UNSHARE(2)                (2020-04-11)                 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 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 pro-
               cess 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 capa-
               bility.

          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

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               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 exist-
               ing 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 call-
               ing 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

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               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 capabil-
               ity.

          CLONE_SYSVSEM (since Linux 2.6.26)
               This flag reverses the effect of the clone(2)
               CLONE_SYSVSEM flag.  Unshare System V semaphore adjust-
               ment (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 refer-
               ence to the process's current semadj list, then the
               adjustments in that list are applied to the correspond-
               ing 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 pro-
          cess 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
          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.

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          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,

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               see namespaces(7).

          EPERM
               The calling process did not have the required privi-
               leges 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 direc-
               tory 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
          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 particular, as at kernel 3.8, unshare() does
          not implement flags that reverse the effects of
          CLONE_SIGHAND, CLONE_THREAD, or CLONE_VM.  Such functional-
          ity may be added in the future, if required.

     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 argu-
          ments.  Here's an example of the use of this program, run-
          ning 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]

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          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 aqerrnoaq 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 aqCaq: flags |= CLONE_NEWCGROUP;      break;
                  case aqiaq: flags |= CLONE_NEWIPC;        break;
                  case aqmaq: flags |= CLONE_NEWNS;         break;
                  case aqnaq: flags |= CLONE_NEWNET;        break;
                  case aqpaq: flags |= CLONE_NEWPID;        break;

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                  case aqtaq: flags |= CLONE_NEWTIME;        break;
                  case aquaq: flags |= CLONE_NEWUTS;        break;
                  case aqUaq: 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),
          namespaces(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 5.10 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/.

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