ioctl_ns(2) Example

ns_show.c

/* ns_show.c
  Licensed under the GNU General Public Lecense v2 or later.
*/
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <errno.h>
#include <sys/sysmacros.h>

#ifndef NS_GET_USERNS
#define NSIO 0xb7
#define NS_GET_USERNS _IO(NSIO, 0x1)
#define NS_GET_PARENT _IO(NSIO, 0x2)
#endif

int
main(int argc, char *argv[])
{
  int fd, userns_fd, parent_fd;
  struct stat sb;
  
  if (argc < 2) {
    fprintf(stderr, "Usage: %s /proc/[pid]/ns/[file] [p|u]\n", argv[0]);
    fprintf(stderr, "\nDisplay the result of one or both "
            "of NS_GET_USERNS (u) or NS_GET_PARENT (p)\n"
            "for the specified /proc/[pid]/ns/[file]. If neither "
            "'p' nor 'u' is specified,\n"
            "NS_GET_USERNS is the default.\n");
    exit(EXIT_FAILURE);
  }
  
  /* Obtain a file descriptor for the 'ns' file specified in argv[1] */
  fd = open(argv[1], O_RDONLY);
  if (fd == -1) {
    perror("open");
    exit(EXIT_FAILURE);
  }
  
  /* Obtain a file descriptor for the owning user namespace and 
    then obtain and display the inode number of that namespace */
  if (argc < 3 || strchr(argv[2], 'u')) {
    userns_fd = ioctl(fd, NS_GET_USERNS);
    
    if (userns_fd == -1) {
      if (errno == EPERM)
        printf("The owning user namespace is outside"
               "your namespace scope\n");
      else
        perror("ioctl-NS_GET_USERNS");
      exit(EXIT_FAILURE);
    }
    
    if (fstat(userns_fd, &sb) == -1) {
      perror("fstat-userns");
      exit(EXIT_FAILURE);
    }
    
    printf("Device/Inode of owning user namespace is: "
           "[%jx,%jx] / %ju\n",
           (uintmax_t) major(sb.st_dev),
           (uintmax_t) minor(sb.st_dev),
           (uintmax_t) sb.st_ino);
    close(userns_fd);
  }
  
  /* Obtain a file descriptor for the parent namespace and 
     then obtain and display the inode number of that namespace */
  if (argc > 2 && strchr(argv[2], 'p')) {
    parent_fd = ioctl(fd, NS_GET_PARENT);
    
    if (parent_fd == -1) {
      if (errno == EINVAL)
        printf("Can' get parent namespace of a "
               "nonhierarchical namespace\n");
      else if (errno == EPERM)
        printf("The parent namespace is outside "
               "your namespace score\n");
      else
        perror("ioctl-NS_GET_PARENT");
      exit(EXIT_FAILURE);
    }
    
    if (fstat(parent_fd, &sb) == -1) {
      perror("fstat-parents");
      exit(EXIT_FAILURE);
    }
    printf("Device/Inode of parent namespace is: [%jx,%jx] / %ju\n",
           (uintmax_t) major(sb.st_dev),
           (uintmax_t) minor(sb.st_dev),
           (uintmax_t) sb.st_ino);
    close(parent_fd);
  }
  exit(EXIT_SUCCESS);
}