directories. + +* Similar, no slashes, as these have special meaning in file system paths + +* No leading or trailing white-space is permitted; and hence no user/group names + consisting of white-space only either (rationale: this typically indicates + parsing errors, and creates confusion since not visible on screen) + +Note that these relaxed rules are implied by the strict rules above, i.e. all +user/group names accepted by the strict rules are also accepted by the relaxed +rules, but not vice versa. + +Note that this relaxed mode does not refuse a couple of very questionable +syntaxes. For example it permits a leading or embedded period. A leading period +is problematic because the matching home directory would typically be hidden +from the user's/administrator's view. An embedded period is problematic since +it creates ambiguity in traditional `chown` syntax (which is still accepted +today) that uses it to separate user and group names in the command's +parameter: without consulting the user/group databases it is not possible to +determine if a `chown` invocation would change just the owning user or both the +owning user and group. It also allows embedding `@` (which is confusing to +MTAs). + +## Common Core + +Combining all rules listed above, user/group names that shall be considered +valid in all systemd contexts and on all Linux systems should match the +following regular expression (at least according to our understanding): + +`^[a-z][a-z0-9-]{0,30}$` diff --git a/docs/USER_RECORD.md b/docs/USER_RECORD.md new file mode 100644 index 0000000..6cba02a --- /dev/null +++ b/docs/USER_RECORD.md @@ -0,0 +1,1083 @@ +--- +title: JSON User Records +category: Users, Groups and Home Directories +layout: default +--- + +# JSON User Records + +systemd optionally processes user records that go beyond the classic UNIX (or +glibc NSS) `struct passwd`. Various components of systemd are able to provide +and consume records in a more extensible format of a dictionary of key/value +pairs, encoded as JSON. Specifically: + +1. [`systemd-homed.service`](https://www.freedesktop.org/software/systemd/man/systemd-homed.service.html) + manages `human` user home directories and embeds these JSON records + directly in the home directory images (see [Home + Directories](https://systemd.io/HOME_DIRECTORY) for details). + +2. [`pam_systemd`](https://www.freedesktop.org/software/systemd/man/pam_systemd.html) + processes these JSON records for users that log in, and applies various + settings to the activated session, including environment variables, nice + levels and more. + +3. [`systemd-logind.service`](https://www.freedesktop.org/software/systemd/man/systemd-logind.service.html) + processes these JSON records of users that log in, and applies various + resource management settings to the per-user slice units it manages. This + allows setting global limits on resource consumption by a specific user. + +4. [`nss-systemd`](https://www.freedesktop.org/software/systemd/man/nss-systemd.html) + is a glibc NSS module that synthesizes classic NSS records from these JSON + records, providing full backwards compatibility with the classic UNIX APIs + both for look-up and enumeration. + +5. The service manager (PID 1) exposes dynamic users (i.e. users synthesized as + effect of `DynamicUser=` in service unit files) as these advanced JSON + records, making them discoverable to the rest of the system. + +6. [`systemd-userdbd.service`](https://www.freedesktop.org/software/systemd/man/systemd-userdbd.service.html) + is a small service that can translate UNIX/glibc NSS records to these JSON + user records. It also provides a unified [Varlink](https://varlink.org/) API + for querying and enumerating records of this type, optionally acquiring them + from various other services. + +JSON user records may contain various fields that are not available in `struct +passwd`, and are extensible for other applications. For example, the record may +contain information about: + +1. Additional security credentials (PKCS#11 security token information, + biometrical authentication information, SSH public key information) + +2. Additional user metadata, such as a picture, email address, location string, + preferred language or timezone + +3. Resource Management settings (such as CPU/IO weights, memory and tasks + limits, classic UNIX resource limits or nice levels) + +4. Runtime parameters such as environment variables or the `nodev`, `noexec`, + `nosuid` flags to use for the home directory + +5. Information about where to mount the home directory from + +And various other things. The record is intended to be extensible, for example +the following extensions are envisioned: + +1. Windows network credential information + +2. Information about default IMAP, SMTP servers to use for this user + +3. Parental control information to enforce on this user + +4. Default parameters for backup applications and similar + +Similar to JSON User Records there are also [JSON Group +Records](https://systemd.io/GROUP_RECORD) that encapsulate UNIX groups. + +JSON User Records may be transferred or written to disk in various protocols +and formats. To inquire about such records defined on the local system use the +[User/Group Lookup API via Varlink](https://systemd.io/USER_GROUP_API). + +## Why JSON? + +JSON is nicely extensible and widely used. In particular it's easy to +synthesize and process with numerous programming languages. It's particularly +popular in the web communities, which hopefully should make it easy to link +user credential data from the web and from local systems more closely together. + +## General Structure + +The JSON user records generated and processed by systemd follow a general +structure, consisting of seven distinct "sections". Specifically: + +1. Various fields are placed at the top-level of user record (the `regular` + section). These are generally fields that shall apply unconditionally to the + user in all contexts, are portable and not security sensitive. + +2. A number of fields are located in the `privileged` section (a sub-object of + the user record). Fields contained in this object are security sensitive, + i.e. contain information that the user and the administrator should be able + to see, but other users should not. In many ways this matches the data + stored in `/etc/shadow` in classic Linux user accounts, i.e. includes + password hashes and more. Algorithmically, when a user record is passed to + an untrusted client, by monopolizing such sensitive records in a single + object field we can easily remove it from view. + +3. A number of fields are located in objects inside the `perMachine` section + (an array field of the user record). Primarily these are resource + management-related fields, as those tend to make sense on a specific system + only, e.g. limiting a user's memory use to 1G only makes sense on a specific + system that has more than 1G of memory. Each object inside the `perMachine` + array comes with a `matchMachineId` or `matchHostname` field which indicate + which systems to apply the listed settings to. Note that many fields + accepted in the `perMachine` section can also be set at the top level (the + `regular` section), where they define the fallback if no matching object in + `perMachine` is found. + +4. Various fields are located in the `binding` section (a sub-sub-object of the + user record; an intermediary object is inserted which is keyed by the + machine ID of the host). Fields included in this section "bind" the object + to a specific system. They generally include non-portable information about + paths or UID assignments, that are true on a specific system, but not + necessarily on others, and which are managed automatically by some user + record manager (such as `systemd-homed`). Data in this section is considered + part of the user record only in the local context, and is generally not + ported to other systems. Due to that it is not included in the reduced user + record the cryptographic signature defined in the `signature` section is + calculated on. In `systemd-homed` this section is also removed when the + user's record is stored in the `~/.identity` file in the home directory, so + that every system with access to the home directory can manage these + `binding` fields individually. Typically, the binding section is persisted + to the local disk. + +5. Various fields are located in the `status` section (a sub-sub-object of the + user record, also with an intermediary object between that is keyed by the + machine ID, similar to the way the `binding` section is organized). This + section is augmented during runtime only, and never persisted to disk. The + idea is that this section contains information about current runtime + resource usage (for example: currently used disk space of the user), that + changes dynamically but is otherwise immediately associated with the user + record and for many purposes should be considered to be part of the user + record. + +6. The `signature` section contains one