From: Maxime Devos <maximedevos@telenet.be>
To: "Ludovic Courtès" <ludo@gnu.org>,
"Felix Lechner" <felix.lechner@gmail.com>
Cc: Guix Devel <guix-devel@gnu.org>
Subject: Re: A corner case of broken reproducibility
Date: Mon, 30 May 2022 22:50:10 +0200 [thread overview]
Message-ID: <31d13a6014b14c886686bca833d926604ce3379d.camel@telenet.be> (raw)
In-Reply-To: <87leuj108h.fsf@gnu.org>
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Ludovic Courtès schreef op ma 30-05-2022 om 17:58 [+0200]:
> Perhaps it should forcefully “chown -R” home directories at boot time,
> so they have the right UID? This has been discussed a few times,
I haven't seen the "chown -R" suggestion yet in the context of homes
(only for system accounts, and at activation time, not only boot time).
> especially in the context of system accounts; systemd-homed appears to
> be doing exactly that.
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.
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).
* a mechanism for telling Guix ‘I'm renaming the user account, not
creating and removing a new one, so keep the uid’
* 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).
IIUC, Blake Shaw only changed their user name, not the home
directory, so this would have detected the problem
* some mechanism for resolving mistakes
Or maybe something else entirely, whatever works without causing new
problems I guess.
Greetings,
Maxime.
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next prev parent reply other threads:[~2022-05-30 20:50 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 [this message]
2022-06-01 16:38 ` Ludovic Courtès
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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