ns_get_userns(2const)

—

Options · 7

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.
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.
EINVAL	fd refers to a nonhierarchical namespace.
NS_GET_USERNS	Linux 4.9.
NS_GET_PARENT	Linux 4.9.

NAME

NS_GET_USERNS, NS_GET_PARENT - discovering namespace relationships

SYNOPSIS

bash

#include <linux/nsfs.h>\n  /* Definition of \nNS_GET_*\n constants */
\n#include <sys/ioctl.h>

bash

int ioctl(int \nfd\n, unsigned long \nop\n);

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.

HISTORY

NS_GET_USERNS: Linux 4.9.
NS_GET_PARENT: Linux 4.9.

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:

bash

$ \n./ns_show /proc/self/ns/user p\n
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:

bash

$ \nunshare -Uu sleep 1000 &\n
[1] 23235
$ \n./ns_show /proc/23235/ns/uts u\n
Device/Inode of owning user namespace is: [0,3] / 4026532448
$ \nreadlink /proc/23235/ns/user\n
user:[4026532448]

Then show that the parent of the new user namespace in the preceding example is the initial user namespace:

bash

$ \nreadlink /proc/self/ns/user\n
user:[4026531837]
$ \n./ns_show /proc/23235/ns/user p\n
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.

bash

$ \nPS1="sh2$ " unshare -U bash\n
sh2$ \n./ns_show /proc/self/ns/user p\n
The parent namespace is outside your namespace scope
sh2$ \n./ns_show /proc/self/ns/uts u\n
The owning user namespace is outside your namespace scope

Program source

bash

/* ns_show.c
\n
   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[])
{
\n
    int          fd, userns_fd, parent_fd;
\n
    struct stat  sb;
\n
    if (argc < 2) {
\n
        fprintf(stderr, "Usage: %s /proc/[pid]/ns/[file] [p|u]\n",
\n
                argv[0]);
\n
        fprintf(stderr, "\nDisplay the result of one or both "
\n
                "of NS_GET_USERNS (u) or NS_GET_PARENT (p)\n"
\n
                "for the specified /proc/[pid]/ns/[file]. If neither "
\n
                "'p' nor 'u' is specified,\n"
\n
                "NS_GET_USERNS is the default.\n");
\n
        exit(EXIT_FAILURE);
\n
    }
\n
    /* Obtain a file descriptor for the 'ns' file specified
\n
       in argv[1]. */
\n
    fd = open(argv[1], O_RDONLY);
\n
    if (fd == -1) {
\n
        perror("open");
\n
        exit(EXIT_FAILURE);
\n
    }
\n
    /* Obtain a file descriptor for the owning user namespace and
\n
       then obtain and display the inode number of that namespace. */
\n
    if (argc < 3 || strchr(argv[2], 'u')) {
\n
        userns_fd = ioctl(fd, NS_GET_USERNS);
\n
        if (userns_fd == -1) {
\n
            if (errno == EPERM)
\n
                printf("The owning user namespace is outside "
\n
                       "your namespace scope\n");
\n
            else
\n
               perror("ioctl-NS_GET_USERNS");
\n
            exit(EXIT_FAILURE);
\n
         }
\n
        if (fstat(userns_fd, &sb) == -1) {
\n
            perror("fstat-userns");
\n
            exit(EXIT_FAILURE);
\n
        }
\n
        printf("Device/Inode of owning user namespace is: "
\n
               "[%x,%x] / %ju\n",
\n
               major(sb.st_dev),
\n
               minor(sb.st_dev),
\n
               (uintmax_t) sb.st_ino);
\n
        close(userns_fd);
\n
    }
\n
    /* Obtain a file descriptor for the parent namespace and
\n
       then obtain and display the inode number of that namespace. */
\n
    if (argc > 2 && strchr(argv[2], 'p')) {
\n
        parent_fd = ioctl(fd, NS_GET_PARENT);
\n
        if (parent_fd == -1) {
\n
            if (errno == EINVAL)
\n
                printf("Can' get parent namespace of a "
\n
                       "nonhierarchical namespace\n");
\n
            else if (errno == EPERM)
\n
                printf("The parent namespace is outside "
\n
                       "your namespace scope\n");
\n
            else
\n
                perror("ioctl-NS_GET_PARENT");
\n
            exit(EXIT_FAILURE);
\n
        }
\n
        if (fstat(parent_fd, &sb) == -1) {
\n
            perror("fstat-parentns");
\n
            exit(EXIT_FAILURE);
\n
        }
\n
        printf("Device/Inode of parent namespace is: [%x,%x] / %ju\n",
\n
               major(sb.st_dev),
\n
               minor(sb.st_dev),
\n
               (uintmax_t) sb.st_ino);
\n
        close(parent_fd);
\n
    }
\n
    exit(EXIT_SUCCESS);
}