or more cryptographic signatures of a + reduced version of the user record. This is used to ensure that only user + records defined by a specific source are accepted on a system, by validating + the signature against the set of locally accepted signature public keys. The + signature is calculated from the JSON user record with all sections removed, + except for `regular`, `privileged`, `perMachine`. Specifically, `binding`, + `status`, `signature` itself and `secret` are removed first and thus not + covered by the signature. This section is optional, and is only used when + cryptographic validation of user records is required (as it is by + `systemd-homed.service` for example). + +7. The `secret` section contains secret user credentials, such as password or + PIN information. This data is never persisted, and never returned when user + records are inquired by a client, privileged or not. This data should only + be included in a user record very briefly, for example when certain very + specific operations are executed. For example, in tools such as + `systemd-homed` this section may be included in user records, when creating + a new home directory, as passwords and similar credentials need to be + provided to encrypt the home directory with. + +Here's a tabular overview of the sections and their properties: + +| Section | Included in Signature | Persistent | Security Sensitive | Contains Host-Specific Data | +|------------|-----------------------|------------|--------------------|-----------------------------| +| regular | yes | yes | no | no | +| privileged | yes | yes | yes | no | +| perMachine | yes | yes | no | yes | +| binding | no | yes | no | yes | +| status | no | no | no | yes | +| signature | no | yes | no | no | +| secret | no | no | yes | no | + +Note that services providing user records to the local system are free to +manage only a subset of these sections and never include the others in +them. For example, a service that has no concept of signed records (for example +because the records it manages are inherently trusted anyway) does not have to +bother with the `signature` section. A service that only defines records in a +strictly local context and without signatures doesn't have to deal with the +`perMachine` or `binding` sections and can include its data exclusively in the +regular section. A service that uses a separate, private channel for +authenticating users (or that doesn't have a concept of authentication at all) +does not need to be concerned with the `secret` section of user records, as +the fields included therein are only useful when executing authentication +operations natively against JSON user records. + +The `systemd-homed` manager uses all seven sections for various +purposes. Inside the home directories (and if the LUKS2 backend is used, also +in the LUKS2 header) a user record containing the `regular`, `privileged`, +`perMachine` and `signature` sections is stored. `systemd-homed` also stores a +version of the record on the host, with the same four sections and augmented +with an additional, fifth `binding` section. When a local client enquires about +a user record managed by `systemd-homed` the service will add in some +additional information about the user and home directory in the `status` +section — this version is only transferred via IPC and never written to +disk. Finally the `secret` section is used during authentication operations via +IPC to transfer the user record along with its authentication tokens in one go. + +## Fields in the `regular` section + +As mentioned, the `regular` section's fields are placed at the top level +object. The following fields are currently defined: + +`userName` → The UNIX user name for this record. Takes a string with a valid +UNIX user name. This field is the only mandatory field, all others are +optional. Corresponds with the `pw_name` field of of `struct passwd` and the +`sp_namp` field of `struct spwd` (i.e. the shadow user record stored in +`/etc/shadow`). See [User/Group Name Syntax](https://systemd.io/USER_NAMES) for +the (relaxed) rules the various systemd components enforce on user/group names. + +`realm` → The "realm" a user is defined in. This concept allows distinguishing +users with the same name that originate in different organizations or +installations. This should take a string in DNS domain syntax, but doesn't have +to refer to an actual DNS domain (though it is recommended to use one for +this). The idea is that the user `lpoetter` in the `redhat.com` realm might be +distinct from the same user in the `poettering.hq` realm. User records for the +same user name that have different realm fields are considered referring to +different users. When updating a user record it is required that any new +version has to match in both `userName` and `realm` field. This field is +optional, when unset the user should not be considered part of any realm. A +user record with a realm set is never compatible (for the purpose of updates, +see above) with a user record without one set, even if the `userName` field matches. + +`realName` → The real name of the user, a string. This should contain the +user's real ("human") name, and corresponds loosely to the GECOS field of +classic UNIX user records. When converting a `struct passwd` to a JSON user +record this field is initialized from GECOS (i.e. the `pw_gecos` field), and +vice versa when converting back. That said, unlike GECOS this field is supposed +to contain only the real name and no other information. This field must not +contain control characters (such as `\n`) or colons (`:`), since those are used +as record separators in classic `/etc/passwd` files and similar formats. + +`emailAddress` → The email address of the user, formatted as +string. [`pam_systemd`](https://www.freedesktop.org/software/systemd/man/pam_systemd.html) +initializes the `$EMAIL` environment variable from this value for all login +sessions. + +`iconName` → The name of an icon picked by the user, for example for the +purpose of an avatar. This must be a string, and should follow the semantics +defined in the [Icon Naming +Specification](https://standards.freedesktop.org/icon-naming-spec/icon-naming-spec-latest.html). + +`location` → A free-form location string describing the location of the user, +if that is applicable. It's probably wise to use a location string processable +by geo-location subsystems, but this is not enforced nor required. Example: +`Berlin, Germany` or `Basement, Room 3a`. + +`disposition` → A string, one of `intrinsic`, `system`, `dynamic`, `regular`, +`container`, `reserved`. If specified clarifies the disposition of the user, +i.e. the context it is defined in. For regular, "human" users this should be +`regular`, for system users (i.e. users that system services run under, and +similar) this should be `system`. The `intrinsic` disposition should be used +only for the two users that have special meaning to the OS kernel itself, +i.e. the `root` and `nobody` users. The `container` string should be used for +users that are used by an OS container, and hence will show up in `ps` listings +and such, but are only defined in container context. Finally `reserved` should +be used for any users outside of these use-cases. Note that this property is +entirely optional and applications are assumed to be able to derive the +disposition of a user automatically from a record even in absence of this +field, based on other fields, for example the numeric UID. By setting this +field explicitly applications can override this default determination. + +`lastChangeUSec` → An unsigned 64bit integer value, referring to a timestamp in µs +since the epoch 1970, indicating when the user record (specifically, any of the +`regular`, `privileged`, `perMachine` sections) was last changed. This field is +used when comparing two records of the same user to identify the newer one, and +is used for example for automatic updating of user records, where appropriate. + +`lastPasswordChangeUSec` → Similar, also an unsigned 64bit integer value, +indicating the point in time the password (or any authentication token) of the +user was last changed. This corresponds to the `sp_lstchg` field of `struct +spwd`, i.e. the matching field in the user shadow database `/etc/shadow`, +though provides finer resolution. + +`shell` → A string, referring to the shell binary to use for terminal logins of +this user. This corresponds with the `pw_shell` field of `struct passwd`, and +should contain an absolute file system path. For system users not suitable for +terminal log-in this field should not be set. + +`umask` → The `umask` to set for the user's login sessions. Takes an +integer. Note that usually on UNIX the umask is noted in octal, but JSON's +integers are generally written in decimal, hence in this context we denote it +umask in decimal too. The specified value should be in the valid range for +umasks, i.e. 0000…0777 (in octal as typical in UNIX), or 0…511 (in decimal, how +it actually appears in the JSON record). This `umask` is automatically set by +[`pam_systemd`](https://www.freedesktop.org/software/systemd/man/pam_systemd.html) +for all login sessions of the user. + +`environment` → An array of strings, each containing an environment variable +and its value to set for the user's login session, in a format compatible with +[`putenv()`](http://man7.org/linux/man-pages/man3/putenv.3.html). Any +environment variable listed here is automatically set by +[`pam_systemd`](https://www.freedesktop.org/software/systemd/man/pam_systemd.html) +for all login sessions of the user. + +`timeZone` → A string indicating a preferred timezone to use for the user. When +logging in +[`pam_systemd`](https://www.freedesktop.org/software/systemd/man/pam_systemd.html) +will automatically initialize the `$TZ` environment variable from this +string. The string should be a `tzdata` compatible location string, for +example: `Europe/Berlin`. + +`preferredLanguage` → A string indicating the preferred language/locale for the +user. When logging in +[`pam_systemd`](https://www.freedesktop.org/software/systemd/man/pam_systemd.html) +will automatically initialize the `$LANG` environment variable from this +string. The string hence should be in a format compatible with this environment +variable, for example: `de_DE.UTF8`. + +`niceLevel` → An integer value in the range -20…19. When logging in +[`pam_systemd`](https://www.freedesktop.org/software/systemd/man/pam_systemd.html) +will automatically initialize the login process' nice level to this value with, +which is then inherited by all the user's processes, see +[`setpriority()`](http://man7.org/linux/man-pages/man2/setpriority.2.html) for +more information. + +`resourceLimits` → An object, where each key refers to a Linux resource limit +(such as `RLIMIT_NOFILE` and similar). Their values should be an object with +two keys `cur` and `max` for the soft and hard resource limit. When logging in +[`pam_systemd`](https://www.freedesktop.org/software/systemd/man/pam_systemd.html) +will automatically initialize the login process' resource limits to these +values, which is then inherited by all the user's processes, see +[`setrlimit()`](http://man7.org/linux/man-pages/man2/setrlimit.2.html) for more +information. + +`locked` → A boolean value. If true the user account is locked, the user may +not log in. If this field is missing it should be assumed to be false, +i.e. logins are permitted. This field corresponds to the `sp_expire` field of +`struct spwd` (i.e. the `/etc/shadow` data for a user) being set to zero or +one. + +`notBeforeUSec` → An unsigned 64bit integer value, indicating a time in µs since +the UNIX epoch (1970) before which the record should be considered invalid for +the purpose of logging in. + +`notAfterUSec` → Similar, but indicates the point in time *after* which logins +shall not be permitted anymore. This corresponds to the `sp_expire` field of +`struct spwd`, when it is set to a value larger than one, but provides finer +granularity. + +`storage` → A string, one of `classic`, `luks`, `directory`, `subvolume`, +`fscrypt`, `cifs`. Indicates the storage mechanism for the user's home +directory. If `classic` the home directory is a plain directory as in classic +UNIX. When `directory`, the home directory is a regular directory, but the +`~/.identity` file in it contains the user's user record, so that the directory +is self-contained. Similar, `subvolume` is a `btrfs` subvolume that also +contains a `~/.identity` user record; `fscrypt` is an `fscrypt`-encrypted +directory, also containing the `~/.identity` user record; `luks` is a per-user +LUKS volume that is mounted as home directory, and `cifs` a home directory +mounted from a Windows File Share. The five latter types are primarily used by +`systemd-homed` when managing home directories, but may be used if other +managers are used too. If this is not set `classic` is the implied default. + +`diskSize` → An unsigned 64bit integer, indicating the intended home directory +disk space in bytes to assign to the user. Depending on the selected storage +type this might be implement differently: for `luks` this is the intended size +of the file system and LUKS volume, while for the others this likely translates +to classic file system quota settings. + +`diskSizeRelative` → Similar to `diskSize` but takes a relative value, but +specifies a fraction of the available disk space on the selected storage medium +to assign to the user. This unsigned integer value is normalized to 2^32 = +100%. + +`skeletonDirectory` → Takes a string with the absolute path to the skeleton +directory to populate a new home directory from. This is only used when a home +directory is first created, and defaults to `/etc/skel` if not defined. + +`accessMode` → Takes an unsigned integer in the range 0…511 indicating the UNIX +access mask for the home directory when it is first created. + +`tasksMax` → Takes an unsigned 64bit integer indicating the maximum number of +tasks the user may start in parallel during system runtime. This counts +all tasks (i.e. threads, where each process is at least one thread) the user starts or that are +forked from these processes even if the user identity is changed (for example +by setuid binaries/`su`/`sudo` and similar). +[`systemd-logind.service`](https://www.freedesktop.org/software/systemd/man/systemd-logind.service.html) +enforces this by setting the `TasksMax` slice property for the user's slice +`user-$UID.slice`. + +`memoryHigh`/`memoryMax` → These take unsigned 64bit integers indicating upper +memory limits for all processes of the user (plus all processes forked off them +that might have changed user identity), in bytes. Enforced by +[`systemd-logind.service`](https://www.freedesktop.org/software/systemd/man/systemd-logind.service.html), +similar to `tasksMax`. + +`cpuWeight`/`ioWeight` → These take unsigned integers in the range 1…10000 +(defaults to 100) and configure the CPU and IO scheduling weights for the +user's processes as a whole. Also enforced by +[`systemd-logind.service`](https://www.freedesktop.org/software/systemd/man/systemd-logind.service.html), +similar to `tasksMax`, `memoryHigh` and `memoryMax`. + +`mountNoDevices`/`mountNoSuid`/`mountNoExecute` → Three booleans that control +the `nodev`, `nosuid`, `noexec` mount flags of the user's home +directories. Note that these booleans are only honored if the home directory +is managed by a subsystem such as `systemd-homed.service` that automatically +mounts home directories on login. + +`cifsDomain` → A string indicating the Windows File Sharing domain (CIFS) to +use. This is generally useful, but particularly when `cifs` is used as storage +mechanism for the user's home directory, see above. + +`cifsUserName` → A string indicating the Windows File Sharing user name (CIFS) +to associate this user record with. This is generally useful, but particularly +useful when `cifs` is used as storage mechanism for the user's home directory, +see above. + +`cifsService` → A string indicating the Windows File Share service (CIFS) to +mount as home directory of the user on login. + +`imagePath` → A string with an absolute file system path to the file, directory +or block device to use for storage backing the home directory. If the `luks` +storage is used this refers to the loopback file or block device node to store +the LUKS volume on. For `fscrypt`, `directory`, `subvolume` this refers to the +directory to bind mount as home directory on login. Not defined for `classic` +or `cifs`. + +`homeDirectory` → A string with an absolute file system path to the home +directory. This is where the image indicated in `imagePath` is mounted to on +login and thus indicates the application facing home directory while the home +directory is active, and is what the user's `$HOME` environment variable is set +to during log-in. It corresponds to the `pw_dir` field of `struct passwd`. + +`uid` → An unsigned integer in the range 0…4294967295: the numeric UNIX user ID (UID) to +use for the user. This corresponds to the `pw_uid` field of `struct passwd`. + +`gid` → An unsigned integer in the range 0…4294967295: the numeric UNIX group +ID (GID) to use for the user. This corresponds to the `pw_gid` field of +`struct passwd`. + +`memberOf` → An array of strings, each indicating a UNIX group this user shall +be a member of. The listed strings must be valid group names, but it is not +required that all groups listed exist in all contexts: any entry for which no +group exists should be silently ignored. + +`fileSystemType` → A string, one of `ext4`, `xfs`, `btrfs` (possibly others) to +use as file system for the user's home directory. This is primarily relevant +when the storage mechanism used is `luks` as a file system to use inside the +LUKS container must be selected. + +`partitionUuid` → A string containing a lower-case, text-formatted UUID, referencing +the GPT partition UUID the home directory is located in. This is primarily +relevant when the storage mechanism used is `luks`. + +`luksUuid` → A