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.
- 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.
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/$$/ns/mnt
mnt:[4026531840]
$ sudo ./unshare -m /bin/bash
# readlink /proc/$$/ns/mnt
mnt:[4026532325]
The differing output of the two
readlink(1) commands shows that the two
shells are in different mount namespaces.
/* 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/userspace-api/unshare.rst in the Linux kernel source tree
(or
Documentation/unshare.txt before Linux 4.12)