setgid and process group list

[Chris Torek]

In article <339 at rhesus.primate.wisc.edu> bin at rhesus.primate.wisc.edu
(Brain in Neutral) writes:
>Why does setgid screw up the list of groups that getgroups returns?

Whether it does depends on your definition of `screw up'.  setgid(gid)
is equivalent to setregid(gid, gid).  Among other things, this has to
take you out of any group you were in from a previous setregid().
Hence the algorithm is:

	leavegroup(old_real_gid);
	leavegroup(old_effective_gid);
	entergroup(new_real_gid);
	entergroup(new_effective_gid);

If you setgid() to a group you are already in, the two leave and enter
calls effectively do nothing; if, however, you setgid to a group you
are not now in, you lose the old group.  (Solution: use `setregid' to
set only the effective or only the real gid.)

This behaviour is somewhat bogus, but as long as setregid() is used
only to exchange real and effective gid, or to set gid's from a setuid-
root program (login) and followed by an initgroups(), everything works.
Since setgid() is supposed to be mostly irrelevant given the multiple-
group scheme, I guess this has not been considered a serious problem.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris at mimsy.umd.edu	Path:	uunet!mimsy!chris