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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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unshare(2) System Calls Manual unshare(2)
unshare - disassociate parts of the process execution context
Standard C library (libc, -lc)
#define _GNU_SOURCE
#include <sched.h>
int unshare(int flags);
unshare() allows a process (or thread) to disassociate parts of
its execution context that are currently being shared with other
processes (or threads). Part of the execution context, such as
the mount namespace, is shared implicitly when a new process is
created using fork(2) or vfork(2), while other parts, such as
virtual memory, may be shared by explicit request when creating a
process or thread using clone(2).
The main use of unshare() is to allow a process to control its
shared execution context without creating a new process.
The flags argument is a bit mask that specifies which parts of
the execution context should be unshared. This argument is
specified by ORing together zero or more of the following
constants:
CLONE_FILES
Reverse the effect of the clone(2) CLONE_FILES flag.
Unshare the file descriptor table, so that the calling
process no longer shares its file descriptors with any
other process.
CLONE_FS
Reverse the effect of the clone(2) CLONE_FS flag. Unshare
filesystem attributes, so that the calling process no
longer shares its root directory (chroot(2)), current
directory (chdir(2)), or umask (umask(2)) attributes with
any other process.
CLONE_NEWCGROUP (since Linux 4.6)
This flag has the same effect as the clone(2)
CLONE_NEWCGROUP flag. Unshare the cgroup namespace. Use
of CLONE_NEWCGROUP requires the CAP_SYS_ADMIN capability.
CLONE_NEWIPC (since Linux 2.6.19)
This flag has the same effect as the clone(2) CLONE_NEWIPC
flag. Unshare the IPC namespace, so that the calling
process has a private copy of the IPC namespace which is
not shared with any other process. Specifying this flag
automatically implies CLONE_SYSVSEM as well. Use of
CLONE_NEWIPC requires the CAP_SYS_ADMIN capability.
CLONE_NEWNET (since Linux 2.6.24)
This flag has the same effect as the clone(2) CLONE_NEWNET
flag. Unshare the network namespace, so that the calling
process is moved into a new network namespace which is not
shared with any previously existing process. Use of
CLONE_NEWNET requires the CAP_SYS_ADMIN capability.
CLONE_NEWNS
This flag has the same effect as the clone(2) CLONE_NEWNS
flag. Unshare the mount namespace, so that the calling
process has a private copy of its namespace which is not
shared with any other process. Specifying this flag
automatically implies CLONE_FS as well. Use of
CLONE_NEWNS requires the CAP_SYS_ADMIN capability. For
further information, see mount_namespaces(7).
CLONE_NEWPID (since Linux 3.8)
This flag has the same effect as the clone(2) CLONE_NEWPID
flag. Unshare the PID namespace, so that the calling
process has a new PID namespace for its children which is
not shared with any previously existing process. The
calling process is not moved into the new namespace. The
first child created by the calling process will have the
process ID 1 and will assume the role of init(1) in the
new namespace. CLONE_NEWPID automatically implies
CLONE_THREAD as well. Use of CLONE_NEWPID requires the
CAP_SYS_ADMIN capability. For further information, see
pid_namespaces(7).
CLONE_NEWTIME (since Linux 5.6)
Unshare the time namespace, so that the calling process
has a new time namespace for its children which is not
shared with any previously existing process. The calling
process is not moved into the new namespace. Use of
CLONE_NEWTIME requires the CAP_SYS_ADMIN capability. For
further information, see time_namespaces(7).
CLONE_NEWUSER (since Linux 3.8)
This flag has the same effect as the clone(2)
CLONE_NEWUSER flag. Unshare the user namespace, so that
the calling process is moved into a new user namespace
which is not shared with any previously existing process.
As with the child process created by clone(2) with the
CLONE_NEWUSER flag, the caller obtains a full set of
capabilities in the new namespace.
CLONE_NEWUSER requires that the calling process is not
threaded; specifying CLONE_NEWUSER automatically implies
CLONE_THREAD. Since Linux 3.9, CLONE_NEWUSER also
automatically implies CLONE_FS. CLONE_NEWUSER requires
that the user ID and group ID of the calling process are
mapped to user IDs and group IDs in the user namespace of
the calling process at the time of the call.
For further information on user namespaces, see
user_namespaces(7).
CLONE_NEWUTS (since Linux 2.6.19)
This flag has the same effect as the clone(2) CLONE_NEWUTS
flag. Unshare the UTS IPC namespace, so that the calling
process has a private copy of the UTS namespace which is
not shared with any other process. Use of CLONE_NEWUTS
requires the CAP_SYS_ADMIN capability.
CLONE_SYSVSEM (since Linux 2.6.26)
This flag reverses the effect of the clone(2)
CLONE_SYSVSEM flag. Unshare System V semaphore adjustment
(semadj) values, so that the calling process has a new
empty semadj list that is not shared with any other
process. If this is the last process that has a reference
to the process's current semadj list, then the adjustments
in that list are applied to the corresponding semaphores,
as described in semop(2).
In addition, CLONE_THREAD, CLONE_SIGHAND, and CLONE_VM can be
specified in flags if the caller is single threaded (i.e., it is
not sharing its address space with another process or thread).
In this case, these flags have no effect. (Note also that
specifying CLONE_THREAD automatically implies CLONE_VM, and
specifying CLONE_VM automatically implies CLONE_SIGHAND.) If the
process is multithreaded, then the use of these flags results in
an error.
If flags is specified as zero, then unshare() is a no-op; no
changes are made to the calling process's execution context.
On success, zero returned. On failure, -1 is returned and errno
is set to indicate the error.
EINVAL An invalid bit was specified in flags.
EINVAL CLONE_THREAD, CLONE_SIGHAND, or CLONE_VM was specified in
flags, and the caller is multithreaded.
