xenial (2) unshare.2.gz

Provided by: manpages-dev_4.04-2_all bug

NAME

       unshare - disassociate parts of the process execution context

SYNOPSIS

       #include <sched.h>

       int unshare(int flags);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       unshare():
           Since glibc 2.14:
               _GNU_SOURCE
           Before glibc 2.14:
               _BSD_SOURCE || _SVID_SOURCE
                   /* _GNU_SOURCE also suffices */

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_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.

       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.

       ENOMEM Cannot allocate sufficient memory to copy parts of caller's context that need to be unshared.

       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 (since Linux 3.11)
              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).

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/$$/ns/mnt
           mnt:[4026531840]
           $ sudo ./unshare -m /bin/bash
           [sudo] password for cecilia:
           # readlink /proc/$$/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/unshare.txt in the Linux kernel source tree

COLOPHON

       This  page  is  part  of  release  4.04  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
       http://www.kernel.org/doc/man-pages/.