ps(1): '-U' to select processes by real user IDs

[olce]

Summary

This is what POSIX mandates for option '-U' and arguably the behavior
that most users actually need in most cases. Before, '-U' would select
processes by their effective user IDs (which is the behavior mandated by
POSIX for option '-u').

Matching by real user IDs allows to list all processes belonging to the
passed users, including those temporarily having a different effective
user ID, which can happen if launched by a setuid executable or if using
some credentials-changing facility (such as seteuid() for root processes
or mac_do(4)/setcred(2)). Conversely, processes temporarily assuming
the identity of some of the passed users will not be listed anymore
(they do not "belong" to these users).

This change also makes '-U' consistent with '-G', the latter already
matching on real group IDs.

While here, remove the (non-compiled) code for tentative option '-R' as
its proposed behavior was the one established here for '-U'. Also, move
the compiled-out old code for '-U' under '-u' for reference, as this is
what the latter should do according to POSIX, even if it seems unlikely
we will want to change the behavior of '-u'.

Relnotes: yes

[emaste]

I find the commit message a bit confusing because it doesn't explicitly mention that -U's behaviour changes, but I believe the three individual changes are correct:

• removing #if-0'd -R code (it has never been enabled)
• changing -U's behaviour
• documenting what -u should be, but not changing it

There should be an UPDATING/RELNOTES entry for the user-facing -U behaviour change.

bin/ps/ps.c

389–399

404–405

[olce]

bin/ps/ps.c

389–399

I've added the header, but reworked the accompanying text (explaining why this code is not activated; but not preventing it to be so one day if deemed better for the functionality/POSIX compliance).

[olce]

• Address comments.
• Add an UPDATING entry.
• Tag the commit as needing a release note.