Bug report

Bug description:

test.test_posix.PosixTester._test_all_chown_common asserts that an
"unprivileged" process cannot change a file's group to gid 0:

if hasattr(os, 'getgroups'):
    if 0 not in os.getgroups():
        self.assertRaises(OSError, chown_func, first_param, -1, 0)
        check_stat(uid, gid)

The guard uses only os.getgroups() (the supplementary groups). But the
kernel's permission check for changing a file's group accepts the group if it
is the process's effective/filesystem gid or one of the supplementary
groups (in_group_p() checks fsgid, which tracks egid). os.getgroups() does
not necessarily include the effective gid.

So when a process runs with effective gid 0 and a non-zero uid — common
in containers, user namespaces, and CI sandboxes — chown(file, -1, 0)
succeeds (the process is a member of group 0 via its egid), and the assertion
fails:

AssertionError: OSError not raised by chown
AssertionError: OSError not raised by lchown

The owner-changing assertions just above (chown(file, 0, 0) / (0, -1)) still
correctly raise, since changing the owner requires CAP_CHOWN, which such a
process does not have — so only the group-to-0 assertion is affected.

CPython versions tested on:

CPython main branch, 3.16, 3.15, 3.14

Operating systems tested on:

Linux

Linked PRs

• gh-151519: Check effective gid in _test_all_chown_common group-0 guard #151521