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setns(2)
SETNS(2) Linux Programmer's Manual SETNS(2)
NAME
setns - reassociate thread with a namespace
SYNOPSIS
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sched.h>
int setns(int fd, int nstype);
DESCRIPTION
Given a file descriptor referring to a namespace, reassociate the call‐
ing 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 reassoci‐
ated with the corresponding namespace, subject to any constraints im‐
posed 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 an‐
other process and, for example, passed to the caller via a UNIX domain
socket.)
Details for specific namespace types
Note the following details and restrictions when reassociating with
specific namespace types:
User namespaces
A process reassociating itself with a user namespace must have
the CAP_SYS_ADMIN capability in the target user namespace.
(This necessarily implies that it is only possible to join a de‐
scendant user namespace.) Upon successfully joining a user
namespace, a process is granted all capabilities in that name‐
space, 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 cur‐
rent 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).
Mount namespaces
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 user namespace that owns
the target mount namespace.
A process may not be reassociated with a new mount namespace if
it is multithreaded.
See user_namespaces(7) for details on the interaction of user
namespaces and mount namespaces.
PID namespaces
In order to reassociate itself with a new PID namespace, the
caller must have the CAP_SYS_ADMIN capability both in its own
user namespace and in the user namespace that owns the target
PID namespace.
If fd refers to a PID namespace, the semantics are somewhat dif‐
ferent 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).
Cgroup namespaces
In order to reassociate itself with a new cgroup namespace, the
caller must have the CAP_SYS_ADMIN capability both in its own
user namespace and in the user namespace that owns the target
cgroup namespace.
Using setns() to change the caller's cgroup namespace does not
change the caller's cgroup memberships.
Network, IPC, and UTS namespaces
In order to reassociate itself with a new network, IPC, or UTS
namespace, the caller must have the CAP_SYS_ADMIN capability
both in its own user namespace and in the user namespace that
owns the target namespace.
RETURN VALUE
On success, setns() returns 0. On failure, -1 is returned and errno is
set to indicate the error.
ERRORS
EBADF fd is not a valid file descriptor.
EINVAL fd refers to a namespace whose type does not match that speci‐
fied in nstype.
EINVAL There is problem with reassociating the thread with the speci‐
fied 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 name‐
space.
ENOMEM Cannot allocate sufficient memory to change the specified name‐
space.
EPERM The calling thread did not have the required capability for this
operation.
VERSIONS
The setns() system call first appeared in Linux in kernel 3.0; library
support was added to glibc in version 2.14.
CONFORMING TO
The setns() system call is Linux-specific.
NOTES
Not all of the attributes that can be shared when a new thread is cre‐
ated using clone(2) can be changed using setns().
EXAMPLE
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 (com‐
piled 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 back‐
ground. 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. In‐
side 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");
}
SEE ALSO
nsenter(1), clone(2), fork(2), unshare(2), vfork(2), namespaces(7),
unix(7)
COLOPHON
This page is part of release 5.02 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 2019-03-06 SETNS(2)