unshare

       #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 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_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.
              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.


       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 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/$$/ns/mnt
           mnt:[4026531840]
           $ sudo ./unshare -m /bin/bash
           [sudo] password for cecilia:
           # readlink /proc/$$/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

       #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/unshare.txt in the Linux kernel source tree

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