unshare(2) System Calls Manual unshare(2)
NAME
unshare - disassociate parts of the process execution context
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#define _GNU_SOURCE
#include <sched.h>
int unshare(int flags);
DESCRIPTION
unshare() allows a process (or thread) to disassociate parts of its exe-
cution 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 exe-
cution 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 re-
quires 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 pri-
vate 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 pre-
viously 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_name-
spaces(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 pre-
viously existing process. The calling process is not moved into
the new namespace. Use of CLONE_NEWTIME requires the CAP_SYS_AD-
MIN 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 ca-
pabilities 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_name-
spaces(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 sema-
phores, 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 automati-
cally 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.
RETURN VALUE
On success, zero returned. On failure, -1 is returned and errno is set
to indicate the error.
ERRORS
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 con-
figured with the CONFIG_SYSVIPC and CONFIG_IPC_NS options.
EINVAL CLONE_NEWNET was specified in flags, but the kernel was not con-
figured with the CONFIG_NET_NS option.
EINVAL CLONE_NEWPID was specified in flags, but the kernel was not con-
figured with the CONFIG_PID_NS option.
EINVAL CLONE_NEWUSER was specified in flags, but the kernel was not con-
figured with the CONFIG_USER_NS option.
EINVAL CLONE_NEWUTS was specified in flags, but the kernel was not con-
figured with the CONFIG_UTS_NS option.
EINVAL CLONE_NEWPID was specified in flags, but the process has previ-
ously called unshare() with the CLONE_NEWPID flag.
ENOMEM Cannot allocate sufficient memory to copy parts of caller's con-
text 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_name-
spaces(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 ch-
root environment (i.e., the caller's root directory does not
match the root directory of the mount namespace in which it re-
sides).
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.
STANDARDS
Linux.
HISTORY
Linux 2.6.16.
NOTES
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 particu-
lar, as at kernel 3.8, unshare() does not implement flags that reverse
the effects of CLONE_SIGHAND, CLONE_THREAD, or CLONE_VM. Such function-
ality 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 au-
tomatically 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.
EXAMPLES
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 name-
spaces:
$ 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");
}
SEE ALSO
unshare(1), clone(2), fork(2), kcmp(2), setns(2), vfork(2), name-
spaces(7)
Documentation/userspace-api/unshare.rst in the Linux kernel source tree
(or Documentation/unshare.txt before Linux 4.12)
Linux man-pages 6.9.1 2024-06-15 unshare(2)
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