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  name-
       spaces allow containers to provide functionality such as suspending/re-
       suming the set of processes in the container  and  migrating  the  con-
       tainer  to a new host while the processes inside the container maintain
       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)).   This process becomes the parent of any child processes that
       are orphaned because a process that resides in this PID namespace  ter-
       minated (see below for further details).

       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  process  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  ac-
       tions  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 de-

   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  name-

       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  an-
       cestor  namespace through to the root PID namespace.  System calls that
       operate on process IDs always operate using the process ID that is vis-
       ible  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 an-
       other namespace.  Likewise, the direct children of a process that  uses
       setns(2) to cause its children to join a PID namespace are in a differ-
       ent PID namespace from the caller of setns(2).  Calls to getppid(2) for
       such processes return 0.

       While processes may freely descend into child PID namespaces (e.g., us-
       ing 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  name-
       spaces 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   name-
       spaces(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  de-
       termined  when the process is created and cannot be changed thereafter.
       Among other things, this means that the parental  relationship  between
       processes mirrors the parental relationship between PID namespaces: the
       parent of a process is either in the same namespace or resides  in  the
       immediate parent PID namespace.

       A  process  may  call  unshare(2) with the CLONE_NEWPID flag only once.
       After it has performed this operation,  its  /proc/PID/ns/pid_for_chil-
       dren  symbolic  link  will be empty until the first child is created in
       the namespace.

   Adoption of orphaned children
       When a child process becomes orphaned, it is reparented to  the  "init"
       process  in  the  PID namespace of its parent (unless one of the nearer
       ancestors of the parent employed  the  prctl(2)  PR_SET_CHILD_SUBREAPER
       command to mark itself as the reaper of orphaned descendant processes).
       Note that because of the setns(2) and  unshare(2)  semantics  described
       above,  this may be the "init" process in the PID namespace that is the
       parent of the child's PID namespace, rather than the "init" process  in
       the child's own 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 de-
       scription 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 (be-
       fore 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

   /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 name-
       space 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 re-
       ceiving 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                             2019-03-06                 PID_NAMESPACES(7)
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