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 will 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) termi-
       nates, then subsequent calls to fork(2) will 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.

       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 CLONE_NEWPID), they may not move in the other
       direction.  That is to say, processes may not enter any ancestor namespaces (parent, grandparent, etc.).  Changing PID namespaces is a  one
       way operation.

   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.  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 applications 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
       CLONE_NEWPID can't be combined with some other CLONE_* flags:

       *  CLONE_THREAD requires being in the same PID namespace in order 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.

       *  CLONE_SIGHAND  requires  being in the same PID namespace; otherwise the process ID of the process sending a signal could not be meaning-
	  fully encoded when a signal is sent (see the description of the siginfo_t type in sigaction(2)).  A signal queue shared by processes	in
	  multiple PID namespaces will defeat that.

       *  CLONE_VM  requires  all of the threads to be in the same PID namespace, because, from the point of view of a core dump, if two processes
	  share the same address space then they are threads and will be core dumped together.	When a core dump  is  written,	the  PID  of  each
	  thread  is written into the core dump.  Writing the process IDs could not meaningfully succeed if some of the process IDs were in a par-
	  ent PID namespace.

       To summarize: there is a technical requirement for each of CLONE_THREAD, CLONE_SIGHAND, and CLONE_VM to share a PID namespace.  (Note  fur-
       thermore  that  in clone(2) requires CLONE_VM to be specified if CLONE_THREAD or CLONE_SIGHAND is specified.)  Thus, call sequences such as
       the following will fail (with the error EINVAL):

	   unshare(CLONE_NEWPID);
	   clone(..., CLONE_VM, ...);	 /* Fails */

	   setns(fd, CLONE_NEWPID);
	   clone(..., CLONE_VM, ...);	 /* Fails */

	   clone(..., CLONE_VM, ...);
	   setns(fd, CLONE_NEWPID);	 /* Fails */

	   clone(..., CLONE_VM, ...);
	   unshare(CLONE_NEWPID);	 /* Fails */

   /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.

   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), setns(2), unshare(2), proc(5), credentials(7), capabilities(7), user_namespaces(7), switch_root(8)

Linux								    2015-01-10							 PID_NAMESPACES(7)