setuid experiment

setuid-experiment.c

#include <stdio.h>
// provides printf()

#include <stdlib.h>
// provides system()

#include <unistd.h>
// provides getuid(), geteuid(), setuid()

int main () 
{
  // https://en.wikipedia.org/wiki/Setuid

  // setuid will not work on script executables (ex. files begin with #!/bin/bash)
  // because the real executable is /bin/bash (or node/ruby...)
  // and sub-process will be run as original user

  // root:
  //   gcc setuid-experiment.c -o setuid-experiment
  //   chmod 4711 setuid-experiment
  // user:
  //   ./setuid-experiment
  
  // at first a.out is started with uid: user and effective uid: 0
  printf("uid: %d, euid: %d\n", getuid(), geteuid());
  system("id"); // when running system call, bash runs using uid, not euid
  // https://unix.stackexchange.com/questions/369883/setuid-root-does-not-work

  // we need this to make the process really running as root
  setuid(0);

  // now we are really root
  printf("uid: %d, euid: %d\n", getuid(), geteuid());
  system("id");

  // finally this will work
  system("ls /root");
  return 0; 
}