setgid() fails - operation not permitted
c, linux, permissions, setuid
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`.
Problem
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 ```