EINVAL CLONE_NEWIPC was specified in flags, but the kernel was
not configured with the CONFIG_SYSVIPC and CONFIG_IPC_NS
options.
EINVAL CLONE_NEWNET was specified in flags, but the kernel was
not configured with the CONFIG_NET_NS option.
EINVAL CLONE_NEWPID was specified in flags, but the kernel was
not configured with the CONFIG_PID_NS option.
EINVAL CLONE_NEWUSER was specified in flags, but the kernel was
not configured with the CONFIG_USER_NS option.
EINVAL CLONE_NEWUTS was specified in flags, but the kernel was
not configured with the CONFIG_UTS_NS option.
EINVAL CLONE_NEWPID was specified in flags, but the process has
previously called unshare() with the CLONE_NEWPID flag.
ENOMEM Cannot allocate sufficient memory to copy parts of
caller's context that need to be unshared.
ENOSPC (since Linux 3.7)
CLONE_NEWPID was specified in flags, but the limit on the
nesting depth of PID namespaces would have been exceeded;
see pid_namespaces(7).
ENOSPC (since Linux 4.9; beforehand EUSERS)
CLONE_NEWUSER was specified in flags, and the call would
cause the limit on the number of nested user namespaces to
be exceeded. See user_namespaces(7).
From Linux 3.11 to Linux 4.8, the error diagnosed in this
case was EUSERS.
ENOSPC (since Linux 4.9)
One of the values in flags specified the creation of a new
user namespace, but doing so would have caused the limit
defined by the corresponding file in /proc/sys/user to be
exceeded. For further details, see namespaces(7).
EPERM The calling process did not have the required privileges
for this operation.
EPERM CLONE_NEWUSER was specified in flags, but either the
effective user ID or the effective group ID of the caller
does not have a mapping in the parent namespace (see
user_namespaces(7)).
EPERM (since Linux 3.9)
CLONE_NEWUSER was specified in flags and the caller is in
a chroot environment (i.e., the caller's root directory
does not match the root directory of the mount namespace
in which it resides).
EUSERS (from Linux 3.11 to Linux 4.8)
CLONE_NEWUSER was specified in flags, and the limit on the
number of nested user namespaces would be exceeded. See
the discussion of the ENOSPC error above.
Linux.
Linux 2.6.16.
Not all of the process attributes that can be shared when a new
process is created using clone(2) can be unshared using
unshare(). In particular, as at kernel 3.8, unshare() does not
implement flags that reverse the effects of CLONE_SIGHAND,
CLONE_THREAD, or CLONE_VM. Such functionality may be added in
the future, if required.
Creating all kinds of namespace, except user namespaces, requires
the CAP_SYS_ADMIN capability. However, since creating a user
namespace automatically confers a full set of capabilities,
creating both a user namespace and any other type of namespace in
the same unshare() call does not require the CAP_SYS_ADMIN
capability in the original namespace.
The program below provides a simple implementation of the
unshare(1) command, which unshares one or more namespaces and
executes the command supplied in its command-line arguments.
Here's an example of the use of this program, running a shell in
a new mount namespace, and verifying that the original shell and
the new shell are in separate mount namespaces:
$ readlink /proc/$$/ns/mnt
mnt:[4026531840]
$ sudo ./unshare -m /bin/bash
# readlink /proc/$$/ns/mnt
mnt:[4026532325]
The differing output of the two readlink(1) commands shows that
the two shells are in different mount namespaces.
Program source
/* unshare.c
A simple implementation of the unshare(1) command: unshare
namespaces and execute a command.
*/
#define _GNU_SOURCE
#include <err.h>
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
static void
usage(char *pname)
{
fprintf(stderr, "Usage: %s [options] program [arg...]\n", pname);
fprintf(stderr, "Options can be:\n");
fprintf(stderr, " -C unshare cgroup namespace\n");
fprintf(stderr, " -i unshare IPC namespace\n");
fprintf(stderr, " -m unshare mount namespace\n");
fprintf(stderr, " -n unshare network namespace\n");
fprintf(stderr, " -p unshare PID namespace\n");
fprintf(stderr, " -t unshare time namespace\n");
fprintf(stderr, " -u unshare UTS namespace\n");
fprintf(stderr, " -U unshare user namespace\n");
exit(EXIT_FAILURE);
}
int
main(int argc, char *argv[])
{
int flags, opt;
flags = 0;
while ((opt = getopt(argc, argv, "CimnptuU")) != -1) {
switch (opt) {
case 'C': flags |= CLONE_NEWCGROUP; break;
case 'i': flags |= CLONE_NEWIPC; break;
case 'm': flags |= CLONE_NEWNS; break;
case 'n': flags |= CLONE_NEWNET; break;
case 'p': flags |= CLONE_NEWPID; break;
case 't': flags |= CLONE_NEWTIME; break;
case 'u': flags |= CLONE_NEWUTS; break;
case 'U': flags |= CLONE_NEWUSER; break;
default: usage(argv[0]);
}
}
if (optind >= argc)
usage(argv[0]);
if (unshare(flags) == -1)
err(EXIT_FAILURE, "unshare");
execvp(argv[optind], &argv[optind]);
err(EXIT_FAILURE, "execvp");
}
unshare(1), clone(2), fork(2), kcmp(2), setns(2), vfork(2),
namespaces(7)
Documentation/userspace-api/unshare.rst in the Linux kernel
source tree (or Documentation/unshare.txt before Linux 4.12)
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
fetched from
⟨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
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-06-15 unshare(2)
Pages that refer to this page: unshare(1), clone(2), fork(2), kcmp(2), mount(2), mount_setattr(2), NS_GET_NSTYPE(2const), setns(2), syscalls(2), vfork(2), 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)
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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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