ioctl_ns(2) — Linux manual page

NAME | DESCRIPTION | ERRORS | CONFORMING TO | EXAMPLES | 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 EXAMPLES 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         top

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

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

CONFORMING TO         top

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

EXAMPLES         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 <stdint.h>
       #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: "
                       "[%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 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: [%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);
       }

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.10 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                          2020-11-01                    IOCTL_NS(2)

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