bug#78225: Testsuite failure relating to chgrp in (unprivileged) user namespaces

On 03/05/2025 04:38, keinflue wrote:
> 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.

Thanks for the details.

I pushed the attached to avoid this issue.

Marking this as done.

cheers,
Padraig.

From 6218cb18b0b7bfdb78dbdd20f3c7ca513bca919c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?P=C3=A1draig=20Brady?= <[email protected]>
Date: Sat, 3 May 2025 09:59:16 +0100
Subject: [PATCH] tests: avoid false failures in user namespaces

* init.cfg (require_membership_in_two_groups_): Skip
overflow gids used in user namespaces, as one can't
chgrp() to these.
Fixes https://bugs.gnu.org/78225
---
 init.cfg | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/init.cfg b/init.cfg
index 7e21f96c6..982418900 100644
--- a/init.cfg
+++ b/init.cfg
@@ -504,6 +504,10 @@ require_membership_in_two_groups_()
 {
   test $# = 0 || framework_failure_
 
+  # Skip overflow gids used in user namespaces
+  overflow_gid=$(cat /proc/sys/kernel/overflowgid 2>/dev/null)
+  : "${overflow_gid:=1}"
+
   groups=
   for group_ in 1 \
     ${COREUTILS_GROUPS-$( (id -G || /usr/xpg4/bin/id -G) 2>/dev/null)}
@@ -511,7 +515,7 @@ require_membership_in_two_groups_()
     # Skip group numbers equal to 2**N - 1 for common N,
     # as they are possibly reserved groups like 'nogroup'.
     case $group_ in
-      1 | 32767 | 65535 | 2147483647 | 4294967295) ;;
+      $overflow_gid | 1 | 32767 | 65535 | 2147483647 | 4294967295) ;;
       9223372036854775807 | 18446744073709551615) ;;
       *) test -z "$groups" || groups="$groups "
          groups="$groups$group_";;
-- 
2.49.0