IOCTL_NS(2)               (2020-11-01)                IOCTL_NS(2)

     NAME
          ioctl_ns - ioctl() operations for Linux namespaces

     DESCRIPTION
        Discovering namespace relationships
          The following ioctl(2) operations are provided to allow dis-
          covery 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 names-
          pace.

          Either of these ioctl(2) operations can fail with the fol-
          lowing 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.

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          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
          The NS_GET_OWNER_UID operation (available since Linux 4.11)
          can be used to discover the owner user ID of a user names-
          pace (i.e., the effective user ID of the process that cre-
          ated 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.

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     EXAMPLES
          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 dis-
          covered.  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>

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          #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 "
                          "aqpaq nor aquaq is specified,\n"
                          "NS_GET_USERNS is the default.\n");
                  exit(EXIT_FAILURE);
              }

              /* Obtain a file descriptor for the aqnsaq 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], aquaq)) {
                  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);

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                   }

                  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], aqpaq)) {
                  parent_fd = ioctl(fd, NS_GET_PARENT);

                  if (parent_fd == -1) {
                      if (errno == EINVAL)
                          printf("Canaq 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
          fstat(2), ioctl(2), proc(5), namespaces(7)

     COLOPHON

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          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/.

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