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docs/AUTOMATIC_BOOT_ASSESSMENT: describe how to integrate a DE viability check

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I reordered the component list to match chronological order: we first install
an entry, then boot it, then the checks happen, etc. Before it was
ordered by "importance", but that is harder to follow.
This commit is contained in:
Zbigniew Jędrzejewski-Szmek 2022-06-09 12:21:23 +02:00
parent db82e667c7
commit 82a0ffe585
1 changed files with 45 additions and 28 deletions
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73
docs/AUTOMATIC_BOOT_ASSESSMENT.md
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@ -20,12 +20,32 @@ 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 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.
boot loader optionally maintains a per-boot-loader-entry counter described by
the [Boot Loader Specification](BOOT_LOADER_SPECIFICATION.md#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)
@ -38,26 +58,6 @@ Here's a brief overview of the complete set of components:
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)`](https://www.freedesktop.org/software/systemd/man/systemd-boot-check-no-failures.service.html)
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)`](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 (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)`](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`).
## Details
As described in [Boot Loader Specification](BOOT_LOADER_SPECIFICATION.md#boot-counting),
@ -132,21 +132,31 @@ scenario the first 4 steps are the same as above:
that are required to succeed for the boot process to be considered
successful. One such unit is `systemd-boot-check-no-failures.service`.
9. `systemd-boot-check-no-failures.service` is run after all its own
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.
10. This allows `boot-complete.target` to be reached. This signifies to the
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.
11. Which in turn permits `systemd-bless-boot.service` to run. It now
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.
12. On the following boot (and all subsequent boots after that) the entry is
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
@ -176,6 +186,13 @@ are a couple of recommendations.
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.