string containing a lower-case, text-formatted UUID, referencing +the LUKS volume UUID the home directory is located in. This is primarily +relevant when the storage mechanism used is `luks`. + +`fileSystemUuid` → A string containing a lower-case, text-formatted UUID, +referencing the file system UUID the home directory is located in. This is +primarily relevant when the storage mechanism used is `luks`. + +`luksDiscard` → A boolean. If true and `luks` storage is used controls whether +the loopback block devices, LUKS and the file system on top shall be used in +`discard` mode, i.e. erased sectors should always be returned to the underlying +storage. If false and `luks` storage is used turns this behavior off. In +addition, depending on this setting an `FITRIM` or `fallocate()` operation is +executed to make sure the image matches the selected option. + +`luksOfflineDiscard` → A boolean. Similar to `luksDiscard`, it controls whether +to trim/allocate the file system/backing file when deactivating the home +directory. + +`luksCipher` → A string, indicating the cipher to use for the LUKS storage mechanism. + +`luksCipherMode` → A string, selecting the cipher mode to use for the LUKS storage mechanism. + +`luksVolumeKeySize` → An unsigned integer, indicating the volume key length in +bytes to use for the LUKS storage mechanism. + +`luksPbkdfHashAlgorithm` → A string, selecting the hash algorithm to use for +the PBKDF operation for the LUKS storage mechanism. + +`luksPbkdfType` → A string, indicating the PBKDF type to use for the LUKS storage mechanism. + +`luksPbkdfTimeCostUSec` → An unsigned 64bit integer, indicating the intended +time cost for the PBKDF operation, when the LUKS storage mechanism is used, in +µs. + +`luksPbkdfMemoryCost` → An unsigned 64bit integer, indicating the intended +memory cost for the PBKDF operation, when LUKS storage is used, in bytes. + +`luksPbkdfParallelThreads` → An unsigned 64bit integer, indicating the intended +required parallel threads for the PBKDF operation, when LUKS storage is used. + +`service` → A string declaring the service that defines or manages this user +record. It is recommended to use reverse domain name notation for this. For +example, if `systemd-homed` manages a user a string of `io.systemd.Home` is +used for this. + +`rateLimitIntervalUSec` → An unsigned 64bit integer that configures the +authentication rate limiting enforced on the user account. This specifies a +timer interval (in µs) within which to count authentication attempts. When the +counter goes above the value configured n `rateLimitIntervalBurst` log-ins are +temporarily refused until the interval passes. + +`rateLimitIntervalBurst` → An unsigned 64bit integer, closely related to +`rateLimitIntervalUSec`, that puts a limit on authentication attempts within +the configured time interval. + +`enforcePasswordPolicy` → A boolean. Configures whether to enforce the system's +password policy when creating the home directory for the user or changing the +user's password. By default the policy is enforced, but if this field is false +it is bypassed. + +`autoLogin` → A boolean. If true the user record is marked as suitable for +auto-login. Systems are supposed to automatically log in a user marked this way +during boot, if there's exactly one user on it defined this way. + +`stopDelayUSec` → An unsigned 64bit integer, indicating the time in µs the +per-user service manager is kept around after the user fully logged out. This +value is honored by +[`systemd-logind.service`](https://www.freedesktop.org/software/systemd/man/systemd-logind.service.html). If +set to zero the per-user service manager is immediately terminated when the +user logs out, and longer values optimize high-frequency log-ins as the +necessary work to set up and tear down a log-in is reduced if the service +manager stays running. + +`killProcesses` → A boolean. If true all processes of the user are +automatically killed when the user logs out. This is enforced by +[`systemd-logind.service`](https://www.freedesktop.org/software/systemd/man/systemd-logind.service.html). If +false any processes left around when the user logs out are left running. + +`passwordChangeMinUSec`/`passwordChangeMaxUSec` → An unsigned 64bit integer, +encoding how much time has to pass at least/at most between password changes of +the user. This corresponds with the `sp_min` and `sp_max` fields of `struct +spwd` (i.e. the `/etc/shadow` entries of the user), but offers finer +granularity. + +`passwordChangeWarnUSec` → An unsigned 64bit integer, encoding how much time to +warn the user before their password expires, in µs. This corresponds with the +`sp_warn` field of `struct spwd`. + +`passwordChangeInactiveUSec` → An unsigned 64bit integer, encoding how much +time has to pass after the password expired that the account is +deactivated. This corresponds with the `sp_inact` field of `struct spwd`. + +`passwordChangeNow` → A boolean. If true the user has to change their password +on next login. This corresponds with the `sp_lstchg` field of `struct spwd` +being set to zero. + +`pkcs11TokenUri` → An array of strings, each with an RFC 7512 compliant PKCS#11 +URI referring to security token (or smart card) of some form, that shall be +associated with the user and may be used for authentication. The URI is used to +search for an X.509 certificate and associated private key that may be used to +decrypt an encrypted secret key that is used to unlock the user's account (see +below). It's undefined how precise the URI is: during log-in it is tested +against all plugged in security tokens and if there's exactly one matching +private key found with it it is used. + +`fido2HmacCredential` → An array of strings, each with a Base64-encoded FIDO2 +credential ID that shell be used for authentication with FIDO2 devices that +implement the `hmac-secret` extension. The salt to pass to the FIDO2 device is +found in `fido2HmacSalt`. + +`recoveryKeyType` → An array of strings, each indicating the type of one +recovery key. The only supported recovery key type at the moment is `modhex64`, +for details see the description of `recoveryKey` below. An account may have any +number of recovery keys defined, and the array should have one entry for each. + +`privileged` → An object, which contains the fields of the `privileged` section +of the user record, see below. + +`perMachine` → An array of objects, which contain the `perMachine` section of +the user record, and thus fields to apply on specific systems only, see below. + +`binding` → An object, keyed by machine IDs formatted as strings, pointing +to objects that contain the `binding` section of the user record, +i.e. additional fields that bind the user record to a specific machine, see +below. + +`status` → An object, keyed by machine IDs formatted as strings, pointing to +objects that contain the `status` section of the user record, i.e. additional +runtime fields that expose the current status of the user record on a specific +system, see below. + +`signature` → An array of objects, which contain cryptographic signatures of +the user record, i.e. the fields of the `signature` section of the user record, +see below. + +`secret` → An object, which contains the fields of the `secret` section of the +user record, see below. + +## Fields in the `privileged` section + +As mentioned, the `privileged` section is encoded in a sub-object of the user +record top-level object, in the `privileged` field. Any data included in this +object shall only be visible to the administrator and the user themselves, and +be suppressed implicitly when other users get access to a user record. It thus +takes the role of the `/etc/shadow` records for each user, which has similarly +restrictive access semantics. The following fields are currently defined: + +`passwordHint` → A user-selected password hint in free-form text. This should +be a string like "What's the name of your first pet?", but is entirely for the +user to choose. + +`hashedPassword` → An array of strings, each containing a hashed UNIX password +string, in the format +[`crypt(3)`](http://man7.org/linux/man-pages/man3/crypt.3.html) generates. This +corresponds with `sp_pwdp` field of `struct spwd` (and in a way the `pw_passwd` +field of `struct passwd`). + +`sshAuthorizedKeys` → An array of strings, each listing an SSH public key that +is authorized to access the account. The strings should follow the same format +as the lines in the traditional `~/.ssh/authorized_key` file. + +`pkcs11EncryptedKey` → An array of objects. Each element of the array should be +an object consisting of three string fields: `uri` shall contain a PKCS#11 +security token URI, `data` shall contain a Base64-encoded encrypted key and +`hashedPassword` shall contain a UNIX password hash to test the key +against. Authenticating with a security token against this account shall work +as follows: the encrypted secret key is converted from its Base64 +representation into binary, then decrypted with the PKCS#11 `C_Decrypt()` +function of the PKCS#11 module referenced by the specified URI, using the +private key found on the same token. The resulting decrypted key is then +Base64-encoded and tested against the specified UNIX hashed password. The +Base64-encoded decrypted key may also be used to unlock further resources +during log-in, for example the LUKS or `fscrypt` storage backend. It is +generally recommended that for each entry in `pkcs11EncryptedKey` there's also +a matching one in `pkcs11TokenUri` and vice versa, with the same URI, appearing +in the same order, but this should not be required by applications processing +user records. + +`fido2HmacSalt` → An array of objects, implementing authentication support with +FIDO2 devices that implement the `hmac-secret` extension. Each element of the +array should be an object consisting of three string fields: `credential`, +`salt`, `hashedPassword`. The first two shall contain Base64-encoded binary +data: the FIDO2 credential ID and the salt value to pass to the FIDO2 +device. During authentication this salt along with the credential ID is sent to +the FIDO2 token, which will HMAC hash the salt with its internal secret key and +return the result. This resulting binary key should then be Base64-encoded and +used as string password for the further layers of the stack. The +`hashedPassword` field of the `fido2HmacSalt` field shall be a UNIX password +hash to test this derived secret key against for authentication. It is +generally recommended that for each entry in `fido2HmacSalt` there's also a +matching one in `fido2HmacCredential`, and vice versa, with the same credential +ID, appearing in the same order, but this should not be required by +applications processing user records. + +`recoveryKey`→ An array of objects, each defining a recovery key. The object +has two mandatory fields: `type` indicates the type of recovery key. The only +currently permitted value is the string `modhex64`. The `hashedPassword` field +contains a UNIX password hash of the normalized recovery key. Recovery keys are +in most ways similar to regular passwords, except that they are generated by +the computer, not chosen by the user, and are longer. Currently, the only +supported recovery key format is `modhex64`, which consists of 64 +[modhex](https://developers.yubico.com/yubico-c/Manuals/modhex.1.html) +characters (i.e. 256bit of information), in groups of 8 chars separated by +dashes, +e.g. `lhkbicdj-trbuftjv-tviijfck-dfvbknrh-uiulbhui-higltier-kecfhkbk-egrirkui`. Recovery +keys should be accepted wherever regular passwords are. The `recoveryKey` field +should always be accompanied by a `recoveryKeyType` field (see above), and each +entry in either should map 1:1 to an entry in the other, in the same order and +matching the type. When accepting a recovery key it should be brought +automatically into normalized form, i.e. the dashes inserted when missing, and +converted into lowercase before tested against the UNIX password hash, so that +recovery keys are effectively case-insensitive. + +## Fields in the `perMachine` section + +As mentioned, the `perMachine` section contains settings that shall apply to +specific systems only. This is primarily interesting for resource management +properties as they tend to require a per-system focus, however they may be used +for other purposes too. + +The `perMachine` field in the top-level object is an array of objects. When +processing the user record first the various fields on the top-level object +should be used. Then this array should be iterated in order, and the various +settings be applied that match either the indicated machine ID or host +name. There may be multiple array entries that match a specific system, in +which case all the object's setting should be applied. If the same option is +set in the top-level object as in a per-machine object the latter wins and +entirely undoes the setting in the top-level object (i.e. no merging of +properties that are arrays themselves is done). If the same option is set in +multiple per-machine objects the one specified later in the array wins (and +here too no merging of individual fields is done, the later field always wins +in full). + +The following fields are defined in this section: + +`matchMachineId` → An array of strings with each a formatted 128bit ID in +hex. If any of the specified IDs match the system's local machine ID +(i.e. matches `/etc/machine-id`) the fields in this object are honored. + +`matchHostname` → An array of string with a each a valid hostname. If any of +the specified hostnames match the system's local hostname, the fields in this +object are honored. If both `matchHostname` and `matchMachineId` are used +within the same array entry, the object is honored when either match succeeds, +i.e. the two match types are combined in OR, not in AND. + +These two are the only two fields specific to this section. All other fields +that may be used in this section are identical to the equally named ones in the +`regular` section (i.e. at the top-level object). Specifically, these are: + +`iconName`, `location`, `shell`, `umask`, `environment`, `timeZone`, +`preferredLanguage`, `niceLevel`, `resourceLimits`, `locked`, `notBeforeUSec`, +`notAfterUSec`, `storage`, `diskSize`, `diskSizeRelative`, `skeletonDirectory`, +`accessMode`, `tasksMax`, `memoryHigh`, `memoryMax`, `cpuWeight`, `ioWeight`, +`mountNoDevices`, `mountNoSuid`, `mountNoExecute`, `cifsDomain`, +`cifsUserName`, `cifsService`, `imagePath`, `uid`, `gid`, `memberOf`, +`fileSystemType`, `partitionUuid`, `luksUuid`, `fileSystemUuid`, `luksDiscard`, +`luksOfflineDiscard`, `luksCipher`, `luksCipherMode`, `luksVolumeKeySize`, +`luksPbkdfHashAlgorithm`, `luksPbkdfType`, `luksPbkdfTimeCostUSec`, +`luksPbkdfMemoryCost`, `luksPbkdfParallelThreads`, `rateLimitIntervalUSec`, +`rateLimitBurst`, `enforcePasswordPolicy`, `autoLogin`, `stopDelayUSec`, +`killProcesses`, `passwordChangeMinUSec`, `passwordChangeMaxUSec`, +`passwordChangeWarnUSec`, `passwordChangeInactiveUSec`, `passwordChangeNow`, +`pkcs11TokenUri`, `fido2HmacCredential`. + +## Fields in the `binding` section + +As mentioned, the `binding` section contains additional fields about the user +record, that bind it to the local system. These fields are generally used by a +local user manager (such as `systemd-homed.service`) to add in fields that make +sense in a local context but not necessarily in a global one. For example, a +user record that contains no `uid` field in the regular section is likely +extended with one in the `binding` section to assign a local UID if no global +UID is defined. + +All fields in the `binding` section only make sense in a local context and are +suppressed when the user record is ported between systems. The `binding` section +is generally persisted on the system but not in the home directories themselves +and the home directory is supposed to be fully portable and thus not contain +the information that `binding` is supposed to contain that binds the portable +record to a specific system. + +The `binding` sub-object on the top-level user record object is keyed by the +machine ID the binding is intended for, which point to an object with the +fields of the bindings. These fields generally match fields that may also be +defined in the `regular` and `perMachine` sections, however override +both. Usually, the `binding` value should not contain settings different from +those set via `regular` or `perMachine`, however this might happen if some +settings are not supported locally (think: `fscrypt` is recorded as intended +storage mechanism in the `regular` section, but the local kernel does not +support `fscrypt`, hence `directory` was chosen as implicit fallback), or have +been changed in the `regular` section through updates (e.g. a home directory +was created with `luks` as storage mechanism but later the user record was +updated to prefer `subvolume`, which however doesn't change the actual storage +used already which is pinned in the `binding` section). + +The following fields are defined in the `binding` section. They all have an +identical format and override their equally named counterparts in the `regular` +and `perMachine` sections: + +`imagePath`, `homeDirectory`, `partitionUuid`, `luksUuid`, `fileSystemUuid`, +`uid`, `gid`, `storage`, `fileSystemType`, `luksCipher`, `luksCipherMode`, +`luksVolumeKeySize`. + +## Fields in the `status` section + +As mentioned, the `status` section contains additional fields about the user +record that are exclusively acquired during runtime, and that expose runtime +metrics of the user and similar metadata that shall not be persisted but are +only acquired "on-the-fly" when requested. + +This section is arranged similarly to the `binding` section: the `status` +sub-object of the top-level user record object is keyed by the machine ID, +which points to the object with the fields defined here. The following fields +are defined: + +`diskUsage` → An unsigned 64bit integer. The currently used disk space of