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.

STANDARDS

       Linux.

HISTORY

       Linux 2.6.16.

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");
       }