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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | VERSIONS | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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setns(2) System Calls Manual setns(2)
setns - reassociate thread with a namespace
Standard C library (libc, -lc)
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sched.h>
int setns(int fd, int nstype);
The setns() system call allows the calling thread to move into
different 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 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.
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.
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
namespace.
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).
Linux.
Linux 3.0, glibc 2.14.
For further information on the /proc/pid/ns/ magic links, see
namespaces(7).
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 <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");
}
nsenter(1), clone(2), fork(2), unshare(2), vfork(2),
namespaces(7), unix(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
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man-pages@man7.org
Linux man-pages 6.9.1 2024-06-15 setns(2)
Pages that refer to this page: nsenter(1), clone(2), pidfd_open(2), syscalls(2), unshare(2), proc_pid_ns(5), systemd.exec(5), capabilities(7), cgroup_namespaces(7), ipc_namespaces(7), mount_namespaces(7), namespaces(7), pid_namespaces(7), time_namespaces(7), user_namespaces(7), uts_namespaces(7), ip-netns(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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