the +home directory in bytes. This value might be determined in different ways, +depending on the selected storage mechanism. For LUKS storage this is the file +size of the loopback file or block device size. For the +directory/subvolume/fscrypt storage this is the current disk space used as +reported by the file system quota subsystem. + +`diskFree` → An unsigned 64bit integer, denoting the number of "free" bytes in +the disk space allotment, i.e. usually the difference between the disk size as +reported by `diskSize` and the used already as reported in `diskFree`, but +possibly skewed by metadata sizes, disk compression and similar. + +`diskSize` → An unsigned 64bit integer, denoting the disk space currently +allotted to the user, in bytes. Depending on the storage mechanism this can mean +different things (see above). In contrast to the top-level field of the same +(or the one in the `perMachine` section), this field reports the current size +allotted to the user, not the intended one. The values may differ when user +records are updated without the home directory being re-sized. + +`diskCeiling`/`diskFloor` → Unsigned 64bit integers indicating upper and lower +bounds when changing the `diskSize` value, in bytes. These values are typically +derived from the underlying data storage, and indicate in which range the home +directory may be re-sized in, i.e. in which sensible range the `diskSize` value +should be kept. + +`state` → A string indicating the current state of the home directory. The +precise set of values exposed here are up to the service managing the home +directory to define (i.e. are up to the service identified with the `service` +field below). However, it is recommended to stick to a basic vocabulary here: +`inactive` for a home directory currently not mounted, `absent` for a home +directory that cannot be mounted currently because it does not exist on the +local system, `active` for a home directory that is currently mounted and +accessible. + +`service` → A string identifying the service that manages this user record. For +example `systemd-homed.service` sets this to `io.systemd.Home` to all user +records it manages. This is particularly relevant to define clearly the context +in which `state` lives, see above. Note that this field also exists on the +top-level object (i.e. in the `regular` section), which it overrides. The +`regular` field should be used if conceptually the user record can only be +managed by the specified service, and this `status` field if it can +conceptually be managed by different managers, but currently is managed by the +specified one. + +`signedLocally` → A boolean. If true indicates that the user record is signed +by a public key for which the private key is available locally. This means that +the user record may be modified locally as it can be re-signed with the private +key. If false indicates that the user record is signed by a public key +recognized by the local manager but whose private key is not available +locally. This means the user record cannot be modified locally as it couldn't +be signed afterwards. + +`goodAuthenticationCounter` → An unsigned 64bit integer. This counter is +increased by one on every successful authentication attempt, i.e. an +authentication attempt where a security token of some form was presented and it +was correct. + +`badAuthenticationCounter` → An unsigned 64bit integer. This counter is +increased by one on every unsuccessfully authentication attempt, i.e. an +authentication attempt where a security token of some form was presented and it +was incorrect. + +`lastGoodAuthenticationUSec` → An unsigned 64bit integer, indicating the time +of the last successful authentication attempt in µs since the UNIX epoch (1970). + +`lastBadAuthenticationUSec` → Similar, but the timestamp of the last +unsuccessfully authentication attempt. + +`rateLimitBeginUSec` → An unsigned 64bit integer: the µs timestamp since the +UNIX epoch (1970) where the most recent rate limiting interval has been +started, as configured with `rateLimitIntervalUSec`. + +`rateLimitCount` → An unsigned 64bit integer, counting the authentication +attempts in the current rate limiting interval, see above. If this counter +grows beyond the value configured in `rateLimitBurst` authentication attempts +are temporarily refused. + +`removable` → A boolean value. If true the manager of this user record +determined the home directory being on removable media. If false it was +determined the home directory is in internal built-in media. (This is used by +`systemd-logind.service` to automatically pick the right default value for +`stopDelayUSec` if the field is not explicitly specified: for home directories +on removable media the delay is selected very low to minimize the chance the +home directory remains in unclean state if the storage device is removed from +the system by the user). + +## Fields in the `signature` section + +As mentioned, the `signature` section of the user record may contain one or +more cryptographic signatures of the user record. Like all others, this section +is optional, and only used when cryptographic validation of user records shall +be used. Specifically, all user records managed by `systemd-homed.service` will +carry such signatures and the service refuses managing user records that come +without signature or with signatures not recognized by any locally defined +public key. + +The `signature` field in the top-level user record object is an array of +objects. Each object encapsulates one signature and has two fields: `data` and +`key` (both are strings). The `data` field contains the actual signature, +encoded in Base64, the `key` field contains a copy of the public key whose +private key was used to make the signature, in PEM format. Currently only +signatures with Ed25519 keys are defined. + +Before signing the user record should be brought into "normalized" form, +i.e. the keys in all objects should be sorted alphabetically. All redundant +white-space and newlines should be removed and the JSON text then signed. + +The signatures only cover the `regular`, `perMachine` and `privileged` sections +of the user records, all other sections (include `signature` itself), are +removed before the signature is calculated. + +Rationale for signing and threat model: while a multi-user operating system +like Linux strives for being sufficiently secure even after a user acquired a +local login session reality tells us this is not the case. Hence it is +essential to restrict carefully which users may gain access to a system and +which ones shall not. A minimal level of trust must be established between +system, user record and the user themselves before a log-in request may be +permitted. In particular if the home directory is provided in its own LUKS2 +encapsulated file system it is essential this trust is established before the +user logs in (and hence the file system mounted), since file system +implementations on Linux are well known to be relatively vulnerable to rogue +disk images. User records and home directories in many context are expected to +be something shareable between multiple systems, and the transfer between them +might not happen via exclusively trusted channels. Hence it's essential that +the user record is not manipulated between uses. Finally, resource management +(which may be done by the various fields of the user record) is security +sensitive, since it should forcefully lock the user into the assigned resource +usage and not allow them to use more. The requirement of being able to trust +the user record data combined with the potential transfer over untrusted +channels suggest a cryptographic signature mechanism where only user records +signed by a recognized key are permitted to log in locally. + +Note that other mechanisms for establishing sufficient trust exist too, and are +perfectly valid as well. For example, systems like LDAP/ActiveDirectory +generally insist on user record transfer from trusted servers via encrypted TLS +channels only. Or traditional UNIX users created locally in `/etc/passwd` never +exist outside of the local trusted system, hence transfer and trust in the +source are not an issue. The major benefit of operating with signed user +records is that they are self-sufficiently trusted, not relying on a secure +channel for transfer, and thus being compatible with a more distributed model +of home directory transfer, including on USB sticks and such. + +## Fields in the `secret` section + +As mentioned, the `secret` section of the user record should never be persisted +nor transferred across machines. It is only defined in short-lived operations, +for example when a user record is first created or registered, as the secret +key data needs to be available to derive encryption keys from and similar. + +The `secret` field of the top-level user record contains the following fields: + +`password` → an array of strings, each containing a plain text password. + +`tokenPin` → an array of strings, each containing