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377 lines
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Groff
377 lines
12 KiB
Groff
.\" Copyright (c) 2013 by Michael Kerrisk <mtk.manpages@gmail.com>
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.\" and Copyright (c) 2012 by Eric W. Biederman <ebiederm@xmission.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\"
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.TH PID_NAMESPACES 7 2017-11-26 "Linux" "Linux Programmer's Manual"
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.SH NAME
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pid_namespaces \- overview of Linux PID namespaces
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.SH DESCRIPTION
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For an overview of namespaces, see
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.BR namespaces (7).
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.PP
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PID namespaces isolate the process ID number space,
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meaning that processes in different PID namespaces can have the same PID.
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PID namespaces allow containers to provide functionality
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such as suspending/resuming the set of processes in the container and
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migrating the container to a new host
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while the processes inside the container maintain the same PIDs.
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.PP
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PIDs in a new PID namespace start at 1,
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somewhat like a standalone system, and calls to
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.BR fork (2),
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.BR vfork (2),
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or
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.BR clone (2)
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will produce processes with PIDs that are unique within the namespace.
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.PP
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Use of PID namespaces requires a kernel that is configured with the
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.B CONFIG_PID_NS
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option.
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.\"
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.\" ============================================================
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.\"
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.SS The namespace "init" process
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The first process created in a new namespace
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(i.e., the process created using
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.BR clone (2)
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with the
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.BR CLONE_NEWPID
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flag, or the first child created by a process after a call to
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.BR unshare (2)
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using the
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.BR CLONE_NEWPID
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flag) has the PID 1, and is the "init" process for the namespace (see
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.BR init (1)).
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A child process that is orphaned within the namespace will be reparented
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to this process rather than
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.BR init (1)
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(unless one of the ancestors of the child
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in the same PID namespace employed the
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.BR prctl (2)
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.B PR_SET_CHILD_SUBREAPER
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command to mark itself as the reaper of orphaned descendant processes).
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.PP
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If the "init" process of a PID namespace terminates,
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the kernel terminates all of the processes in the namespace via a
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.BR SIGKILL
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signal.
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This behavior reflects the fact that the "init" process
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is essential for the correct operation of a PID namespace.
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In this case, a subsequent
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.BR fork (2)
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into this PID namespace fail with the error
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.BR ENOMEM ;
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it is not possible to create a new processes in a PID namespace whose "init"
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process has terminated.
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Such scenarios can occur when, for example,
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a process uses an open file descriptor for a
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.I /proc/[pid]/ns/pid
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file corresponding to a process that was in a namespace to
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.BR setns (2)
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into that namespace after the "init" process has terminated.
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Another possible scenario can occur after a call to
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.BR unshare (2):
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if the first child subsequently created by a
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.BR fork (2)
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terminates, then subsequent calls to
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.BR fork (2)
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fail with
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.BR ENOMEM .
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.PP
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Only signals for which the "init" process has established a signal handler
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can be sent to the "init" process by other members of the PID namespace.
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This restriction applies even to privileged processes,
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and prevents other members of the PID namespace from
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accidentally killing the "init" process.
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.PP
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Likewise, a process in an ancestor namespace
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can\(emsubject to the usual permission checks described in
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.BR kill (2)\(emsend
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signals to the "init" process of a child PID namespace only
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if the "init" process has established a handler for that signal.
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(Within the handler, the
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.I siginfo_t
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.I si_pid
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field described in
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.BR sigaction (2)
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will be zero.)
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.B SIGKILL
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or
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.B SIGSTOP
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are treated exceptionally:
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these signals are forcibly delivered when sent from an ancestor PID namespace.
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Neither of these signals can be caught by the "init" process,
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and so will result in the usual actions associated with those signals
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(respectively, terminating and stopping the process).
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.PP
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Starting with Linux 3.4, the
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.BR reboot (2)
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system call causes a signal to be sent to the namespace "init" process.
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See
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.BR reboot (2)
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for more details.
