Re: chpst -u and supplementary groups

From: Steve Litt <>
Date: Mon, 19 Aug 2019 15:33:04 -0400

On Mon, 19 Aug 2019 14:08:07 +0200
Jan Braun <> wrote:

> Hello list!
> Yesterday, I spent way too much time chasing down a permissions
> problem caused by the fact that "chpst -u acc prog..." only sets the
> account's primary group, and ignores any supplementary groups the
> account may be a member of.
> TFM mentions "All initial supplementary groups are removed.", but I
> failed to memorize that. (Also, what does "initial" signify here?)
> My inability to see the issue came from the fact that all other
> similar programs (I'm aware of) do in fact add the supplementary
> groups. Watch:
> | # chpst -u test id
> | uid=1003(test) gid=1003(test) groups=1003(test)
> | # runuser -u test id
> | uid=1003(test) gid=1003(test) groups=1003(test),4(adm)
> | # s6-setuidgid test id
> | uid=1003(test) gid=1003(test) groups=1003(test),4(adm)
> | # su - test -c id
> | uid=1003(test) gid=1003(test) groups=1003(test),4(adm)
> | # su test -c id
> | uid=1003(test) gid=1003(test) groups=1003(test),4(adm)
> | # sudo -u test id
> | uid=1003(test) gid=1003(test) groups=1003(test),4(adm)
> | #
> So now I'm wondering:
> What are the use cases for not applying existing supplementary groups?
> Should chpst apply them by default?
> Should chpst grow an option to (not) apply them?
> "chpst -u acc: prog..." is still free.
> Or is everything as it's supposed to be, and people might need to
> munge the output of "getent initgroups acc" and feed it to the -u
> option?
> I'll be happy to try to come up with a patch (even if it's still a
> fatter warning in the manpage) if people can agree here what the right
> thing to do is.
> regards,
> Jan

Thanks Jan,

I was having a similar problem, and inspired by your post, I solved it
the following way, using Runit's chpst:

chpst -u slitt:audio:disk /tmp/

In the preceding, /tmp/ was permissioned 750 owner,
and contained the following:

cat /d/
cat /d/

file /d/ was 640, while /d/ was 640
root.disk. Both cat statements printed out the contents of the files,
proving that I was acting with both groups disk and audio. The output
of the whoami proved I was acting as user slitt.

You're obviously right that having all the groups slitt belongs to,
which include both disk and audio, would have been easier, but as you
say, it doesn't work right now, so I used this method for the time

Thanks for helping me solve my problem.


Steve Litt
August 2019 featured book: Twenty Eight Tales of Troubleshooting
Received on Mon Aug 19 2019 - 19:33:04 UTC

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