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204 lines
10 KiB
Markdown
204 lines
10 KiB
Markdown
# 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. This
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support is built into various of its components. When used together these
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components provide a complete solution on UEFI systems, built as add-on to the
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[Boot Loader
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Specification](https://systemd.io/BOOT_LOADER_SPECIFICATION). However, the
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different components may also be used independently, and in combination with
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other software, to implement similar schemes, for example with other boot
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loaders or for non-UEFI systems. Here's a brief overview of the complete set of
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components:
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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 that is
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decreased by one on each attempt to boot the entry, prioritizing entries that
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have non-zero counters over those which already reached a counter of zero
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when choosing the entry to boot.
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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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* 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 health check tool that determines whether the boot
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completed successfully. When enabled it becomes an indirect dependency of
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`systemd-bless-boot.service` (by means of `boot-complete.target`, see
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below), ensuring that the boot will not be considered successful if there are
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any failed services.
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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 shall be considered
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necessary to consider a boot successful on one side (example:
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`systemd-boot-check-no-failures.service` as described above), and units that
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want to act only if the boot is successful on the other (example:
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`systemd-bless-boot.service` as described above).
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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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# Details
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The boot counting data `systemd-boot` and `systemd-bless-boot.service`
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manage is stored in the name of the boot loader entries. If a boot loader entry
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file name contains `+` followed by one or two numbers (if two numbers, then
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those need to be separated by `-`) right before the `.conf` suffix, then boot
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counting is enabled for it. The first number is the "tries left" counter
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encoding how many attempts to boot this entry shall still be made. The second
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number is the "tries done" counter, encoding how many failed attempts to boot
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it have already been made. Each time a boot loader entry marked this way is
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booted the first counter is decreased by one, and the second one increased by
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one. (If the second counter is missing, then it is assumed to be equivalent to
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zero.) If the "tries left" counter is above zero the entry is still considered
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for booting (the entry's state is considered to be "indeterminate"), as soon as
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it reached zero the entry is not tried anymore (entry state "bad"). If the boot
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attempt completed successfully the entry's counters are removed from the name
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(entry state "good"), thus 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 is
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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 and thus only one less
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is left. 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 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 hence will
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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 the
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tag `+0-3`, i.e. the counter reached zero. At this point the entry will be
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considered "bad", and ordered to the end of the list of entries. The next
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newest boot entry is now tried, i.e. the system automatically reverted back
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to an 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. `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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10. 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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11. 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.
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12. 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 simply
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implement an equivalent file rename based logic, and thus integrate fully
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with 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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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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# FAQ
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1. *Why do you use file renames to store the counter? Why not a regular file?*
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— Mainly two reasons: it's relatively likely that renames can be implemented
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atomically even in simpler file systems, while writing to file contents has
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a much bigger chance to be result in incomplete or corrupt data, as renaming
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generally avoids allocating or releasing data blocks. Moreover it has the
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benefit that the boot count metadata is directly attached to the boot loader
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entry file, and thus the lifecycle of the metadata and the entry itself are
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bound together. This means no additional clean-up needs to take place to
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drop the boot loader counting information for an entry when it is removed.
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2. *Why not use EFI variables for storing the boot counter?* — The memory chips
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used to back the persistent EFI variables are generally not of the highest
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quality, hence shouldn't be written to more than necessary. This means we
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can't really use it for changes made regularly during boot, but can use it
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only for seldom made configuration changes.
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3. *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?* — Well, that's orthogonal to
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the above, please use `FailureAction=` in the unit file for this.
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4. *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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