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.\"
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.\" ============================================================
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.\"
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.SS Nesting PID namespaces
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PID namespaces can be nested:
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each PID namespace has a parent,
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except for the initial ("root") PID namespace.
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The parent of a PID namespace is the PID namespace of the process that
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created the namespace using
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.BR clone (2)
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or
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.BR unshare (2).
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PID namespaces thus form a tree,
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with all namespaces ultimately tracing their ancestry to the root namespace.
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Since Linux 3.7,
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.\" commit f2302505775fd13ba93f034206f1e2a587017929
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.\" The kernel constant MAX_PID_NS_LEVEL
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the kernel limits the maximum nesting depth for PID namespaces to 32.
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.PP
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A process is visible to other processes in its PID namespace,
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and to the processes in each direct ancestor PID namespace
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going back to the root PID namespace.
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In this context, "visible" means that one process
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can be the target of operations by another process using
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system calls that specify a process ID.
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Conversely, the processes in a child PID namespace can't see
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processes in the parent and further removed ancestor namespaces.
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More succinctly: a process can see (e.g., send signals with
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.BR kill (2),
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set nice values with
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.BR setpriority (2),
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etc.) only processes contained in its own PID namespace
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and in descendants of that namespace.
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.PP
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A process has one process ID in each of the layers of the PID
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namespace hierarchy in which is visible,
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and walking back though each direct ancestor namespace
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through to the root PID namespace.
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System calls that operate on process IDs always
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operate using the process ID that is visible in the
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PID namespace of the caller.
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A call to
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.BR getpid (2)
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always returns the PID associated with the namespace in which
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the process was created.
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.PP
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Some processes in a PID namespace may have parents
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that are outside of the namespace.
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For example, the parent of the initial process in the namespace
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(i.e., the
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.BR init (1)
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process with PID 1) is necessarily in another namespace.
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Likewise, the direct children of a process that uses
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.BR setns (2)
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to cause its children to join a PID namespace are in a different
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PID namespace from the caller of
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.BR setns (2).
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Calls to
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.BR getppid (2)
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for such processes return 0.
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.PP
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While processes may freely descend into child PID namespaces
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(e.g., using
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.BR setns (2)
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with a PID namespace file descriptor),
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they may not move in the other direction.
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That is to say, processes may not enter any ancestor namespaces
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(parent, grandparent, etc.).
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Changing PID namespaces is a one-way operation.
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.PP
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The
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.BR NS_GET_PARENT
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.BR ioctl (2)
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operation can be used to discover the parental relationship
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between PID namespaces; see
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.BR ioctl_ns (2).
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.\"
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.\" ============================================================
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.\"
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.SS setns(2) and unshare(2) semantics
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Calls to
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.BR setns (2)
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that specify a PID namespace file descriptor
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and calls to
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.BR unshare (2)
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with the
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.BR CLONE_NEWPID
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flag cause children subsequently created
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by the caller to be placed in a different PID namespace from the caller.
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(Since Linux 4.12, that PID namespace is shown via the
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.IR /proc/[pid]/ns/pid_for_children
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file, as described in
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.BR namespaces (7).)
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These calls do not, however,
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change the PID namespace of the calling process,
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because doing so would change the caller's idea of its own PID
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(as reported by
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.BR getpid ()),
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which would break many applications and libraries.
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.PP
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To put things another way:
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a process's PID namespace membership is determined when the process is created
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and cannot be changed thereafter.
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Among other things, this means that the parental relationship
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between processes mirrors the parental relationship between PID namespaces:
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the parent of a process is either in the same namespace
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or resides in the immediate parent PID namespace.
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.SS Compatibility of CLONE_NEWPID with other CLONE_* flags
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In current versions of Linux,
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.BR CLONE_NEWPID
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can't be combined with
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.BR CLONE_THREAD .
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Threads are required to be in the same PID namespace such that
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the threads in a process can send signals to each other.
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Similarly, it must be possible to see all of the threads
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of a processes in the
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.BR proc (5)
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filesystem.
