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 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)
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)
boot loader optionally maintains a per-boot-loader-entry counter described by
the Boot Loader Specification
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))
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)
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)
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.
As described in Boot Loader Specification,
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.
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 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.
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 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:
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.
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.
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.
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.
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 for this entry.
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 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.
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,
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.
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.
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.
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.