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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. |