NAME | DESCRIPTION | CONFORMING TO | ERRORS | EXAMPLE | SEE ALSO | COLOPHON

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

NAME         top

       ioctl_ns - ioctl() operations for Linux namespaces

DESCRIPTION         top

   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.

CONFORMING TO         top

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

ERRORS         top

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

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

EXAMPLE         top

       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         top

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

COLOPHON         top

       This page is part of release 4.11 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                            2017-05-03                      IOCTL_NS(2)

Pages that refer to this page: ioctl(2)namespaces(7)pid_namespaces(7)user_namespaces(7)