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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON |
SETNS(2) Linux Programmer's Manual SETNS(2)
setns - reassociate thread with a namespace
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sched.h>
int setns(int fd, int nstype);
Given a file descriptor referring to a namespace, reassociate the
calling thread with that namespace.
The fd argument is a file descriptor referring to one of the
namespace entries in a /proc/[pid]/ns/ directory; see namespaces(7)
for further information on /proc/[pid]/ns/. The calling thread will
be reassociated with the corresponding namespace, subject to any
constraints imposed by the nstype argument.
The nstype argument specifies which type of namespace the calling
thread may be reassociated with. This argument can have one of the
following values:
0 Allow any type of namespace to be joined.
CLONE_NEWCGROUP (since Linux 4.6)
fd must refer to a cgroup namespace.
CLONE_NEWIPC (since Linux 3.0)
fd must refer to an IPC namespace.
CLONE_NEWNET (since Linux 3.0)
fd must refer to a network namespace.
CLONE_NEWNS (since Linux 3.8)
fd must refer to a mount namespace.
CLONE_NEWPID (since Linux 3.8)
fd must refer to a descendant PID namespace.
CLONE_NEWUSER (since Linux 3.8)
fd must refer to a user namespace.
CLONE_NEWUTS (since Linux 3.0)
fd must refer to a UTS namespace.
Specifying nstype as 0 suffices if the caller knows (or does not
care) what type of namespace is referred to by fd. Specifying a
nonzero value for nstype is useful if the caller does not know what
type of namespace is referred to by fd and wants to ensure that the
namespace is of a particular type. (The caller might not know the
type of the namespace referred to by fd if the file descriptor was
opened by another process and, for example, passed to the caller via
a UNIX domain socket.)
If fd refers to a PID namespaces, the semantics are somewhat
different from other namespace types: reassociating the calling
thread with a PID namespace changes only the PID namespace that
subsequently created child processes of the caller will be placed in;
it does not change the PID namespace of the caller itself.
Reassociating with a PID namespace is allowed only if the PID
namespace specified by fd is a descendant (child, grandchild, etc.)
of the PID namespace of the caller. For further details on PID
namespaces, see pid_namespaces(7).
A process reassociating itself with a user namespace must have the
CAP_SYS_ADMIN capability in the target user namespace. Upon
successfully joining a user namespace, a process is granted all
capabilities in that namespace, regardless of its user and group IDs.
A multithreaded process may not change user namespace with setns().
It is not permitted to use setns() to reenter the caller's current
user namespace. This prevents a caller that has dropped capabilities
from regaining those capabilities via a call to setns(). For
security reasons, a process can't join a new user namespace if it is
sharing filesystem-related attributes (the attributes whose sharing
is controlled by the clone(2) CLONE_FS flag) with another process.
For further details on user namespaces, see user_namespaces(7).
A process may not be reassociated with a new mount namespace if it is
multithreaded. Changing the mount namespace requires that the caller
possess both CAP_SYS_CHROOT and CAP_SYS_ADMIN capabilities in its own
user namespace and CAP_SYS_ADMIN in the target mount namespace. See
user_namespaces(7) for details on the interaction of user namespaces
and mount namespaces.
Using setns() to change the caller's cgroup namespace does not change
the caller's cgroup memberships.
On success, setns() returns 0. On failure, -1 is returned and errno
is set to indicate the error.
EBADF fd is not a valid file descriptor.
EINVAL fd refers to a namespace whose type does not match that
specified in nstype.
EINVAL There is problem with reassociating the thread with the
specified namespace.
EINVAL The caller tried to join an ancestor (parent, grandparent, and
so on) PID namespace.
EINVAL The caller attempted to join the user namespace in which it is
already a member.
EINVAL The caller shares filesystem (CLONE_FS) state (in particular,
the root directory) with other processes and tried to join a
new user namespace.
EINVAL The caller is multithreaded and tried to join a new user
namespace.
ENOMEM Cannot allocate sufficient memory to change the specified
namespace.
EPERM The calling thread did not have the required capability for
this operation.
The setns() system call first appeared in Linux in kernel 3.0;
library support was added to glibc in version 2.14.
The setns() system call is Linux-specific.
Not all of the attributes that can be shared when a new thread is
created using clone(2) can be changed using setns().
The program below takes two or more arguments. The first argument
specifies the pathname of a namespace file in an existing
/proc/[pid]/ns/ directory. The remaining arguments specify a command
and its arguments. The program opens the namespace file, joins that
namespace using setns(), and executes the specified command inside
that namespace.
The following shell session demonstrates the use of this program
(compiled as a binary named ns_exec) in conjunction with the
CLONE_NEWUTS example program in the clone(2) man page (complied as a
binary named newuts).
We begin by executing the example program in clone(2) in the
background. That program creates a child in a separate UTS
namespace. The child changes the hostname in its namespace, and then
both processes display the hostnames in their UTS namespaces, so that
we can see that they are different.
$ su # Need privilege for namespace operations
Password:
# ./newuts bizarro &
[1] 3549
clone() returned 3550
uts.nodename in child: bizarro
uts.nodename in parent: antero
# uname -n # Verify hostname in the shell
antero
We then run the program shown below, using it to execute a shell.
Inside that shell, we verify that the hostname is the one set by the
child created by the first program:
# ./ns_exec /proc/3550/ns/uts /bin/bash
# uname -n # Executed in shell started by ns_exec
bizarro
Program source
#define _GNU_SOURCE
#include <fcntl.h>
#include <sched.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
int
main(int argc, char *argv[])
{
int fd;
if (argc < 3) {
fprintf(stderr, "%s /proc/PID/ns/FILE cmd args...\n", argv[0]);
exit(EXIT_FAILURE);
}
fd = open(argv[1], O_RDONLY); /* Get file descriptor for namespace */
if (fd == -1)
errExit("open");
if (setns(fd, 0) == -1) /* Join that namespace */
errExit("setns");
execvp(argv[2], &argv[2]); /* Execute a command in namespace */
errExit("execvp");
}
clone(2), fork(2), unshare(2), vfork(2), namespaces(7), unix(7)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-03-15 SETNS(2)
Pages that refer to this page: nsenter(1), clone(2), syscalls(2), unshare(2), proc(5), systemd.exec(5), capabilities(7), cgroup_namespaces(7), mount_namespaces(7), namespaces(7), pid_namespaces(7), user_namespaces(7), ip-netns(8)