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

NAME

       pid_namespaces - overview of Linux PID namespaces

DESCRIPTION

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

       PID  namespaces  isolate the process ID number space, meaning that processes in different PID namespaces can have the same PID.  PID names-
       paces allow containers to provide functionality such as suspending/resuming 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 system, and calls to fork(2), vfork(2), or clone(2) will produce pro-
       cesses 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  cre-
       ated  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 terminates 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 subse-
       quent 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 corresponding 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 subsequently 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 handler 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 established 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
       details.

   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 names-
       pace 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
       ancestor  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 namespace.

       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 created.

       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 namespaces(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 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.

   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 different 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 processes visible in the PID namespace of the  process  that  performed  the
       mount, even if the /proc filesystem is viewed from processes in other namespaces.

       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
       namespace 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 PID.

              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.

   Miscellaneous
       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.

CONFORMING TO

       Namespaces are a Linux-specific feature.

EXAMPLE

       See user_namespaces(7).

SEE ALSO

       clone(2), reboot(2), setns(2),  unshare(2),  proc(5),  capabilities(7),  credentials(7),  mount_namespaces(7),  namespaces(7),  user_names-
       paces(7), switch_root(8)

COLOPHON

       This  page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Linux                                                               2017-11-26                                                   PID_NAMESPACES(7)