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NAME |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |STANDARDS |HISTORY |EXAMPLES |SEE ALSO |COLOPHON | |
NS_GET_USERNS(2const)NS_GET_USERNS(2const)NS_GET_USERNS, NS_GET_PARENT - discovering namespace relationships
#include <linux/nsfs.h>/* Definition ofNS_GET_*constants */#include <sys/ioctl.h>int ioctl(intfd, unsigned longop);
The followingioctl(2) operations are provided to allow discovery of namespace relationships (seeuser_namespaces(7) andpid_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 byfd.NS_GET_PARENT Returns a file descriptor that refers to the parent namespace of the namespace referred to byfd. This operation is valid only for hierarchical namespaces (i.e., PID and user namespaces). For user namespaces,NS_GET_PARENTis synonymous withNS_GET_USERNS. The new file descriptor returned by these operations is opened with theO_RDONLYandO_CLOEXEC(close-on-exec; seefcntl(2)) flags. By applyingfstat(2) to the returned file descriptor, one obtains astat structure whosest_dev (resident device) andst_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, anderrno is set to indicate the error.
EPERMThe 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.ENOTTYThe operation is not supported by this kernel version. Additionally, theNS_GET_PARENToperation can fail with the following error:EINVALfd refers to a nonhierarchical namespace.
Linux.
NS_GET_USERNS Linux 4.9.NS_GET_PARENT Linux 4.9.
The example shown below uses theioctl(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 runningsleep(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 scopeProgram 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 theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17NS_GET_USERNS(2const)Pages that refer to this page:ioctl_nsfs(2)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |