Ubuntu Manpage: unshare - disassociate parts of the process execution context

NAME

       unshare - disassociate parts of the process execution context

LIBRARY

       Standard C library (libc, -lc)

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_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
              calling 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 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 in Linux 2.6.16.

STANDARDS

       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.

       Creating  all  kinds  of  namespace,  except  user  namespaces, requires the CAP_SYS_ADMIN
       capability.  However, since creating a user namespace automatically confers a full set  of
       capabilities,  creating  both a user namespace and any other type of namespace in the same
       unshare() call does not require the CAP_SYS_ADMIN capability in the original namespace.

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
       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 <err.h>
       #include <sched.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

       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 'C': flags |= CLONE_NEWCGROUP;      break;
               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 't': flags |= CLONE_NEWTIME;        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)
               err(EXIT_FAILURE, "unshare");

           execvp(argv[optind], &argv[optind]);
           err(EXIT_FAILURE, "execvp");
       }