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 another 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 descendant 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).

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.

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");
}