[systemd-devel] Fwd: Systemd boot

Riccardo Paolo Bestetti pbl at bestov.io
Sat Nov 28 19:41:06 UTC 2020

I'm trying to fully make sense of the boot process with systemd.

I've read various pages from the manual, including bootup(7). There are
two points I don't fully understand.

* Filesystem mounts during initrd
The man page, under the initrd section, says: "systemd detects that it
is run within an initrd [...]. The bootup process begins identical to
the system manager bootup (see above) until it reaches basic.target.
[...] Before any file systems are mounted, it must be determined whether
the system will resume from hibernation or proceed with normal boot."

In my mind, that part self-contradicts when both saying that 1) the
bootup proceeds identical to the system manager bootup and 2) a
determination on whether to mount any file systems is made /after/
basic.target. This is because some file systems (including, possibly the
root file systems) would have been mounted before local-fs.target, which
is ordered before basic.target.

So either the process is not really identical until basic.target, or I'm
missing something.

* Instances
I gather that the systemd which runs inside the initrd is (at least
functionally) a completely separate instance from the one which then
runs in the booted system.  Which implies - as discussed above - that
the system initialization sequence from beginning to basic.target
actually happens twice.

Does this means than any initialization units which could potentially be
run twice - once in the initrd and once in the booted system - should be
instrumented to avoid running their logic twice (in the cases where
that's not needed or even harmful)?

Let's consider for example an hypothetical service with
"WantedBy=cryptsetup-pre.target", which decrypts a keyfile using an
hardware token. This keyfile is then consumed by
systemd-cryptsetup-generator with keyfile-erase=on. What happens if the
service is run twice, but the key is consumed only the first time? A
decrypted keyfile remains in the system.

Is this an issue or am I once again missing something? What would be the
correct approach to avoid such a scenario?


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