bug#35996: User account password got locked when booting old generation

["Ludovic Courtès" <ludo@gnu.org>]

Hi Florian,

"pelzflorian (Florian Pelz)" <pelzflorian@pelzflorian.de> skribis:

> On Sat, Jun 01, 2019 at 07:52:38AM +0200, pelzflorian (Florian Pelz) wrote:
>> I wonder what would change /etc/shadow.
>> 
>
> If the error occurred on common non-Guix distros, it hopefully would
> have been fixed before, maybe.  Of course Guix recreates /etc/shadow
> much more frequently.

Definitely.

> Guix appears to add shadow files atomically in gnu/build/accounts.scm.
> I do not know if there could have been an error reading the old shadow
> file, e.g. because it is locked or something?

(gnu build accounts) doesn’t care at all about /etc/.pwd.lock, the lock
file used by libc’s ‘lckpwdf’ function.

This is definitely not a problem when booting.  It could be a problem if
you’re concurrently running ‘guix system reconfigure’ (which runs
activation snippets, including the account updating code) and some other
program, such as ‘passwd’, that assumes it holds an exclusive lock on
the file.  Though in that case, the worst that could happen is that the
changes made by Guix would be undoed by that other program.

> The elogind source code in src/basic/user-util.c contains code for
> locking /etc/shadow, with a comment that explains why its lckpwdf is
> implemented differently from shadow-utils.
>
> AccountsService appears to only be usable for reading /etc/shadow, not
> for writing it, contrary to what the Guix manual claims (??). 

That might be a bug.

> For writing passwords, gnome-control-center does not use
> AccountsService, it calls /usr/bin/passwd directly in its source code
> in panels/user-accounts/run-passwd.c.

That’s definitely a bug to fix: it should invoke
/run/setuid-programs/passwd instead.

Thanks for investigating,
Ludo’.