For review: pid_namespaces(7) man page

Michael Kerrisk (man-pages) mtk.manpages at
Wed Aug 20 23:38:15 UTC 2014

Hello Eric et al.

Here is the current draft of the pid_namespaces(7) man page, which
described PID namespaces. The rendered version is below, and the
source is attached.

Review comments/suggestions for improvements / bug fixes welcome.




       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
       processes in different PID namespaces can have the same PID.  PID
       namespaces allow containers to provide functionality such as sus‐
       pending/resuming the  set  of  processes  in  the  container  and
       migrating  the container 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  stand‐
       alone  system,  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_GET_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 subsequent fork(2) into this PID namespace will fail
       with the error ENOMEM; it is not possible to create  a  new  pro‐
       cesses  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 names‐
       pace  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) will fail with ENOMEM.

       Only signals for which the "init" process has established a  sig‐
       nal 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  associ‐
       ated  with  those signals (respectively, terminating and stopping
       the process).

       Starting with Linux 3.4, the reboot(2) system 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  ances‐
       try 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 namespace.  More succinctly:
       a process can see (e.g., send signals with kill(2), set nice val‐
       ues  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  out‐
       side  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 chil‐
       dren 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  getppid(2)  for  such  processes
       return 0.

   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
       subsequently  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  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; other‐
          wise 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)).  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  they  are  threads
          and  will  be core dumped together.  When a core dump is writ‐
          ten, 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 parent PID namespace.

       To summarize: there  is  a  technical  requirement  for  each  of
       CLONE_THREAD,  CLONE_SIGHAND,  and CLONE_VM to share a PID names‐
       pace.  (Note furthermore 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

           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 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 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  nec‐
       essary 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.

       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  corre‐
       sponding PID value in the receiving process's PID namespace.

       Namespaces are a Linux-specific feature.

       See user_namespaces(7).

       clone(2), setns(2), unshare(2), proc(5), credentials(7), capabil‐
       ities(7), user_namespaces(7), switch_root(8)

Michael Kerrisk
Linux man-pages maintainer;
Linux/UNIX System Programming Training:
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