setns(2) System Calls Manual setns(2)
NAME
setns - reassociate thread with a namespace
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sched.h>
int setns(int fd, int nstype);
DESCRIPTION
The setns() system call allows the calling thread to move into differ-
ent namespaces. The fd argument is one of the following:
• a file descriptor referring to one of the magic links in a
/proc/pid/ns/ directory (or a bind mount to such a link);
• a PID file descriptor (see pidfd_open(2)).
The nstype argument is interpreted differently in each case.
fd refers to a /proc/[pid]/ns/ link
If fd refers to a /proc/pid/ns/ link, then setns() reassociates the
calling thread with the namespace associated with that link, subject to
any constraints imposed by the nstype argument. In this usage, each
call to setns() changes just one of the caller's namespace memberships.
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_NEWTIME (since Linux 5.8)
fd must refer to a time 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.)
fd is a PID file descriptor
Since Linux 5.8, fd may refer to a PID file descriptor obtained from
pidfd_open(2) or clone(2). In this usage, setns() atomically moves the
calling thread into one or more of the same namespaces as the thread
referred to by fd.
The nstype argument is a bit mask specified by ORing together one or
more of the CLONE_NEW* namespace constants listed above. The caller is
moved into each of the target thread's namespaces that is specified in
nstype; the caller's memberships in the remaining namespaces are left
unchanged.
For example, the following code would move the caller into the same
user, network, and UTS namespaces as PID 1234, but would leave the
caller's other namespace memberships unchanged:
int fd = pidfd_open(1234, 0);
setns(fd, CLONE_NEWUSER | CLONE_NEWNET | CLONE_NEWUTS);
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 can't join a new mount namespace if it is sharing
filesystem-related attributes (the attributes whose sharing is
controlled by the clone(2) CLONE_FS flag) with another process.
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.
Reassociating the PID namespace has 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 target
PID namespace is a descendant (child, grandchild, etc.) of, or
is the same as, the current 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, time, and UTS namespaces
In order to reassociate itself with a new network, IPC, time, 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.
EINVAL fd is a PID file descriptor and nstype is invalid (e.g., it is
0).
ENOMEM Cannot allocate sufficient memory to change the specified name-
space.
EPERM The calling thread did not have the required capability for this
operation.
ESRCH fd is a PID file descriptor but the process it refers to no
longer exists (i.e., it has terminated and been waited on).
VERSIONS
The setns() system call first appeared in Linux 3.0; library support
was added in glibc 2.14.
STANDARDS
The setns() system call is Linux-specific.
NOTES
For further information on the /proc/pid/ns/ magic links, see name-
spaces(7).
Not all of the attributes that can be shared when a new thread is cre-
ated using clone(2) can be changed using setns().
EXAMPLES
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 argu-
ments. The program opens the namespace file, joins that namespace us-
ing 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 <err.h>
#include <fcntl.h>
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
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);
}
/* Get file descriptor for namespace; the file descriptor is opened
with O_CLOEXEC so as to ensure that it is not inherited by the
program that is later executed. */
fd = open(argv[1], O_RDONLY | O_CLOEXEC);
if (fd == -1)
err(EXIT_FAILURE, "open");
if (setns(fd, 0) == -1) /* Join that namespace */
err(EXIT_FAILURE, "setns");
execvp(argv[2], &argv[2]); /* Execute a command in namespace */
err(EXIT_FAILURE, "execvp");
}
SEE ALSO
nsenter(1), clone(2), fork(2), unshare(2), vfork(2), namespaces(7),
unix(7)
Linux man-pages 6.03 2023-02-05 setns(2)
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