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. 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)
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)
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)
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)
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))
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)
script can optionally create boot loader entries that carry an initial boot
counter (the initial counter is configurable in /etc/kernel/tries).
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.
Here’s an example walkthrough of how this all fits together.
The user runs echo 3 > /etc/kernel/tries to enable boot counting.
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.
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.
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.
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.
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:
as above
as above
as above
as above
Let’s say the second boot succeeds. The kernel initializes properly, systemd is started and invokes all generators.
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.
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.
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.
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.
This allows boot-complete.target to be reached. This signifies to the
system that this boot attempt shall be considered successful.
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.
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.
Of the stack described above many components may be replaced or augmented. Here are a couple of recommendations.
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.
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.
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.
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.
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.
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.
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.