Setuid(geteuid()), help

[Chris Lewis]

In article <2733 at piraat.cs.vu.nl> maart at cs.vu.nl (Maarten Litmaath) writes:
>tony at ajfcal.UUCP (Tony Field) writes:
>\Is there any way to do a   setuid(geteuid())   when geteuid() != 0  ???

>setuid() to either geteuid() or getuid() must ALWAYS succeed. Elementary.

Maarten's usually right, but in this case he isn't.  *Most* versions of
UNIX (eg: Xenix and SV, but I think BSD may be the opposite) do not allow
you to setuid() to anything other than getuid() unless geteuid() == 0.
(SVID and System V also include something called the saved userid, but
this doesn't apply here.).  

On the other hand, BSD can do this quite easily as Maarten
suggests with things like seteuid() and setruid().

Maarten's right in the rest of his article (except for being able
to in general say setuid(geteuid()), however...

The easiest way to get a mkdir to get the ownership the way you want it
is to write a setuid root wrapper that invokes mkdir and then chown's 
the created directory to "news" explicitly.

eg, compile and make setuserid root:

	main(argc, argv) int argc; char **argv; {
	    int pid;
	    if ((pid = fork()) == 0)
		exec("/bin/mkdir", argv[1], 0);
	    while(wait(0) != pid);
	    exec("/bin/chown", "news", argv[1], 0);
	}

(error checking and strong typing left as an exercise to the reader.)

Grotty and inefficient but effective and small.
-- 
Chris Lewis, R.H. Lathwell & Associates: Elegant Communications Inc.
UUCP: {uunet!mnetor, utcsri!utzoo}!lsuc!eci386!clewis
Phone: (416)-595-5425


-- 
Chris Lewis, R.H. Lathwell & Associates: Elegant Communications Inc.
UUCP: {uunet!mnetor, utcsri!utzoo}!lsuc!eci386!clewis
Phone: (416)-595-5425