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;
}

@Yuan-Zhuo how do you run it?

it works for me tho:

$ unshare -U -m -p -f -r bash
root@rabbit:~# sleep 60

-- another terminal

$ ps aux | grep '[s]leep'
user     29536  0.0  0.0   6000   680 pts/1    S+   12:16   0:00 sleep 60

$ sudo ./a.out 29536
moving to: 29536
waiting [fork in 36192ns]
moved in [78651ns]
^C

Thank you for reply. I just noticed that fd as a PID file descriptor in setns is a new feature since Linux 5.8, however my pc' current version is 5.4.☹️

@Yuan-Zhuo do you mean pidfd_open (https://lwn.net/Articles/800379/) (it says 5.3)?

for setns you can replace pid fd with normal fd

a file descriptor referring to one of the magic links in a /proc/[pid]/ns/ directory (or a bind mount to such a link)`

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