setgid() fails - operation not permitted

Question

I created a setuid program in C. The executable looks like this:

-r-s-r-s--- 1 root users 13073 Jun 15 21:56 server

I execute the program as userA/users and try to set the uid/gid to userB/otherUsers. setgid() fails with Operation not permitted. userA is not part of otherUsers How can I change the effective gid?

---

[EDIT] Here is a small summary of what I did. My C program, executed as userA, sets uid and gid to userB and creates a file. Not as expected, the file belongs to the group root, because setgid() fails.

[userA@node uid]$ id
uid=11945(userA) gid=544(users) groups=544(users)
[userA@node uid]$ id userB
uid=11946(userB) gid=10792(otherUsers) groups=10792(otherUsers)
[userA@node uid]$ cat uid.c 
#include <stdio.h>
#include <unistd.h>

int main() {
  setuid(11946);
  setgid(10792);

FILE *f = fopen("userB_file", "w");
fclose(f);

return 0;
}
[userA@node uid]$ ls -l uid
-r-sr-sr-x 1 root root 7130 Jun 17 14:16 uid
[userA@node uid]$ ./uid 
[userA@node uid]$ ls -l userB_file 
-rw-r--r-- 1 userB root 0 Jun 17 14:19 userB_file

Solution

I suspect you're calling setuid before setgid. As soon as you call setuid to change the uid to something other than root, you've forfeited your permission to change the gid to an arbitrary value. You must call setgid first, then setuid.

Comments

[multiholle]

This is as obvious as simple! Damn, you're so right. I changed the order and of course, it works! Thanks. btw: I added my original C program in the question.

[Bemipefe]

It is actually not so obvious. It is not reported in the man page of setgid and neither in the man page of setuid.

[R.. GitHub STOP HELPING ICE]

@Bemipefe: It should be obvious as a consequence of the permission model that you can't change your group id to anything you want when you're a normal unprivileged user.

[Bemipefe]

@R.. I was referring to the fact that the setgid function must be called before the setuid function. This rule applies also to processes wich run as root user.

[R.. GitHub STOP HELPING ICE]

@Bemipefe: That "rule" is just a consequence of the fact that only root can change gid to arbitrary groups. After calling setuid you're no longer root.