setgroups.2: Add SECURITY CONSIDERATIONS, fix the groups limit, rework

[olce]

Summary

Add a new SECURITY CONSIDERATIONS section describing in details what the
new behavior is after commit 9da2fe96ff2e ("kern: fix setgroups(2) and
getgroups(2) to match other platforms"), what setgroups(2) does not
do anymore, and how programs using it are affected.

Fix the groups limit after commit 9da2fe96ff2e ("kern: fix setgroups(2)
and getgroups(2) to match other platforms").

Prefer a terminology referring to POSIX terms, i.e., use "effective
group list" instead of "group access list".

While here, fix some style.

Fixes: 9da2fe96ff2e ("kern: fix setgroups(2) and getgroups(2) to match other platforms")
Sponsored by: The FreeBSD Foundation

[olce]

• Impacts of the paradigm change in D52282 in SECURITY CONSIDERATIONS. Relax the wording about applications passing the additional group/effective GID as the first element in setgroups(2). Mention that this is what initgroups(3) does.
• Add notes about clearing all supplementary groups in HISTORY and SECURITY CONSIDERATIONS.
• Use "calling process" instead of "current process".
• Be explicit on gidset not being used at all when ngroups is 0 (replacing the old ambiguous formulation).
• "GID" => "group ID", consistently with other manual pages.

[emaste]

lib/libsys/setgroups.2

1

I don't think we need this (.\"-) marker anymore