Using setuid Properly

The problem is that setuid and setgid are not sufficient to give your process all the credentials it needs. The authorizations of a process depend on

• its UID
• its GID
• its supplementary groups
• its capabilities.

See man 7 credentials to get a more detailed overview. So, in your case, the problem is that you correctly set the UID and GID, but you don't set the supplementary groups of the process. And group bar has GID 54, no 73 so it is not recognized as a group your process is in.

You should do

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <grp.h>

int main (void) {
    gid_t supplementary_groups[] = {54};

    setgroups(1, supplementary_groups);
    setgid(73);
    setuid(73);
    int fd = open("/test.txt", O_RDONLY);
    fprintf(stderr,"%d\n", fd);
    return 0;
}  

I use runit's chpst tool for tasks like this. For example, from the up script call your unprivileged script:

chpst -u nobody /path/to/script