access(2) (was: Writing to A NON-Existing File in "C")

[J_Allen_Schones]

In article <975 at unmvax.unm.edu> mike at turing.UNM.EDU.UUCP (Michael I. Bushnell) writes:
[ Much stuff deleted. ]
>Not really.  But there is another way it can run under suid
>conditions:
>
>% whoami
>foo
>% su
>Password:
># nifty_program
>
>
>Note that nifty program will now have REAL uid foo and EFFECTIVE uid
>root.
>
[ .signature deleted ]

Is nifty_program(1) :-) setuid or just a regular (non-setuid) program?
I can't speak for BSD systems, but on a 3B15 running System V Rel.
3.1.1 UNIX (trademark of AT&T), nifty_program (running non-setuid)
will have both REAL and EFFECTIVE uid set to root.  nifty_program
(running setuid) will have REAL uid root and EFFECTIVE uid foo.  su(1)
does a "setuid(geteuid())" when you run it.

Two programs:
	nifty.c:

#include <stdio.h>

main()
{
	printf("uid == %d\n", getuid());
	printf("euid == %d\n", geteuid());
}

	setuid.c:

#include <stdio.h>

main()
{
	execlp("./nifty", "nifty", 0);
}

Session follows:

$ ls -l 
total 42
-rwxr-xr-x	1 jal	user1	13536 Apr 22 12:14 nifty
-rw-r--r--	1 jal	user1	  113 Apr 22 12:02 nifty.c
-rwsr-xr-x	1 jal	user1	 5180 Apr 22 12:14 setuid
-rw-r--r--	1 jal	user1	   63 Apr 22 12:14 setuid.c
$ id
uid=326(jal) gid=300(user1)
$ ./nifty
uid == 326
euid == 326
$ ./setuid
uid == 326
euid == 326
$ su
Password: <generic root password>
# id
uid=0(root) gid=3(sys)
# ./nifty
uid == 0
euid == 0
# ./setuid
uid == 0
euid == 326

End of session.
-- 
J. Allen Schones -- AT&T -- Oklahoma City Works
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