unshare

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

NAME
       unshare - disassociate parts of the process execution context

SYNOPSIS
       #define _GNU_SOURCE
       #include <sched.h>

       int unshare(int flags);

DESCRIPTION
       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 names-
       pace, 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
              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 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_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_names-
              paces(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 automat-
       ically 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.

       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  nest-
              ing  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  num-
              ber  of  nested user namespaces would be exceeded.  See the dis-
              cussion of the ENOSPC error above.

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

CONFORMING TO
       The unshare() system call is Linux-specific.

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 func-
       tionality may be added in the future, if required.

EXAMPLE
       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 verify-
       ing 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, "    -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");

       }

SEE ALSO
       unshare(1), clone(2),  fork(2),  kcmp(2),  setns(2),  vfork(2),  names-
       paces(7)

       Documentation/userspace-api/unshare.rst in the Linux kernel source tree
       (or Documentation/unshare.txt before Linux 4.12)

COLOPHON
       This page is part of release 4.15 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                             2018-02-02                        UNSHARE(2)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2021 Hurricane Electric. All Rights Reserved.