# NS_GET_USERNS

## NAME



           NS_GET_USERNS, NS_GET_PARENT - discovering namespace relationships
    


## SYNOPSIS



           **#include <linux/nsfs.h>  **/* Definition of **NS_GET_*** constants */
           **#include <sys/ioctl.h>**
    
           **int ioctl(int** _fd_**, unsigned long** _op_**);**
    


## DESCRIPTION



           The following ioctl(2) operations are provided to allow discovery
           of namespace relationships (see user_namespaces(7) and
           pid_namespaces(7)).
    
           In each case, _fd_ refers to a _/proc/_ pid _/ns/*_ file.  Both operations
           return a new file descriptor on success.
    
           **NS_GET_USERNS**
                  Returns a file descriptor that refers to the owning user
                  namespace for the namespace referred to by _fd_.
    
           **NS_GET_PARENT**
                  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.
    


## RETURN VALUE



           On success, a file descriptor is returned.  Or error, -1 is
           returned, and _errno_ is set to indicate the error.
    


## 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.
    


## STANDARDS



           Linux.
    


## 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 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 <errno.h>
           #include <fcntl.h>
           #include <linux/nsfs.h>
           #include <stdint.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <string.h>
           #include <sys/ioctl.h>
           #include <sys/stat.h>
           #include <sys/sysmacros.h>
           #include <unistd.h>
    
           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: "
                          "[%x,%x] / %ju\n",
                          major(sb.st_dev),
                          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: [%x,%x] / %ju\n",
                          major(sb.st_dev),
                          minor(sb.st_dev),
                          (uintmax_t) sb.st_ino);
    
                   close(parent_fd);
               }
    
               exit(EXIT_SUCCESS);
           }
    


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