set euid with posix_spawn

Question

I have the following code snippet.

char *const parmList[] = {"sh", "-c", "whoami", NULL};
if(geteuid() == 0) {
   seteuid(atoi(getenv("SUDO_UID")));
}
posix_spawn(&pid, "/bin/sh", NULL, NULL, parmList, environ);

From my understanding the default behavior of posix_spawn is:

If the POSIX_SPAWN_RESETIDS flag is not set, the child process shall inherit the parent process' effective user ID.

However, when I run my program with sudo, I still get root as the output from posix_spawn. How do I have it so posix_spawn will run as the original user? Is there a better way to do this?

Answer 1

I ended up accomplishing this by creating a function that forks then execs

pid_t runCmd(char *cmd) {
    if(!cmd) return -1;

    pid_t ans = fork();
    if(ans == 0) {
        if(geteuid() == 0) {
            int uid = atoi(getenv("SUDO_UID"));
            setreuid(uid, uid);
        }
        if(verbose_flag) println("uid %d; euid %d", getuid(), geteuid());
        char *const parmList[] = {"sh", "-c", cmd, NULL};
        execv("/bin/sh", parmList);
    }
    return ans;
}

whoami now returns the original user