Provided by: manpages-dev_4.16-1_all bug

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 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 effective 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]\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 4.16 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/.