cr_canseeothergids(): Use real instead of effective group membership

[olce]

Summary

Using the effective group and not the real one when testing membership has the
consequence that unprivileged processes cannot see setuid commands they launch
until these have relinquished their privileges. This is also in contradiction
with how the similar cr_canseeotheruids() works, i.e., by taking into account
real user IDs.

Fix this by substituting groupmember() with realgroupmember(). While here,
simplify the code.

PR: 272093

[mhorne]

The change requires some elaboration in the description. The text from the PR would be fine.

sys/kern/kern_prot.c

1410–1411

This is a style regression. Even though see_other_gids has an int type, it is a boolean variable and so we should check its truthiness directly.

[olce]

Fix style of the test on see_other_gids.

[mhorne]

sys/kern/kern_prot.c

1412–1422

My only clarification... u1->cr_groups[0] contains the effective gid, right? That is why you have split this statement from the loop?

[olce]

sys/kern/kern_prot.c

1412–1422

Exactly. That's also why next loop starts with index 1.

[olce]

'cr_see*' => 'cr_cansee*' (suppressed a rename in the stack).

[olce]

Impacts of (suppressing) the rename, so re-validating.