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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON |
UNSHARE(2) Linux Programmer's Manual UNSHARE(2)
unshare - disassociate parts of the process execution context
#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_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.
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 (since Linux 3.11)
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.
The unshare() system call was added to Linux in kernel 2.6.16.
The unshare() system call is Linux-specific.
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.
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/$$/ns/mnt
mnt:[4026531840]
$ sudo ./unshare -m /bin/bash
[sudo] password for cecilia:
# readlink /proc/$$/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 <sched.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
/* A simple error-handling function: print an error message based
on the value in 'errno' and terminate the calling process */
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
static void
usage(char *pname)
{
fprintf(stderr, "Usage: %s [options] program [arg...]\n", pname);
fprintf(stderr, "Options can be:\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, " -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, "imnpuU")) != -1) {
switch (opt) {
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 '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)
errExit("unshare");
execvp(argv[optind], &argv[optind]);
errExit("execvp");
}
unshare(1), clone(2), fork(2), kcmp(2), setns(2), vfork(2),
namespaces(7)
Documentation/unshare.txt in the Linux kernel source tree
This page is part of release 4.12 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 2017-05-03 UNSHARE(2)
Pages that refer to this page: unshare(1), clone(2), fork(2), ioctl_ns(2), kcmp(2), mount(2), setns(2), syscalls(2), vfork(2), systemd.exec(5), capabilities(7), cgroup_namespaces(7), mount_namespaces(7), namespaces(7), pid_namespaces(7), user_namespaces(7)