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diff --git a/docs/AUTOMATIC_BOOT_ASSESSMENT.md b/docs/AUTOMATIC_BOOT_ASSESSMENT.md new file mode 100644 index 0000000..2fbf86e --- /dev/null +++ b/docs/AUTOMATIC_BOOT_ASSESSMENT.md @@ -0,0 +1,219 @@ +--- +title: Automatic Boot Assessment +category: Booting +layout: default +SPDX-License-Identifier: LGPL-2.1-or-later +--- + +# 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. The +[Boot Loader Specification](https://uapi-group.org/specifications/specs/boot_loader_specification/#boot-counting) +describes how to annotate boot loader entries with a counter that specifies how +many attempts should be made to boot it. This document describes how systemd +implements this scheme. + +The many different components involved in the implementation may be used +independently and in combination with other software to, for example, support +other boot loaders or take actions outside of the boot loader. + +Here's a brief overview of the complete set of components: + +* 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`). + +* The + [`systemd-boot(7)`](https://www.freedesktop.org/software/systemd/man/systemd-boot.html) + boot loader optionally maintains a per-boot-loader-entry counter described by + the [Boot Loader Specification](https://uapi-group.org/specifications/specs/boot_loader_specification/#boot-counting) + 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 `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 (e.g. `systemd-boot-check-no-failures.service` + described below), and units that want to act only if the boot is + successful (e.g. `systemd-bless-boot.service` described below). + +* 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 service health check tool. 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 + [`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. + +## Details + +As described in the +[Boot Loader Specification](https://uapi-group.org/specifications/specs/boot_loader_specification/#boot-counting), +the boot counting data is stored in the file name of the boot loader entries as +a plus (`+`), followed by a number, optionally followed by `-` and another +number, right before the file name suffix (`.conf` or `.efi`). + +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 decremented, +and the second one incremented. (If the second counter is missing, then it is +assumed to be equivalent to zero.) 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 has been + 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. + 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 will 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 will hence + 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 after all non-bad entries. The next newest + boot entry is now tried, i.e. the system automatically reverted 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. The graphical desktop environment installed on the machine starts a + service called `graphical-session-good.service`, which is also ordered before + `boot-complete.target`, that registers a D-Bus endpoint. + +10. `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. + +11. `graphical-session-good.service` waits for a user to log in. In the user + desktop environment, one minute after the user has logged in and started the + first program, a user service is invoked which makes a D-Bus call to + `graphical-session-good.service`. Upon receiving that call, + `graphical-session-good.service` exits cleanly. + +12. This allows `boot-complete.target` to be reached. This signifies to the + system that this boot attempt shall be considered successful. + +13. 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 for this entry. + +14. 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 the same rename 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. + + Depending on the setup, it may be most convenient to pull in such units + through normal enablement symlinks, or during early boot using a + [`generator`](https://www.freedesktop.org/software/systemd/man/systemd.generator.html), + or even during later boot. In the last case, care must be taken to ensure + that the start job is created before `boot-complete.target` has been + reached. + +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. + + Such unit would be typically wanted (or required) by one of the + [`bootup`](https://www.freedesktop.org/software/systemd/man/bootup.html) targets, + for example, `multi-user.target`. To avoid potential loops due to conflicting + [default dependencies](https://www.freedesktop.org/software/systemd/man/systemd.unit.html#Default%20Dependencies) + ordering, it is recommended to also add an explicit dependency (e.g. + `After=multi-user.target`) to the unit. This overrides the implicit ordering + and allows `boot-complete.target` to start after the given bootup target. + +## FAQ + +1. *I have a service which — when it fails — should immediately cause a + reboot. How does that fit in with the above?* — That's orthogonal to + the above, please use `FailureAction=` in the unit file for this. + +2. *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. |