PID_NAMESPACES(7)          Linux Programmer's Manual         PID_NAMESPACES(7)

       pid_namespaces - overview of Linux PID namespaces

       For an overview of namespaces, see namespaces(7).

       PID  namespaces  isolate the process ID number space, meaning that pro-
       cesses in different PID namespaces can have the same PID.   PID  names-
       paces  allow  containers  to  provide  functionality  such  as suspend-
       ing/resuming the set of processes in the container  and  migrating  the
       container  to a new host while the processes inside the container main-
       tain the same PIDs.

       PIDs in a new PID namespace start at 1, somewhat like a standalone sys-
       tem, and calls to fork(2), vfork(2), or clone(2) will produce processes
       with PIDs that are unique within the namespace.

       Use of PID namespaces requires a kernel that  is  configured  with  the
       CONFIG_PID_NS option.

   The namespace init process
       The first process created in a new namespace (i.e., the process created
       using clone(2) with the CLONE_NEWPID flag, or the first  child  created
       by  a  process  after a call to unshare(2) using the CLONE_NEWPID flag)
       has the PID 1, and  is  the  "init"  process  for  the  namespace  (see
       init(1)).   A  child process that is orphaned within the namespace will
       be reparented to this process rather than init(1) (unless  one  of  the
       ancestors  of the child in the same PID namespace employed the prctl(2)
       PR_SET_CHILD_SUBREAPER command to mark itself as the reaper of orphaned
       descendant processes).

       If  the "init" process of a PID namespace terminates, the kernel termi-
       nates all of the processes in the namespace via a SIGKILL signal.  This
       behavior reflects the fact that the "init" process is essential for the
       correct operation of a PID  namespace.   In  this  case,  a  subsequent
       fork(2)  into  this PID namespace fail with the error ENOMEM; it is not
       possible to create a new processes in  a  PID  namespace  whose  "init"
       process  has terminated.  Such scenarios can occur when, for example, a
       process uses an open file descriptor for a /proc/[pid]/ns/pid file cor-
       responding  to  a process that was in a namespace to setns(2) into that
       namespace after the "init" process has  terminated.   Another  possible
       scenario  can occur after a call to unshare(2): if the first child sub-
       sequently created by a fork(2) terminates,  then  subsequent  calls  to
       fork(2) fail with ENOMEM.

       Only signals for which the "init" process has established a signal han-
       dler can be sent to the "init" process by  other  members  of  the  PID
       namespace.   This restriction applies even to privileged processes, and
       prevents other members of the PID namespace from  accidentally  killing
       the "init" process.

       Likewise,  a process in an ancestor namespace can--subject to the usual
       permission checks described in  kill(2)--send  signals  to  the  "init"
       process  of a child PID namespace only if the "init" process has estab-
       lished a handler for that signal.  (Within the handler,  the  siginfo_t
       si_pid  field  described  in  sigaction(2)  will  be zero.)  SIGKILL or
       SIGSTOP are treated exceptionally: these signals are forcibly delivered
       when sent from an ancestor PID namespace.  Neither of these signals can
       be caught by the "init" process,  and  so  will  result  in  the  usual
       actions  associated  with  those signals (respectively, terminating and
       stopping the process).

       Starting with Linux 3.4, the reboot(2) system call causes a  signal  to
       be  sent  to  the  namespace  "init"  process.   See reboot(2) for more

   Nesting PID namespaces
       PID namespaces can be nested: each PID namespace has a  parent,  except
       for  the initial ("root") PID namespace.  The parent of a PID namespace
       is the PID namespace of the process that created  the  namespace  using
       clone(2)  or  unshare(2).   PID  namespaces  thus form a tree, with all
       namespaces ultimately tracing their ancestry  to  the  root  namespace.
       Since  Linux  3.7,  the kernel limits the maximum nesting depth for PID
       namespaces to 32.

       A process is visible to other processes in its PID  namespace,  and  to
       the  processes  in each direct ancestor PID namespace going back to the
       root PID namespace.  In this context, "visible" means that one  process
       can  be  the target of operations by another process using system calls
       that specify a process ID.  Conversely, the processes in  a  child  PID
       namespace  can't see processes in the parent and further removed ances-
       tor namespaces.  More succinctly: a process can see (e.g., send signals
       with kill(2), set nice values with setpriority(2), etc.) only processes
       contained in its own PID namespace and in descendants  of  that  names-

       A process has one process ID in each of the layers of the PID namespace
       hierarchy in which is visible, and  walking  back  though  each  direct
       ancestor  namespace  through  to  the root PID namespace.  System calls
       that operate on process IDs always operate using the process ID that is
       visible in the PID namespace of the caller.  A call to getpid(2) always
       returns the PID associated with the namespace in which the process  was

       Some  processes in a PID namespace may have parents that are outside of
       the namespace.  For example, the parent of the initial process  in  the
       namespace  (i.e.,  the  init(1)  process  with PID 1) is necessarily in
       another namespace.  Likewise, the direct children  of  a  process  that
       uses  setns(2)  to  cause its children to join a PID namespace are in a
       different PID namespace from the caller of setns(2).   Calls  to  getp-
       pid(2) for such processes return 0.

