From: "Ludovic Courtès" <ludo@gnu.org>
To: Maxime Devos <maximedevos@telenet.be>
Cc: Felix Lechner <felix.lechner@gmail.com>,
Guix Devel <guix-devel@gnu.org>
Subject: Re: A corner case of broken reproducibility
Date: Wed, 01 Jun 2022 18:38:46 +0200 [thread overview]
Message-ID: <87bkvcuynd.fsf@gnu.org> (raw)
In-Reply-To: <31d13a6014b14c886686bca833d926604ce3379d.camel@telenet.be> (Maxime Devos's message of "Mon, 30 May 2022 22:50:10 +0200")
Maxime Devos <maximedevos@telenet.be> 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).
Yeah.
> * some mechanism for resolving mistakes
Sure.
Ludo’.
next prev parent reply other threads:[~2022-06-01 16:39 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-05-25 6:10 A corner case of broken reproducibility Blake Shaw
2022-05-25 11:35 ` Felix Lechner
2022-05-30 15:58 ` Ludovic Courtès
2022-05-30 20:50 ` Maxime Devos
2022-06-01 16:38 ` Ludovic Courtès [this message]
2022-06-01 19:59 ` Maxime Devos
2022-06-02 14:13 ` Ludovic Courtès
2022-06-02 14:43 ` Maxime Devos
2022-06-01 20:09 ` Maxime Devos
2022-06-01 20:41 ` raingloom
2022-06-01 21:50 ` Maxime Devos
2022-06-02 14:08 ` Ludovic Courtès
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