test setns

setns.c

#define _GNU_SOURCE             /* See feature_test_macros(7) */
#include <sched.h>
#include <sys/syscall.h>      /* Definition of SYS_* constants */
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/wait.h>
#include <time.h>

static int
pidfd_open(pid_t pid, unsigned int flags) {
   return syscall(__NR_pidfd_open, pid, flags);
}

int main(int argc, char **argv) {
  struct timespec tstart={0,0}, tend={0,0};

  pid_t pid, child;
  int pidfd, st;

  if (argc < 2) {
    printf("err: invalid num of arguments");
    return 1;
  }

  pid = (pid_t) strtol(argv[1], (char **)NULL, 10);
  printf("moving to: %d\n", pid);

  clock_gettime(CLOCK_MONOTONIC, &tstart);
  pidfd = pidfd_open(pid, 0);
  if (pidfd == -1) {
    printf("err: bad open: %s", strerror(errno));
    return 1;
  }

  if (setns(pidfd, CLONE_NEWPID | CLONE_NEWCGROUP | CLONE_NEWIPC | CLONE_NEWNET | CLONE_NEWNS | CLONE_NEWUTS) != 0) {
    printf("err: bad setns: %s", strerror(errno));
    return 1;
  }

  child = fork();
  if (child > 0) {
    clock_gettime(CLOCK_MONOTONIC, &tend);
    printf("waiting [fork in %ldns]\n", tend.tv_nsec-tstart.tv_nsec);
    if (waitpid(child, &st, 0) == -1) {
      printf("err: bad wait: %s", strerror(errno));
      return 1;
    }
  } else {
    clock_gettime(CLOCK_MONOTONIC, &tend);
    printf("moved in [%ldns]\n", tend.tv_nsec-tstart.tv_nsec);
    getchar();
  }

  close(pidfd);

  return 0;
}

Yes, replace pid fd with normal fd can work. And another reason as follows.