Spawn a new Linux process by inheriting all the kernel namespaces of the existing process specified by PID

ns_exec.c

/*
 * Module      : ns_exec
 * Created     : 20-NOV-2013 20:50
 * Author      : Tomas Morstein (www.IDEA.cz)
 * Description : Spawn a new process by inheriting all the kernel
 *               namespaces of the existing process specified by PID
 * Prereqs     : Linux >=3.8 built with kernel namespace support;
 *               Older kernels does not implement all the namespaces
 *               and even the 3.8 shipped by Ubuntu does not come
 *               with CONFIG_USER_NS enabled:
 *                 $ grep -He 'CONFIG_[A-Z]*_NS=y' /boot/config*
 */

#define _GNU_SOURCE
#include <fcntl.h>
#include <sched.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdarg.h>
#include <stdbool.h>

#define MAX_PATH  256
#define MAX_ERROR 256
#define ERR_LABEL "\%NSEXEC-E-"

void trap_error (bool die, const char * msg, ...) {

  va_list args;
  char errmsg [MAX_ERROR];

  va_start (args, msg);
  vsnprintf (errmsg, MAX_ERROR, msg, args);
  va_end (args);

  perror (errmsg);
  if (die) exit (EXIT_FAILURE);
}

void join_ns (char * pid, char * ns) {

  int fd;
  char path [MAX_PATH];

  snprintf (path, MAX_PATH, "/proc/%s/ns/%s", pid, ns);

  /* Open the namespace */
  if ((fd = open (path, O_RDONLY)) < 0) {
    trap_error (false, ERR_LABEL "OPEN, %s", path);
    return;
  }

  /* Join it after all */
  if (setns (fd, 0) < 0)
    trap_error (false, ERR_LABEL "SETNS, %s", path);
}

void exec_ns (char * parent_pid, char * cmd, char * args []) {

  /* Join all the namespaces */
  /* NOTE: since the 'mnt' namespace may not allow us
   * to see the original host's '/proc' filesystem,
   * we have to join it at last.
   */
  join_ns (parent_pid, "ipc");
  join_ns (parent_pid, "net");
  join_ns (parent_pid, "pid");
  join_ns (parent_pid, "uts");
  join_ns (parent_pid, "user");
  join_ns (parent_pid, "mnt");

  /* Execute a command within all the joined namespaces;
   * execvp returns only if the error occurs...
   */
  execvp (cmd, args);
  trap_error (true, ERR_LABEL "EXECVP, %s", cmd);
}

int main (int argc, char * argv []) {

  if (argc < 3) {
    fprintf (stderr, "%s <pid> <cmd> [args...]\n", argv [0]);
    exit (EXIT_FAILURE);
  }

  exec_ns (argv [1], argv [2], &argv [2]);

  /* Just a formality, exec_ns should not return/exit */
  return (EXIT_SUCCESS);
}