unshare(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

UNSHARE(2)              Linux Programmer's Manual             UNSHARE(2)

NAME         top

       unshare - disassociate parts of the process execution context

SYNOPSIS         top

       #define _GNU_SOURCE
       #include <sched.h>

       int unshare(int flags);

DESCRIPTION         top

       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.

RETURN VALUE         top

       On success, zero returned.  On failure, -1 is returned and errno
       is set to indicate the error.

ERRORS         top

       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.

VERSIONS         top

       The unshare() system call was added to Linux in kernel 2.6.16.

CONFORMING TO         top

       The unshare() system call is Linux-specific.

NOTES         top

       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.

EXAMPLES         top

       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 <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, "    -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)
               errExit("unshare");

           execvp(argv[optind], &argv[optind]);
           errExit("execvp");
       }

SEE ALSO         top

       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)

COLOPHON         top

       This page is part of release 5.11 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                          2021-03-22                     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)ipc_namespaces(7)mount_namespaces(7)namespaces(7)pid_namespaces(7)time_namespaces(7)user_namespaces(7)uts_namespaces(7)