Why my own suid-ed program still holds the original uid?

Question

I'm using the following program, and I've suid-ed it (by running chown root XXX; chmod 4755 XXX as root), but the output is still ruid 1000, euid 1000, suid 1000, shouldn't effect uid be zero here?

#define _GNU_SOURCE
#include <stdio.h>
#include <unistd.h>

int main()
{
        uid_t ruid, euid, suid;
        if (! getresuid (&ruid, &euid, &suid))
                printf ("ruid %d, euid %d, suid %d\n", ruid, euid, suid);
        else
                perror ("getresuid");

        return 0;
}

Output of ls -l:

-rwsr-xr-x 1 root root 9.7K May 1 11:36 test*

Answer 1

Please check the mount command output, your file system could be mounted with nosuid option.

From mount man page

nosuid: Do not allow set-user-identifier or set-group-identifier bits to take effect.