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_names-
       paces(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 namespace 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  number  can  be  matched  with  the  inode  number   of   another
       /proc/[pid]/ns/{pid,user} file to determine whether that is the owning/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  initial  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.

   Discovering the namespace type
       The  NS_GET_NSTYPE  operation (available since Linux 4.11) can be used to discover the type of namespace referred to by the file descriptor
       fd:

	   nstype = ioctl(fd, NS_GET_NSTYPE);

       fd refers to a /proc/[pid]/ns/* file.

       The return value is one of the CLONE_NEW* values that can be specified to clone(2) or unshare(2) in order to create a namespace.

   Discovering the owner of a user namespace
       The NS_GET_OWNER_UID operation (available since Linux 4.11) can be used to discover the owner user ID of a user namespace (i.e., the effec-
       tive user ID of the process that created the user namespace).  The form of the call is:

	   uid_t uid;
	   ioctl(fd, NS_GET_OWNER_UID, &uid);

       fd refers to a /proc/[pid]/ns/user file.

       The owner user ID is returned in the uid_t pointed to by the third argument.

       This operation can fail with the following error:

       EINVAL fd does not refer to a user namespace.

ERRORS

       Any of the above ioctl() operations can return the following errors:

       ENOTTY fd does not refer to a /proc/[pid]/ns/* file.

CONFORMING TO

       Namespaces and the operations described on this page are a Linux-specific.

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]
",
		       argv[0]);
	       fprintf(stderr, "
Display the result of one or both "
		       "of NS_GET_USERNS (u) or NS_GET_PARENT (p)
"
		       "for the specified /proc/[pid]/ns/[file]. If neither "
		       "'p' nor 'u' is specified,
"
		       "NS_GET_USERNS is the default.
");
	       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
");
		   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
",
		       (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
");
		   else if (errno == EPERM)
		       printf("The parent namespace is outside "
			       "your namespace scope
");
		   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
",
		       (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)

Linux								    2019-03-06							       IOCTL_NS(2)