pid_namespaces(7) — Linux manual page


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

NAME         top

       pid_namespaces - overview of Linux PID namespaces

DESCRIPTION         top

       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
       suspending/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 standalone
       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)).  This process becomes the parent of any
       child processes that are orphaned because a process that resides in
       this PID namespace terminated (see below for further details).

       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 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 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

   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 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
       getppid(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
       namespaces 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
       subsequently 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 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.

       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_children 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 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 computed 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 use‐
       ful 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 (which is virtualized per PID namespace) displays
              the last PID that was allocated in this PID namespace.  When
              the next PID is allocated, the kernel will search for the low‐
              est 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 the user namespace that owns the PID names‐
              pace.  This makes it possible to determine the PID that is
              allocated to the next process that is created inside this PID

       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         top

       Namespaces are a Linux-specific feature.

EXAMPLES         top

       See user_namespaces(7).

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.07 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

Linux                            2020-06-09                PID_NAMESPACES(7)

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