Bug#862992: systemd: avoid attempt to re-create /etc/mtab by systemd-tmpfiles-setup.service

[Michael Biebl]

Am 19.05.2017 um 19:58 schrieb Maximilian Stein:
> Package: systemd
> Version: 232-23
> Severity: normal
> 
> Dear Maintainer,
> 
> After having upgraded to Stretch, I noticed that
> systemd-tmpfiles-setup.service attempts to create /etc/mtab as
> configured in /usr/lib/tmpfiles.d/debian.conf. Unfortunately, this
> does not work on some of my machines where / is mounted read-only and
> consequently, systemd-tmpfiles-setup.service fails.
> 
> Moreover, this step does not seem to be necessary, as /etc/mtab is
> symlinked to /proc/mounts after a fresh debootstrap. /proc/mounts in
> turn symlinks to /proc/self/mounts, the same file as configured in
> /usr/lib/tmpfiles.d/debian.conf.
> 
> What exactly are the reasons for re-creating /etc/mtab on every boot? Is
> this step really necessary?

Well, it's not really re-created on every boot, just if /etc/mtab is
*not* a relative symlink to ../proc/self/mounts.

debootstrap from sid creates /etc/mtab as relative symlink to
../proc/mounts afaics (the version from jessie seems to create it as an
absolute symlink to /proc/mounts)

So I guess the problem here is that there is a mismatch between what
debootstrap creates and what systemd-tmpfiles (i.e. debian.conf) expects.

In jessie we handled this slightly differently [1]. We had a dedicated
service unit which checked if /etc/mtab was a symlink. So we didn't run
into the issue there, that the symlink can be absolute or relative and
point to either /proc/mounts or /proc/self/mounts.

We chose ../proc/self/mounts in debian.conf since that's also what's
used by systemd upstream [2], i.e. we are consistent with other distros
in that aspect.

Maybe we can change debootstrap to use ../proc/self/mounts or is there a
good reason why it should point to ../proc/mounts?

CCed the debootstrap maintainers for their input.

Regards,
Michael



[1]
https://anonscm.debian.org/cgit/pkg-systemd/systemd.git/commit/?id=b3ba0e60
[2]
https://github.com/systemd/systemd/blob/master/tmpfiles.d/etc.conf.m4#L12
-- 
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