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Re: A corner case of broken reproducibility

From: Ludovic Courtès
Subject: Re: A corner case of broken reproducibility
Date: Wed, 01 Jun 2022 18:38:46 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/28.1 (gnu/linux)

Maxime Devos <> skribis:

> I don't think the problem is that the uid of /home/... was wrong,
> rather I think the problem is that Guix has forgotten the uid and hence
> invents a new one to put in /etc/passwd instead of keeping the old one.
> A pitfall (noticed in the context of system accounts): the user could
> have created files outside /home (e.g. in /tmp).  IIUC, this would also
> require a reboot to keep name<->uid consistent after "guix system
> reconfigure".
> A 'chown -R' doesn't seem great to me from a security perspective
> (seems very easy to get something wrong, and the TOCTTOU-free chownat
> hasn't been merged yet in Guile), a performance perspective (what if
> you have a huge $HOME).  Also extra io -> slower boot + disk wear.
> It also destroys some information, it's possible to intentionally have
> files owned by other users inside $HOME.

There’s a talk by Lennart Poettering where he explains that, contrary to
what one might think, “chown -R $HOME” turns out to be fast enough that
systemd-homed can do that unconditionally (off the of my head).

> Things that seem missing here to me:
>   * a mechanism for remembering that an uid is still in use even though
>     the user has been removed (previously mentioned solutions: keep the
>     uid in /etc/passwd even though it is ‘removed’, or keep a separate
>     /etc/passwd-graveyard or such, etc.).  For system accounts and user
>     accounts.  Won't help in this particular case but would make more
>     general adding/removing user accounts less fragile (avoid 
>     accidental reuse).

How do you know that user “maxime” created today is “the same” as that
“maxime” deleted a while back?  You can’t.

(gnu build accounts) is stateful in that it makes sure UIDs aren’t
reused.  (This is roughly the same algorithm as used by Shadow.)

>   * a mechanism for telling Guix ‘I'm renaming the user account, not
>     creating and removing a new one, so keep the uid’

Every system generation stands alone though; it’s functional, stateless,
and all that.  What does “rename” mean in this context?

>   * some heuristics for detecting mistakes (e.g.: if Guix thinks it
>     should create a directory /home/foo for uid 1234, but it notices
>     there is already a directory /home/bar with that uid 1234, then
>     that's super suspicious.  Likewise, if Guix thinks the home
>     /home/foo should be owned by uid 1234, but it notices it's already
>     owned by 1235!=1234, that's also suspicious).


>   * some mechanism for resolving mistakes 



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