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
       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.

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
              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
       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
       functionality 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
       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, "    -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),
       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 5.03 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                             2019-03-06                        UNSHARE(2)