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Additionally, if two threads were in different PID
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namespaces, the process ID of the process sending a signal
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could not be meaningfully encoded when a signal is sent
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(see the description of the
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.I siginfo_t
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type in
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.BR sigaction (2)).
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Since this is computed when a signal is enqueued,
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a signal queue shared by processes in multiple PID namespaces
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would defeat that.
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.PP
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.\" Note these restrictions were all introduced in
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.\" 8382fcac1b813ad0a4e68a838fc7ae93fa39eda0
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.\" when CLONE_NEWPID|CLONE_VM was disallowed
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In earlier versions of Linux,
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.BR CLONE_NEWPID
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was additionally disallowed (failing with the error
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.BR EINVAL )
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in combination with
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.BR CLONE_SIGHAND
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.\" (restriction lifted in faf00da544045fdc1454f3b9e6d7f65c841de302)
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(before Linux 4.3) as well as
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.\" (restriction lifted in e79f525e99b04390ca4d2366309545a836c03bf1)
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.BR CLONE_VM
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(before Linux 3.12).
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The changes that lifted these restrictions have also been ported to
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earlier stable kernels.
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.\"
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.\" ============================================================
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.\"
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.SS /proc and PID namespaces
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A
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.I /proc
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filesystem shows (in the
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.I /proc/[pid]
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directories) only processes visible in the PID namespace
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of the process that performed the mount, even if the
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.I /proc
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filesystem is viewed from processes in other namespaces.
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.PP
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After creating a new PID namespace,
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it is useful for the child to change its root directory
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and mount a new procfs instance at
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.I /proc
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so that tools such as
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.BR ps (1)
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work correctly.
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If a new mount namespace is simultaneously created by including
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.BR CLONE_NEWNS
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in the
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.IR flags
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argument of
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.BR clone (2)
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or
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.BR unshare (2),
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then it isn't necessary to change the root directory:
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a new procfs instance can be mounted directly over
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.IR /proc .
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.PP
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From a shell, the command to mount
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.I /proc
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is:
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.PP
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.in +4n
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.EX
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$ mount -t proc proc /proc
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.EE
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.in
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.PP
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Calling
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.BR readlink (2)
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on the path
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.I /proc/self
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yields the process ID of the caller in the PID namespace of the procfs mount
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(i.e., the PID namespace of the process that mounted the procfs).
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This can be useful for introspection purposes,
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when a process wants to discover its PID in other namespaces.
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.\"
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.\" ============================================================
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.\"
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.SS /proc files
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.TP
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.BR /proc/sys/kernel/ns_last_pid " (since Linux 3.3)"
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.\" commit b8f566b04d3cddd192cfd2418ae6d54ac6353792
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This file displays the last PID that was allocated in this PID namespace.
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When the next PID is allocated,
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the kernel will search for the lowest unallocated PID
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that is greater than this value,
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and when this file is subsequently read it will show that PID.
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.IP
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This file is writable by a process that has the
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.B CAP_SYS_ADMIN
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capability inside its user namespace.
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.\" This ability is necessary to support checkpoint restore in user-space
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This makes it possible to determine the PID that is allocated
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to the next process that is created inside this PID namespace.
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.\"
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.\" ============================================================
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.\"
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.SS Miscellaneous
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When a process ID is passed over a UNIX domain socket to a
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process in a different PID namespace (see the description of
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.B SCM_CREDENTIALS
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in
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.BR unix (7)),
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it is translated into the corresponding PID value in
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the receiving process's PID namespace.
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.SH CONFORMING TO
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Namespaces are a Linux-specific feature.
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.SH EXAMPLE
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See
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.BR user_namespaces (7).
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.SH SEE ALSO
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.BR clone (2),
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.BR reboot (2),
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.BR setns (2),
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.BR unshare (2),
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.BR proc (5),
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.BR capabilities (7),
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.BR credentials (7),
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.BR mount_namespaces (7),
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.BR namespaces (7),
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.BR user_namespaces (7),
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.BR switch_root (8)
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