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220 lines
11 KiB
Markdown
220 lines
11 KiB
Markdown
---
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title: Automatic Boot Assessment
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category: Booting
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layout: default
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SPDX-License-Identifier: LGPL-2.1-or-later
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---
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# Automatic Boot Assessment
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systemd provides support for automatically reverting back to the previous
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version of the OS or kernel in case the system consistently fails to boot. The
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[Boot Loader Specification](https://uapi-group.org/specifications/specs/boot_loader_specification/#boot-counting)
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describes how to annotate boot loader entries with a counter that specifies how
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many attempts should be made to boot it. This document describes how systemd
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implements this scheme.
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The many different components involved in the implementation may be used
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independently and in combination with other software to, for example, support
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other boot loaders or take actions outside of the boot loader.
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Here's a brief overview of the complete set of components:
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* The
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[`kernel-install(8)`](https://www.freedesktop.org/software/systemd/man/kernel-install.html)
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script can optionally create boot loader entries that carry an initial boot
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counter (the initial counter is configurable in `/etc/kernel/tries`).
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* The
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[`systemd-boot(7)`](https://www.freedesktop.org/software/systemd/man/systemd-boot.html)
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boot loader optionally maintains a per-boot-loader-entry counter described by
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the [Boot Loader Specification](https://uapi-group.org/specifications/specs/boot_loader_specification/#boot-counting)
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that is decreased by one on each attempt to boot the entry, prioritizing
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entries that have non-zero counters over those which already reached a
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counter of zero when choosing the entry to boot.
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* The `boot-complete.target` target unit (see
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[`systemd.special(7)`](https://www.freedesktop.org/software/systemd/man/systemd.special.html))
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serves as a generic extension point both for units that are necessary to
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consider a boot successful (e.g. `systemd-boot-check-no-failures.service`
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described below), and units that want to act only if the boot is
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successful (e.g. `systemd-bless-boot.service` described below).
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* The
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[`systemd-boot-check-no-failures.service(8)`](https://www.freedesktop.org/software/systemd/man/systemd-boot-check-no-failures.service.html)
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service is a simple service health check tool. When enabled it becomes an
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indirect dependency of `systemd-bless-boot.service` (by means of
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`boot-complete.target`, see below), ensuring that the boot will not be
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considered successful if there are any failed services.
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* The
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[`systemd-bless-boot.service(8)`](https://www.freedesktop.org/software/systemd/man/systemd-bless-boot.service.html)
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service automatically marks a boot loader entry, for which boot counting as
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mentioned above is enabled, as "good" when a boot has been determined to be
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successful, thus turning off boot counting for it.
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* The
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[`systemd-bless-boot-generator(8)`](https://www.freedesktop.org/software/systemd/man/systemd-bless-boot-generator.html)
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generator automatically pulls in `systemd-bless-boot.service` when use of
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`systemd-boot` with boot counting enabled is detected.
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## Details
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As described in the
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[Boot Loader Specification](https://uapi-group.org/specifications/specs/boot_loader_specification/#boot-counting),
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the boot counting data is stored in the file name of the boot loader entries as
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a plus (`+`), followed by a number, optionally followed by `-` and another
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number, right before the file name suffix (`.conf` or `.efi`).
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The first number is the "tries left" counter encoding how many attempts to boot
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this entry shall still be made. The second number is the "tries done" counter,
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encoding how many failed attempts to boot it have already been made. Each time
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a boot loader entry marked this way is booted the first counter is decremented,
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and the second one incremented. (If the second counter is missing, then it is
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assumed to be equivalent to zero.) If the boot attempt completed successfully
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the entry's counters are removed from the name (entry state "good"), thus
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turning off boot counting for the future.
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## Walkthrough
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Here's an example walkthrough of how this all fits together.
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1. The user runs `echo 3 >/etc/kernel/tries` to enable boot counting.
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2. A new kernel is installed. `kernel-install` is used to generate a new boot
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loader entry file for it. Let's say the version string for the new kernel is
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`4.14.11-300.fc27.x86_64`, a new boot loader entry
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`/boot/loader/entries/4.14.11-300.fc27.x86_64+3.conf` is hence created.
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3. The system is booted for the first time after the new kernel has been
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installed. The boot loader now sees the `+3` counter in the entry file
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name. It hence renames the file to `4.14.11-300.fc27.x86_64+2-1.conf`
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indicating that at this point one attempt has started.
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After the rename completed, the entry is booted as usual.
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4. Let's say this attempt to boot fails. On the following boot the boot loader
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will hence see the `+2-1` tag in the name, and will hence rename the entry file to
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`4.14.11-300.fc27.x86_64+1-2.conf`, and boot it.
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5. Let's say the boot fails again. On the subsequent boot the loader will hence
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see the `+1-2` tag, and rename the file to
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`4.14.11-300.fc27.x86_64+0-3.conf` and boot it.