a plain text PIN, suitable +for unlocking security tokens that require that. (The field `pkcs11Pin` should +be considered a compatibility alias for this field, and merged with `tokenPin` +in case both are set.) + +`pkcs11ProtectedAuthenticationPathPermitted` → a boolean. If set to true allows +the receiver to use the PKCS#11 "protected authentication path" (i.e. a +physical button/touch element on the security token) for authenticating the +user. If false or unset, authentication this way shall not be attempted. + +`fido2UserPresencePermitted` → a boolean. If set to true allows the receiver to +use the FIDO2 "user presence" flag. This is similar to the concept of +`pkcs11ProtectedAuthenticationPathPermitted`, but exposes the FIDO2 concept +behind it. If false or unset authentication this way shall not be attempted. + +## Mapping to `struct passwd` and `struct spwd` + +When mapping classic UNIX user records (i.e. `struct passwd` and `struct spwd`) +to JSON user records the following mappings should be applied: + +| Structure | Field | Section | Field | Condition | +|-----------------|-------------|--------------|------------------------------|----------------------------| +| `struct passwd` | `pw_name` | `regular` | `userName` | | +| `struct passwd` | `pw_passwd` | `privileged` | `password` | (See notes below) | +| `struct passwd` | `pw_uid` | `regular` | `uid` | | +| `struct passwd` | `pw_gid` | `regular` | `gid` | | +| `struct passwd` | `pw_gecos` | `regular` | `realName` | | +| `struct passwd` | `pw_dir` | `regular` | `homeDirectory` | | +| `struct passwd` | `pw_shell` | `regular` | `shell` | | +| `struct spwd` | `sp_namp` | `regular` | `userName` | | +| `struct spwd` | `sp_pwdp` | `privileged` | `password` | (See notes below) | +| `struct spwd` | `sp_lstchg` | `regular` | `lastPasswordChangeUSec` | (if `sp_lstchg` > 0) | +| `struct spwd` | `sp_lstchg` | `regular` | `passwordChangeNow` | (if `sp_lstchg` == 0) | +| `struct spwd` | `sp_min` | `regular` | `passwordChangeMinUSec` | | +| `struct spwd` | `sp_max` | `regular` | `passwordChangeMaxUSec` | | +| `struct spwd` | `sp_warn` | `regular` | `passwordChangeWarnUSec` | | +| `struct spwd` | `sp_inact` | `regular` | `passwordChangeInactiveUSec` | | +| `struct spwd` | `sp_expire` | `regular` | `locked` | (if `sp_expire` in [0, 1]) | +| `struct spwd` | `sp_expire` | `regular` | `notAfterUSec` | (if `sp_expire` > 1) | + +At this time almost all Linux machines employ shadow passwords, thus the +`pw_passwd` field in `struct passwd` is set to `"x"`, and the actual password +is stored in the shadow entry `struct spwd`'s field `sp_pwdp`. + +## Extending These Records + +User records following this specifications are supposed to be extendable for +various applications. In general, subsystems are free to introduce their own +keys, as long as: + +* Care should be taken to place the keys in the right section, i.e. the most + appropriate for the data field. + +* Care should be taken to avoid namespace clashes. Please prefix your fields + with a short identifier of your project to avoid ambiguities and + incompatibilities. + +* This specification is supposed to be a living specification. If you need + additional fields, please consider submitting them upstream for inclusion in + this specification. If they are reasonably universally useful, it would be + best to list them here. + +## Examples + +The shortest valid user record looks like this: + +```json +{ + "userName" : "u" +} +``` + +A reasonable user record for a system user might look like this: + +```json +{ + "userName" : "httpd", + "uid" : 473, + "gid" : 473, + "disposition" : "system", + "locked" : true +} +``` + +A fully featured user record associated with a home directory managed by +`systemd-homed.service` might look like this: + +```json +{ + "autoLogin" : true, + "binding" : { + "15e19cf24e004b949ddaac60c74aa165" : { + "fileSystemType" : "ext4", + "fileSystemUuid" : "758e88c8-5851-4a2a-b88f-e7474279c111", + "gid" : 60232, + "homeDirectory" : "/home/grobie", + "imagePath" : "/home/grobie.home", + "luksCipher" : "aes", + "luksCipherMode" : "xts-plain64", + "luksUuid" : "e63581ba-79fb-4226-b9de-1888393f7573", + "luksVolumeKeySize" : 32, + "partitionUuid" : "41f9ce04-c827-4b74-a981-c669f93eb4dc", + "storage" : "luks", + "uid" : 60232 + } + }, + "disposition" : "regular", + "enforcePasswordPolicy" : false, + "lastChangeUSec" : 1565950024279735, + "memberOf" : [ + "wheel" + ], + "privileged" : { + "hashedPassword" : [ + "$6$WHBKvAFFT9jKPA4k$OPY4D4TczKN/jOnJzy54DDuOOagCcvxxybrwMbe1SVdm.Bbr.zOmBdATp.QrwZmvqyr8/SafbbQu.QZ2rRvDs/" + ] + }, + "signature" : [ + { + "data" : "LU/HeVrPZSzi3MJ0PVHwD5m/xf51XDYCrSpbDRNBdtF4fDVhrN0t2I2OqH/1yXiBidXlV0ptMuQVq8KVICdEDw==", + "key" : "-----BEGIN PUBLIC KEY-----\nMCowBQYDK2VwAyEA/QT6kQWOAMhDJf56jBmszEQQpJHqDsGDMZOdiptBgRk=\n-----END PUBLIC KEY-----\n" + } + ], + "userName" : "grobie", + "status" : { + "15e19cf24e004b949ddaac60c74aa165" : { + "goodAuthenticationCounter" : 16, + "lastGoodAuthenticationUSec" : 1566309343044322, + "rateLimitBeginUSec" : 1566309342340723, + "rateLimitCount" : 1, + "state" : "inactive", + "service" : "io.systemd.Home", + "diskSize" : 161118667776, + "diskCeiling" : 190371729408, + "diskFloor" : 5242880, + "signedLocally" : true + } + } +} +``` + +When `systemd-homed.service` manages a home directory it will also include a +version of the user record in the home directory itself in the `~/.identity` +file. This version lacks the `binding` and `status` sections which are used for +local management of the user, but are not intended to be portable between +systems. It would hence look like this: + +```json +{ + "autoLogin" : true, + "disposition" : "regular", + "enforcePasswordPolicy" : false, + "lastChangeUSec" : 1565950024279735, + "memberOf" : [ + "wheel" + ], + "privileged" : { + "hashedPassword" : [ + "$6$WHBKvAFFT9jKPA4k$OPY4D4TczKN/jOnJzy54DDuOOagCcvxxybrwMbe1SVdm.Bbr.zOmBdATp.QrwZmvqyr8/SafbbQu.QZ2rRvDs/" + ] + }, + "signature" : [ + { + "data" : "LU/HeVrPZSzi3MJ0PVHwD5m/xf51XDYCrSpbDRNBdtF4fDVhrN0t2I2OqH/1yXiBidXlV0ptMuQVq8KVICdEDw==", + "key" : "-----BEGIN PUBLIC KEY-----\nMCowBQYDK2VwAyEA/QT6kQWOAMhDJf56jBmszEQQpJHqDsGDMZOdiptBgRk=\n-----END PUBLIC KEY-----\n" + } + ], + "userName" : "grobie", +} +``` diff --git a/docs/_config.yml b/docs/_config.yml new file mode 100644 index 0000000..925b4ad --- /dev/null +++ b/docs/_config.yml @@ -0,0 +1,9 @@ +# Site settings +title: systemd +baseurl: "" # the subpath of your site, e.g. /blog/ +url: "http://systemd.io" # the base hostname & protocol for your site + +permalink: /:title/ + +# Build settings +markdown: kramdown diff --git a/docs/_data/extra_pages.json b/docs/_data/extra_pages.json new file mode 100644 index 0000000..5ede56c --- /dev/null +++ b/docs/_data/extra_pages.json @@ -0,0 +1,10 @@ +[ + { "category": "Project", "title": "Brand", "url": "https://brand.systemd.io/" }, + { "category": "Project", "title": "Releases", "url": "https://github.com/systemd/systemd/releases" }, + { "category": "Project", "title": "GitHub Project Page", "url": "https://github.com/systemd/systemd" }, + { "category": "Project", "title": "Issues", "url": "https://github.com/systemd/systemd/issues" }, + { "category": "Project", "title": "Pull Requests", "url": "https://github.com/systemd/systemd/pulls" }, + { "category": "Project", "title": "Mailing List", "url": "https://lists.freedesktop.org/mailman/listinfo/systemd-devel" }, + { "category": "Manual Pages", "title": "Index", "url": "https://www.freedesktop.org/software/systemd/man/" }, + { "category": "Manual Pages", "title": "Directives", "url": "https://www.freedesktop.org/software/systemd/man/systemd.directives.html" } +] diff --git a/docs/_includes/footer.html b/docs/_includes/footer.html new file mode 100644 index 0000000..9528973 --- /dev/null +++ b/docs/_includes/footer.html @@ -0,0 +1,5 @@ +<footer class="site-footer"> + <p>© systemd, 2020</p> + + <p><a href="https://github.com/systemd/systemd">Website source</a></p> +</footer> diff --git a/docs/_includes/head.html b/docs/_includes/head.html new file mode 100644 index 0000000..3ff4bbe --- /dev/null +++ b/docs/_includes/head.html @@ -0,0 +1,16 @@ +<head> + <meta charset="utf-8"> + <meta http-equiv="X-UA-Compatible" content="IE=edge"> + <meta name="viewport" content="width=device-width, initial-scale=1"> + <meta name="theme-color" content="#0021D8"> + + <title>{% if page.title %}{{ page.title }}{% else %}{{ site.title }}{% endif %}</title> + + <link rel="canonical" href="{{ page.url | replace:'index.html','' | prepend: site.baseurl | prepend: site.url 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100644 index 0000000..1e45115 --- /dev/null +++ b/docs/fonts/heebo-bold.woff diff --git a/docs/fonts/heebo-regular.woff b/docs/fonts/heebo-regular.woff Binary files differnew file mode 100644 index 0000000..484eae4 --- /dev/null +++ b/docs/fonts/heebo-regular.woff diff --git a/docs/index.md b/docs/index.md new file mode 100644 index 0000000..c853d80 --- /dev/null +++ b/docs/index.md @@ -0,0 +1,95 @@ +--- +layout: default +--- + +systemd is a suite of basic building blocks for a Linux system. It provides a system and service manager that runs as PID 1 and starts the rest of the system. + +systemd provides aggressive parallelization capabilities, uses socket and D-Bus activation for starting services, offers on-demand starting of daemons, keeps track of processes using Linux control groups, maintains mount and automount points, and implements an elaborate transactional dependency-based service control logic. systemd supports SysV and LSB init scripts and works as a replacement for sysvinit. + +Other parts include a logging daemon, utilities to control basic system configuration like the hostname, date, locale, maintain a list of logged-in users and running containers and virtual machines, system accounts, runtime directories and settings, and daemons to manage simple network configuration, network time synchronization, log forwarding, and name resolution. + +--- + +{% assign by_category = site.pages | group_by:"category" %} +{% assign extra_pages = site.data.extra_pages | group_by:"category" %} +{% assign merged = by_category | concat: extra_pages | sort:"name" %} + +{% for pair in merged %} + {% if pair.name != "" %} +## {{ pair.name }} +{% assign sorted = pair.items | sort:"title" %}{% for page in sorted %} +* [{{ page.title }}]({{ page.url | relative_url }}){% endfor %} + {% endif %} +{% endfor %} + +## See also + +* [Introductory blog story](http://0pointer.de/blog/projects/systemd.html) +* [Three](http://0pointer.de/blog/projects/systemd-update.html) [status](http://0pointer.de/blog/projects/systemd-update-2.html) [updates](http://0pointer.de/blog/projects/systemd-update-3.html) +* The [Wikipedia article](https://en.wikipedia.org/wiki/systemd) + +--- + +<pre style="color:white; background-color:black; font-size:smaller; padding:6pt 8pt"> +Welcome to <span style="color:blue">Fedora 20 (Heisenbug)</span>! + +[ <span style="color:green">OK</span> ] Reached target Remote File Systems. +[ <span style="color:green">OK</span> ] Listening on Delayed Shutdown Socket. +[ <span style="color:green">OK</span> ] Listening on /dev/initctl Compatibility Named Pipe. +[ <span style="color:green">OK</span> ] Reached target Paths. +[ <span style="color:green">OK</span> ] Reached target Encrypted Volumes. +[ <span style="color:green">OK</span> ] Listening on Journal Socket. + Mounting Huge Pages File System... + Mounting POSIX Message Queue File System... + Mounting Debug File System... + Starting Journal Service... +[ <span style="color:green">OK</span> ] Started Journal Service. + Mounting Configuration File System... + Mounting FUSE Control File System... +[ <span style="color:green">OK</span> ] Created slice Root Slice. +[ <span style="color:green">OK</span> ] Created slice User and Session Slice. +[ <span style="color:green">OK</span> ] Created slice System Slice. +[ <span style="color:green">OK</span> ] Reached target Slices. +[ <span style="color:green">OK</span> ] Reached target Swap. + Mounting Temporary Directory... +[ <span style="color:green">OK</span> ] Reached target Local File Systems (Pre). + Starting Load Random Seed... + Starting Load/Save Random Seed... +[ <span style="color:green">OK</span> ] Mounted Huge Pages File System. +[ <span style="color:green">OK</span> ] Mounted POSIX Message Queue File System. +[ <span style="color:green">OK</span> ] Mounted Debug File System. +[ <span style="color:green">OK</span> ] Mounted Configuration File System. +[ <span style="color:green">OK</span> ] Mounted FUSE Control File System. +[ <span style="color:green">OK</span> ] Mounted Temporary Directory. +[ <span style="color:green">OK</span> ] Started Load Random Seed. +[ <span style="color:green">OK</span> ] Started Load/Save Random Seed. +[ <span style="color:green">OK</span> ] Reached target Local File Systems. + Starting Recreate Volatile Files and Directories... + Starting Trigger Flushing of Journal to Persistent Storage... +[ <span style="color:green">OK</span> ] Started Recreate Volatile Files and Directories. + Starting Update UTMP about System Reboot/Shutdown... +[ <span style="color:green">OK</span> ] Started Trigger Flushing of Journal to Persistent Storage. +[ <span style="color:green">OK</span> ] Started Update UTMP about System Reboot/Shutdown. +[ <span style="color:green">OK</span> ] Reached target System Initialization. +[ <span style="color:green">OK</span> ] Reached target Timers. +[ <span style="color:green">OK</span> ] Listening on D-Bus System Message Bus Socket. +[ <span style="color:green">OK</span> ] Reached target Sockets. +[ <span style="color:green">OK</span> ] Reached target Basic System. + Starting Permit User Sessions... + Starting D-Bus System Message Bus... +[ <span style="color:green">OK</span> ] Started D-Bus System Message Bus. + Starting Login Service... + Starting Cleanup of Temporary Directories... +[ <span style="color:green">OK</span> ] Started Permit User Sessions. +[ <span style="color:green">OK</span> ] Started Cleanup of Temporary Directories. + Starting Console Getty... +[ <span style="color:green">OK</span> ] Started Console Getty. +[ <span style="color:green">OK</span> ] Reached target Login Prompts. +[ <span style="color:green">OK</span> ] Started Login Service. +[ <span style="color:green">OK</span> ] Reached target Multi-User System. + +Fedora release 20 (Heisenbug) +Kernel 3.9.2-200.fc18.x86_64 on an x86_64 (console) + +fedora login: +</pre> diff --git a/docs/style.css b/docs/style.css new file mode 100644 index 0000000..8a2ae71 --- /dev/null +++ b/docs/style.css @@ -0,0 +1,347 @@ +@font-face { + font-family: 'Heebo'; 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+ background-color: rgba(0,0,0, 0.07); +} diff --git a/docs/sysvinit/README.in b/docs/sysvinit/README.in new file mode 100644 index 0000000..de5d80d --- /dev/null +++ b/docs/sysvinit/README.in @@ -0,0 +1,27 @@ +You are looking for the traditional init scripts in @SYSTEM_SYSVINIT_PATH@, +and they are gone? + +Here's an explanation on what's going on: + +You are running a systemd-based OS where traditional init scripts have +been replaced by native systemd services files. Service files provide +very similar functionality to init scripts. To make use of service +files simply invoke "systemctl", which will output a list of all +currently running services (and other units). Use "systemctl +list-unit-files" to get a listing of all known unit files, including +stopped, disabled and masked ones. Use "systemctl start +foobar.service" and "systemctl stop foobar.service" to start or stop a +service, respectively. For further details, please refer to +systemctl(1). + +Note that traditional init scripts continue to function on a systemd +system. An init script @SYSTEM_SYSVINIT_PATH@/foobar is implicitly mapped +into a service unit foobar.service during system initialization. + +Thank you! + +Further reading: + man:systemctl(1) + man:systemd(1) + http://0pointer.de/blog/projects/systemd-for-admins-3.html + https://www.freedesktop.org/wiki/Software/systemd/Incompatibilities diff --git a/docs/sysvinit/meson.build b/docs/sysvinit/meson.build new file mode 100644 index 0000000..261bd68 --- /dev/null +++ b/docs/sysvinit/meson.build @@ -0,0 +1,11 @@ +# SPDX-License-Identifier: LGPL-2.1-or-later + +file = configure_file( + input : 'README.in', + output : 'README', + configuration : substs) + +if conf.get('HAVE_SYSV_COMPAT') == 1 + install_data(file, + install_dir : sysvinit_path) +endif diff --git a/docs/var-log/README b/docs/var-log/README new file mode 100644 index 0000000..5c6c592 --- /dev/null +++ b/docs/var-log/README @@ -0,0 +1,25 @@ +You are looking for the traditional text log files in /var/log, and they are +gone? + +Here's an explanation on what's going on: + +You are running a systemd-based OS where traditional syslog has been replaced +with the Journal. The journal stores the same (and more) information as classic +syslog. To make use of the journal and access the collected log data simply +invoke "journalctl", which will output the logs in the identical text-based +format the syslog files in /var/log used to be. For further details, please +refer to journalctl(1). + +Alternatively, consider installing one of the traditional syslog +implementations available for your distribution, which will generate the +classic log files for you. Syslog implementations such as syslog-ng or rsyslog +may be installed side-by-side with the journal and will continue to function +the way they always did. + +Thank you! + +Further reading: + man:journalctl(1) + man:systemd-journald.service(8) + man:journald.conf(5) + http://0pointer.de/blog/projects/the-journal.html diff --git a/docs/var-log/meson.build b/docs/var-log/meson.build new file mode 100644 index 0000000..ba79577 --- /dev/null +++ b/docs/var-log/meson.build @@ -0,0 +1,6 @@ +# SPDX-License-Identifier: LGPL-2.1-or-later + +if conf.get('HAVE_SYSV_COMPAT') == 1 and get_option('create-log-dirs') + install_data('README', + install_dir : '/var/log') +endif |
