setuidtest explicit w/syscalls

setuidtest.c

#define _GNU_SOURCE
#include <stdio.h>
#include <unistd.h>
#include <sys/syscall.h>

void printid(void) {
  uid_t r, e, s;
  getresuid(&r, &e, &s);
  printf("real: %d eff: %d saved: %d\n", r, e, s);
}

int main(void) {
  printid();
  if (seteuid(0) != 0) {
    puts("ERROR Cannot gain root priviledges");
    printid();
    return 1;
  } else
    puts("Gained root priviledges");
  printid();
  // if (setuid(getuid()) != 0) {
  if (syscall(SYS_setuid, syscall(SYS_getuid)) != 0) {
    puts("ERROR Dropping priviledges failed");
    printid();
    return 1;
  } else
    puts("Dropped root priviledges");
  printid();
  // if (setuid(0) == 0) {
  if (syscall(SYS_setuid, 0) == 0) {
    puts("ERROR Still able to recover root");
    printid();
    return 1;
  } else
    puts("Unable to recover root");
  printid();
  return 0;
}