Subject: Testsuite failure relating to chgrp in (unprivileged) user namespaces

From: keinflue <[email-masked]>
Date: Sat, 03 May 2025 03:38:11 +0000

Hello,

noticed this on Guix (https://issues.guix.gnu.org/77862#5) with 
coreutils 9.1 and also verified with latest release 9.7.

When building and running the testsuite of coreutils on Linux in a user 
namespace as unprivileged user the latter may fail chgrp test cases:

> FAIL: tests/chgrp/default-no-deref.sh
> FAIL: tests/chgrp/no-x.sh
> FAIL: tests/chgrp/posix-H.sh
> FAIL: tests/chgrp/recurse.sh
> FAIL: tests/chgrp/basic.sh

The cause for this are supplementary groups of the build process which 
are not mapped in the user namespace via /proc/pid/gid_map.

Inside the user namespace these groups are reported as the overflow gid 
(by default 65534). require_membership_in_two_groups_ in init.cfg has no 
exemption for this gid and the chgrp tests will attempt to change 
ownership to this gid, assuming this to be valid as usually is the case 
when changing ownership to a supplementary group. However, this is not 
allowed for the unmapped overflow gid and the syscall will fail.

The same problem occurs in gnulib-tests, but I suppose I should report 
this to the bug-gnulib list.

This was noticed during experimentation with Guix's new feature to run 
the build daemon as unprivileged user process, which relies on 
unprivileged user namespaces to construct the build container. As 
discussed in the linked issue it isn't really an option to drop the 
supplementary groups in this setting.

I think the overflow gid should be exempt in 
require_membership_in_two_groups_ as was already implemented for special 
gids on MacOS.

Best,
keinflue