POS36-C. Observe correct revocation order while relinquishing privileges

/* Drop superuser privileges in incorrect order */

if (setuid(getuid()) == -1) {
  /* handle error condition */
}
if (setgid(getgid()) == -1) {
  /* handle error condition */
}

/* It is still possible to regain group privileges due to
 * incorrect relinquishment order */

/*  Drop superuser privileges in correct order */

if (setgid(getgid()) == -1) {
  /* handle error condition */
}
if (setuid(getuid()) == -1) {
  /* handle error condition */
}

/*
 * Not possible to regain group privileges due to correct relinquishment order
 */

/* Returns nonzero if the two group lists are equivalent (taking into
   account that the lists may differ wrt the egid */
int eql_sups(const int cursups_size, const gid_t* const cursups_list,
	     const int targetsups_size, const gid_t* const targetsups_list) {
  int i;
  int j;
  const int n = targetsups_size;
  const int diff = cursups_size - targetsups_size;
  const gid_t egid = getegid();
  if (diff > 1 || diff < 0 ) {
    return 0;
  }
  for (i=0, j=0; i < n; i++, j++) {
    if (cursups_list[j] != targetsups_list[i]) {
      if (cursups_list[j] == egid) {
	i--; /* skipping j */
      } else {
	return 0;
      }
    }
  }
  /* If reached here, we're sure i==targetsups_size. Now, either
     j==cursups_size (skipped the egid or it wasn't there), or we didn't
     get to the egid yet because it's the last entry in cursups */
  return j == cursups_size ||
    (j+1 == cursups_size && cursups_list[j] == egid);
}


/* Sets the suplimentary group list, returns 0 if successful  */
int set_sups(const int target_sups_size,const gid_t* const target_sups_list) {
#ifdef __FreeBSD__
  const int targetsups_size = target_sups_size + 1;
  gid_t* const targetsups_list = (gid_t* const) malloc(sizeof(gid_t) * targetsups_size);
  if (targetsups_list == NULL) {
    /* handle error */
  }
  memcpy(targetsups_list+1, target_sups_list, target_sups_size * sizeof(gid_t) );
  targetsups_list[0] = getegid();
#else
  const int targetsups_size = target_sups_size;
  const gid_t* const targetsups_list = target_sups_list;
#endif
  if (geteuid() == 0) { /* allowed to setgroups, let's not take any chances */
    if (-1 == setgroups(targetsups_size, targetsups_list)) {
      /* handle error */
    }
  } else {
    int cursups_size = getgroups( 0, NULL);
    gid_t* cursups_list = (gid_t*) malloc( sizeof(gid_t) * cursups_size);
    if (cursups_list == NULL) {
      /* handle error */
    }
    if (-1 == getgroups( cursups_size, cursups_list)) {
      /* handle error */
    }
    if (!eql_sups(cursups_size, cursups_list, targetsups_size, targetsups_list)) {
      if (-1 == setgroups(targetsups_size, targetsups_list)) { /* will probably fail... :( */
	/* handle error */
      }
    }
    free( cursups_list);
  }

#ifdef __FreeBSD__
  free( targetsups_list);
#endif
  return 0;
}