Subject: bug#36508: GDM files have incorrect owner after temporarily removing service

From: Ludovic Courtès <[email-masked]>
Date: Wed, 14 Apr 2021 12:32:48 +0200

Hi Mark,

Mark H Weaver <[email-masked]> skribis:

> Brendan Tildesley via Bug reports for GNU Guix <[email-masked]>
> writes:
>
>> I recently encountered what is likely the same bug. The directory /var/lib/gdm
>> had the correct permissions gdm:gdm, but all the files inside had something like
>> 973:gdm
>
> The underlying problem here, which I've also experienced, is that if you
> reconfigure your system with fewer users/groups, and then later add
> those users/groups back, there is no guarantee that they will be
> assigned the same UIDs and GIDs.

Yes.

The patch Brendan posted LGTM (though I’m surprised the directory itself
can have the right UID/GID while files inside it don’t; perhaps this was
made possible by 2161820ebbbab62a5ce76c9101ebaec54dc61586, which chowns
the home directory unconditionally.)

Note that there are other places, in addition to GDM, where we
forcefully reset the UID/GID of the home directory (e.g., for the
‘knot-resolver’ service.)

My preferred solution to this would be to unconditionally chown -R home
directories upon activation (for efficiency, it would be best if we
could do that if and only if the home directory itself has wrong
ownership).  Thoughts?

systemd-homed does something like that.  The intuition here is that
UIDs/GIDs are implementation details that should get out of the way.

> There's some discussion of this issue at <https://bugs.gnu.org/44944>,
> although I'm not sure that Danny's suggested solution is practical.
>
> Here's one idea: when activating a system, *never* delete users or
> groups if files still exist that are owned by those users/groups.
> Checking all filesystems would likely be too expensive, but perhaps it
> would be sufficient to check certain directories such as /var, /etc, and
> possibly the top directory of /home.

How would you determine which directories to look at though?  What if we
miss an important one?

Note that the ID allocation strategy in (gnu build accounts) ensures
UIDs/GIDs aren’t reused right away (same strategy as implemented by
Shadow, etc.).  So if you remove “bob”, then add “alice”, “alice” won’t
be able to access the left-behind /home/bob because it has a different
UID.

Ludo’.