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6. If this boot also fails, on the next boot the boot loader will see the tag
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`+0-3`, i.e. the counter reached zero. At this point the entry will be
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considered "bad", and ordered after all non-bad entries. The next newest
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boot entry is now tried, i.e. the system automatically reverted to an
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earlier version.
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The above describes the walkthrough when the selected boot entry continuously
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fails. Let's have a look at an alternative ending to this walkthrough. In this
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scenario the first 4 steps are the same as above:
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1. *as above*
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2. *as above*
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3. *as above*
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4. *as above*
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5. Let's say the second boot succeeds. The kernel initializes properly, systemd
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is started and invokes all generators.
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6. One of the generators started is `systemd-bless-boot-generator` which
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detects that boot counting is used. It hence pulls
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`systemd-bless-boot.service` into the initial transaction.
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7. `systemd-bless-boot.service` is ordered after and `Requires=` the generic
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`boot-complete.target` unit. This unit is hence also pulled into the initial
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transaction.
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8. The `boot-complete.target` unit is ordered after and pulls in various units
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that are required to succeed for the boot process to be considered
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successful. One such unit is `systemd-boot-check-no-failures.service`.
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9. The graphical desktop environment installed on the machine starts a
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service called `graphical-session-good.service`, which is also ordered before
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`boot-complete.target`, that registers a D-Bus endpoint.
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10. `systemd-boot-check-no-failures.service` is run after all its own
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dependencies completed, and assesses that the boot completed
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successfully. It hence exits cleanly.
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11. `graphical-session-good.service` waits for a user to log in. In the user
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desktop environment, one minute after the user has logged in and started the
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first program, a user service is invoked which makes a D-Bus call to
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`graphical-session-good.service`. Upon receiving that call,
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`graphical-session-good.service` exits cleanly.
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12. This allows `boot-complete.target` to be reached. This signifies to the
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system that this boot attempt shall be considered successful.
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13. Which in turn permits `systemd-bless-boot.service` to run. It now
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determines which boot loader entry file was used to boot the system, and
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renames it dropping the counter tag. Thus
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`4.14.11-300.fc27.x86_64+1-2.conf` is renamed to
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`4.14.11-300.fc27.x86_64.conf`. From this moment boot counting is turned
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off for this entry.
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14. On the following boot (and all subsequent boots after that) the entry is
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now seen with boot counting turned off, no further renaming takes place.
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## How to adapt this scheme to other setups
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Of the stack described above many components may be replaced or augmented. Here
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are a couple of recommendations.
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1. To support alternative boot loaders in place of `systemd-boot` two scenarios
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are recommended:
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a. Boot loaders already implementing the Boot Loader Specification can
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simply implement the same rename logic, and thus integrate fully with
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the rest of the stack.
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b. Boot loaders that want to implement boot counting and store the counters
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elsewhere can provide their own replacements for
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`systemd-bless-boot.service` and `systemd-bless-boot-generator`, but should
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continue to use `boot-complete.target` and thus support any services
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ordered before that.
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2. To support additional components that shall succeed before the boot is
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considered successful, simply place them in units (if they aren't already)
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and order them before the generic `boot-complete.target` target unit,
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combined with `Requires=` dependencies from the target, so that the target
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cannot be reached when any of the units fail. You may add any number of
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units like this, and only if they all succeed the boot entry is marked as
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good. Note that the target unit shall pull in these boot checking units, not
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the other way around.
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Depending on the setup, it may be most convenient to pull in such units
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through normal enablement symlinks, or during early boot using a
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[`generator`](https://www.freedesktop.org/software/systemd/man/systemd.generator.html),
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or even during later boot. In the last case, care must be taken to ensure
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that the start job is created before `boot-complete.target` has been
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reached.
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3. To support additional components that shall only run on boot success, simply
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wrap them in a unit and order them after `boot-complete.target`, pulling it
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in.
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Such unit would be typically wanted (or required) by one of the
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[`bootup`](https://www.freedesktop.org/software/systemd/man/bootup.html) targets,
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for example, `multi-user.target`. To avoid potential loops due to conflicting
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[default dependencies](https://www.freedesktop.org/software/systemd/man/systemd.unit.html#Default%20Dependencies)
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ordering, it is recommended to also add an explicit dependency (e.g.
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`After=multi-user.target`) to the unit. This overrides the implicit ordering
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and allows `boot-complete.target` to start after the given bootup target.
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## FAQ
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1. *I have a service which — when it fails — should immediately cause a
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reboot. How does that fit in with the above?* — That's orthogonal to
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the above, please use `FailureAction=` in the unit file for this.
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2. *Under some condition I want to mark the current boot loader entry as bad
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right-away, so that it never is tried again, how do I do that?* — You may
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invoke `/usr/lib/systemd/systemd-bless-boot bad` at any time to mark the
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current boot loader entry as "bad" right-away so that it isn't tried again
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on later boots.
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