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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | EXAMPLES | SEE ALSO | COLOPHON |
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NS_GET_USERNS(2const) NS_GET_USERNS(2const)
NS_GET_USERNS, NS_GET_PARENT - discovering namespace relationships
#include <linux/nsfs.h> /* Definition of NS_GET_* constants */
#include <sys/ioctl.h>
int ioctl(int fd, unsigned long op);
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.
On success, a file descriptor is returned. Or error, -1 is
returned, and errno is set to indicate the error.
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.
Linux.
NS_GET_USERNS
Linux 4.9.
NS_GET_PARENT
Linux 4.9.
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);
}
ioctl(2), ioctl_nsfs(2)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
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for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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2025-08-11. If you discover any rendering problems in this HTML
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Linux man-pages 6.15 2025-05-17 NS_GET_USERNS(2const)
Pages that refer to this page: ioctl_nsfs(2)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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