IOCTL_NS(2)		   Linux Programmer's Manual		   IOCTL_NS(2)



NAME
       ioctl_ns - ioctl() operations for Linux namespaces

DESCRIPTION
   Discovering namespace relationships
       The  following  ioctl(2)	 operations are provided to allow discovery of
       namespace relationships (see user_namespaces(7) and pid_namespaces(7)).
       The form of the calls is:

	   new_fd = ioctl(fd, request);

       In  each	 case,	fd refers to a /proc/[pid]/ns/* file.  Both operations
       return a new file descriptor on success.

       NS_GET_USERNS (since Linux 4.9)
	      Returns a file descriptor that refers to the owning user	names-
	      pace for the namespace referred to by fd.

       NS_GET_PARENT (since Linux 4.9)
	      Returns a file descriptor that refers to the parent namespace of
	      the namespace referred to by fd.	This operation is  valid  only
	      for  hierarchical	 namespaces  (i.e.,  PID and user namespaces).
	      For  user	  namespaces,	NS_GET_PARENT	is   synonymous	  with
	      NS_GET_USERNS.

       The new file descriptor returned by these operations is opened with the
       O_RDONLY and O_CLOEXEC (close-on-exec; see fcntl(2)) flags.

       By applying fstat(2) to the returned file  descriptor,  one  obtains  a
       stat structure whose st_dev (resident device) and st_ino (inode number)
       fields together identify the owning/parent namespace.  This inode  num-
       ber    can    be	  matched   with   the	 inode	 number	  of   another
       /proc/[pid]/ns/{pid,user} file to determine whether that	 is  the  own-
       ing/parent namespace.

       Either of these ioctl(2) operations can fail with the following errors:

       EPERM  The  requested  namespace	 is  outside of the caller's namespace
	      scope.  This error can occur if, for example,  the  owning  user
	      namespace is an ancestor of the caller's current user namespace.
	      It can also occur on attempts to obtain the parent of  the  ini-
	      tial user or PID namespace.

       ENOTTY The operation is not supported by this kernel version.

       Additionally,  the  NS_GET_PARENT operation can fail with the following
       error:

       EINVAL fd refers to a nonhierarchical namespace.

       See the EXAMPLE section for an example of the use of these operations.

CONFORMING TO
       Namespaces and the operations described on this page are	 a  Linux-spe-
       cific.

EXAMPLE
       The example shown below uses the ioctl(2) operations described above to
       perform simple discovery of  namespace  relationships.	The  following
       shell sessions show various examples of the use of this program.

       Trying  to get the parent of the initial user namespace fails, since it
       has no parent:

	   $ ./ns_show /proc/self/ns/user p
	   The parent namespace is outside your namespace scope

       Create a process running sleep(1) that resides  in  new	user  and  UTS
       namespaces,  and show that the new UTS namespace is associated with the
       new user namespace:

	   $ unshare -Uu sleep 1000 &
	   [1] 23235
	   $ ./ns_show /proc/23235/ns/uts u
	   Device/Inode of owning user namespace is: [0,3] / 4026532448
	   $ readlink /proc/23235/ns/user
	   user:[4026532448]

       Then show that the parent of the new user namespace  in	the  preceding
       example is the initial user namespace:

	   $ readlink /proc/self/ns/user
	   user:[4026531837]
	   $ ./ns_show /proc/23235/ns/user p
	   Device/Inode of parent namespace is: [0,3] / 4026531837

       Start  a	 shell in a new user namespace, and show that from within this
       shell, the parent user namespace can't be discovered.   Similarly,  the
       UTS  namespace  (which  is  associated with the initial user namespace)
       can't be discovered.

	   $ PS1="sh2$ " unshare -U bash
	   sh2$ ./ns_show /proc/self/ns/user p
	   The parent namespace is outside your namespace scope
	   sh2$ ./ns_show /proc/self/ns/uts u
	   The owning user namespace is outside your namespace scope

   Program source

       /* ns_show.c

	  Licensed under the GNU General Public License v2 or later.
       */
       #include <stdlib.h>
       #include <unistd.h>
       #include <stdio.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: "
		       "[%lx,%lx] / %ld\n",
		       (long) major(sb.st_dev), (long) minor(sb.st_dev),
		       (long) 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 scope\n");
		   else
		       perror("ioctl-NS_GET_PARENT");
		   exit(EXIT_FAILURE);
	       }

	       if (fstat(parent_fd, &sb) == -1) {
		   perror("fstat-parentns");
		   exit(EXIT_FAILURE);
	       }
	       printf("Device/Inode of parent namespace is: [%lx,%lx] / %ld\n",
		       (long) major(sb.st_dev), (long) minor(sb.st_dev),
		       (long) sb.st_ino);

	       close(parent_fd);
	   }

	   exit(EXIT_SUCCESS);
       }

SEE ALSO
       fstat(2), ioctl(2), proc(5), namespaces(7)

COLOPHON
       This page is part of release 4.10 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2017-01-09			   IOCTL_NS(2)