.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH NS_GET_USERNS 2const 2025-05-17 "Linux man-pages (unreleased)"
.SH NAME
NS_GET_USERNS,
NS_GET_PARENT
\-
discovering namespace relationships
.SH SYNOPSIS
.nf
.BR "#include <linux/nsfs.h>" "  /* Definition of " NS_GET_* " constants */"
.B #include <sys/ioctl.h>
.P
.BI "int ioctl(int " fd ", unsigned long " op );
.fi
.SH DESCRIPTION
The following
.BR ioctl (2)
operations are provided to allow discovery of namespace relationships (see
.BR user_namespaces (7)
and
.BR pid_namespaces (7)).
.P
In each case,
.I fd
refers to a
.IR /proc/ pid /ns/*
file.
Both operations return a new file descriptor on success.
.TP
.B NS_GET_USERNS
Returns a file descriptor that refers to the owning user namespace
for the namespace referred to by
.IR fd .
.TP
.B NS_GET_PARENT
Returns a file descriptor that refers to the parent namespace of
the namespace referred to by
.IR fd .
This operation is valid only for hierarchical namespaces
(i.e., PID and user namespaces).
For user namespaces,
.B NS_GET_PARENT
is synonymous with
.BR NS_GET_USERNS .
.P
The new file descriptor returned by these operations is opened with the
.B O_RDONLY
and
.B O_CLOEXEC
(close-on-exec; see
.BR fcntl (2))
flags.
.P
By applying
.BR fstat (2)
to the returned file descriptor, one obtains a
.I stat
structure whose
.I st_dev
(resident device) and
.I st_ino
(inode number) fields together identify the owning/parent namespace.
This inode number can be matched with the inode number of another
.IR /proc/ pid /ns/ { pid , user }
file to determine whether that is the owning/parent namespace.
.SH RETURN VALUE
On success,
a file descriptor is returned.
Or error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B 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.
.TP
.B ENOTTY
The operation is not supported by this kernel version.
.P
Additionally, the
.B NS_GET_PARENT
operation can fail with the following error:
.TP
.B EINVAL
.I fd
refers to a nonhierarchical namespace.
.SH STANDARDS
Linux.
.SH HISTORY
.TP
.B NS_GET_USERNS
.\" linux.git bcac25a58bfc6bd79191ac5d7afb49bea96da8c9
.\" linux.git 6786741dbf99e44fb0c0ed85a37582b8a26f1c3b
Linux 4.9.
.TP
.B NS_GET_PARENT
.\" linux.git a7306ed8d94af729ecef8b6e37506a1c6fc14788
Linux 4.9.
.SH EXAMPLES
The example shown below uses the
.BR 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.
.P
Trying to get the parent of the initial user namespace fails,
since it has no parent:
.P
.in +4n
.EX
.RB $ " ./ns_show /proc/self/ns/user p" ;
The parent namespace is outside your namespace scope
.EE
.in
.P
Create a process running
.BR sleep (1)
that resides in new user and UTS namespaces,
and show that the new UTS namespace is associated with the new user namespace:
.P
.in +4n
.EX
.RB $ " unshare \-Uu sleep 1000 &"
[1] 23235
.RB $ " ./ns_show /proc/23235/ns/uts u" ;
Device/Inode of owning user namespace is: [0,3] / 4026532448
.RB $ " readlink /proc/23235/ns/user" ;
user:[4026532448]
.EE
.in
.P
Then show that the parent of the new user namespace in the preceding
example is the initial user namespace:
.P
.in +4n
.EX
.RB $ " readlink /proc/self/ns/user" ;
user:[4026531837]
.RB $ " ./ns_show /proc/23235/ns/user p" ;
Device/Inode of parent namespace is: [0,3] / 4026531837
.EE
.in
.P
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.
.P
.in +4n
.EX
.RB $ " PS1=\[dq]sh2$ \[dq] unshare \-U bash" ;
.RB sh2$ " ./ns_show /proc/self/ns/user p" ;
The parent namespace is outside your namespace scope
.RB sh2$ " ./ns_show /proc/self/ns/uts u" ;
The owning user namespace is outside your namespace scope
.EE
.in
.SS Program source
\&
.\" SRC BEGIN (ns_show.c)
.EX
/* 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]\[rs]n",
                argv[0]);
        fprintf(stderr, "\[rs]nDisplay the result of one or both "
                "of NS_GET_USERNS (u) or NS_GET_PARENT (p)\[rs]n"
                "for the specified /proc/[pid]/ns/[file]. If neither "
                "\[aq]p\[aq] nor \[aq]u\[aq] is specified,\[rs]n"
                "NS_GET_USERNS is the default.\[rs]n");
        exit(EXIT_FAILURE);
    }
\&
    /* Obtain a file descriptor for the \[aq]ns\[aq] 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], \[aq]u\[aq])) {
        userns_fd = ioctl(fd, NS_GET_USERNS);
\&
        if (userns_fd == \-1) {
            if (errno == EPERM)
                printf("The owning user namespace is outside "
                       "your namespace scope\[rs]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\[rs]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], \[aq]p\[aq])) {
        parent_fd = ioctl(fd, NS_GET_PARENT);
\&
        if (parent_fd == \-1) {
            if (errno == EINVAL)
                printf("Can\[aq] get parent namespace of a "
                       "nonhierarchical namespace\[rs]n");
            else if (errno == EPERM)
                printf("The parent namespace is outside "
                       "your namespace scope\[rs]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\[rs]n",
               major(sb.st_dev),
               minor(sb.st_dev),
               (uintmax_t) sb.st_ino);
\&
        close(parent_fd);
    }
\&
    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR ioctl (2),
.BR ioctl_nsfs (2)