       While  processes  may  freely  descend into child PID namespaces (e.g.,
       using setns(2) with a PID namespace file descriptor), they may not move
       in  the  other  direction.  That is to say, processes may not enter any
       ancestor namespaces (parent, grandparent, etc.).  Changing  PID  names-
       paces is a one-way operation.

       The  NS_GET_PARENT  ioctl(2)  operation  can  be  used  to discover the
       parental relationship between PID namespaces; see ioctl_ns(2).

   setns(2) and unshare(2) semantics
       Calls to setns(2) that specify a  PID  namespace  file  descriptor  and
       calls  to  unshare(2)  with the CLONE_NEWPID flag cause children subse-
       quently created by the caller to be placed in a different PID namespace
       from  the  caller.   (Since Linux 4.12, that PID namespace is shown via
       the  /proc/[pid]/ns/pid_for_children  file,  as  described  in   names-
       paces(7).)   These  calls  do not, however, change the PID namespace of
       the calling process, because doing so would change the caller's idea of
       its  own PID (as reported by getpid()), which would break many applica-
       tions and libraries.

       To put things another way: a  process's  PID  namespace  membership  is
       determined  when  the  process  is created and cannot be changed there-
       after.  Among other things, this means that the  parental  relationship
       between  processes mirrors the parental relationship between PID names-
       paces: the parent of a process is  either  in  the  same  namespace  or
       resides in the immediate parent PID namespace.

   Compatibility of CLONE_NEWPID with other CLONE_* flags
       In  current  versions  of  Linux,  CLONE_NEWPID  can't be combined with
       CLONE_THREAD.  Threads are required to be in  the  same  PID  namespace
       such  that  the  threads  in  a process can send signals to each other.
       Similarly, it must be possible to see all of the threads of a processes
       in  the  proc(5) filesystem.  Additionally, if two threads were in dif-
       ferent PID namespaces, the process ID of the process sending  a  signal
       could  not  be  meaningfully  encoded  when  a  signal is sent (see the
       description of the siginfo_t type in sigaction(2)).  Since this is com-
       puted  when a signal is enqueued, a signal queue shared by processes in
       multiple PID namespaces would defeat that.

       In earlier versions of Linux, CLONE_NEWPID was additionally  disallowed
       (failing  with  the  error  EINVAL)  in  combination with CLONE_SIGHAND
       (before Linux 4.3) as  well  as  CLONE_VM  (before  Linux  3.12).   The
       changes that lifted these restrictions have also been ported to earlier
       stable kernels.

   /proc and PID namespaces
       A /proc filesystem shows (in the  /proc/[pid]  directories)  only  pro-
       cesses  visible  in the PID namespace of the process that performed the
       mount, even if the /proc filesystem is viewed from processes  in  other

       After  creating  a  new  PID  namespace,  it is useful for the child to
       change its root directory and mount a new procfs instance at  /proc  so
       that  tools  such as ps(1) work correctly.  If a new mount namespace is
       simultaneously created by including CLONE_NEWNS in the  flags  argument
       of  clone(2)  or unshare(2), then it isn't necessary to change the root
       directory: a new procfs instance can be mounted directly over /proc.

       From a shell, the command to mount /proc is:

           $ mount -t proc proc /proc

       Calling readlink(2) on the path /proc/self yields the process ID of the
       caller  in  the PID namespace of the procfs mount (i.e., the PID names-
       pace of the process that mounted the procfs).  This can be  useful  for
       introspection  purposes,  when  a  process wants to discover its PID in
       other namespaces.

   /proc files
       /proc/sys/kernel/ns_last_pid (since Linux 3.3)
              This file displays the last PID that was allocated in  this  PID
              namespace.   When  the  next  PID  is allocated, the kernel will
              search for the lowest unallocated PID that is greater than  this
              value, and when this file is subsequently read it will show that

              This file is writable by a process that  has  the  CAP_SYS_ADMIN
              capability inside its user namespace.  This makes it possible to
              determine the PID that is allocated to the next process that  is
              created inside this PID namespace.

       When a process ID is passed over a UNIX domain socket to a process in a
       different PID namespace (see  the  description  of  SCM_CREDENTIALS  in
       unix(7)),  it  is  translated  into  the corresponding PID value in the
       receiving process's PID namespace.

       Namespaces are a Linux-specific feature.

       See user_namespaces(7).

       clone(2), reboot(2), setns(2),  unshare(2),  proc(5),  capabilities(7),
       credentials(7), mount_namespaces(7), namespaces(7), user_namespaces(7),

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Linux                             2017-11-26                 PID_NAMESPACES(7)
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