mirror of https://github.com/mkerrisk/man-pages
409 lines
13 KiB
Groff
409 lines
13 KiB
Groff
.\" Copyright (c) 2013 by Michael Kerrisk <mtk.manpages@gmail.com>
|
|
.\" and Copyright (c) 2012 by Eric W. Biederman <ebiederm@xmission.com>
|
|
.\"
|
|
.\" %%%LICENSE_START(VERBATIM)
|
|
.\" Permission is granted to make and distribute verbatim copies of this
|
|
.\" manual provided the copyright notice and this permission notice are
|
|
.\" preserved on all copies.
|
|
.\"
|
|
.\" Permission is granted to copy and distribute modified versions of this
|
|
.\" manual under the conditions for verbatim copying, provided that the
|
|
.\" entire resulting derived work is distributed under the terms of a
|
|
.\" permission notice identical to this one.
|
|
.\"
|
|
.\" Since the Linux kernel and libraries are constantly changing, this
|
|
.\" manual page may be incorrect or out-of-date. The author(s) assume no
|
|
.\" responsibility for errors or omissions, or for damages resulting from
|
|
.\" the use of the information contained herein. The author(s) may not
|
|
.\" have taken the same level of care in the production of this manual,
|
|
.\" which is licensed free of charge, as they might when working
|
|
.\" professionally.
|
|
.\"
|
|
.\" Formatted or processed versions of this manual, if unaccompanied by
|
|
.\" the source, must acknowledge the copyright and authors of this work.
|
|
.\" %%%LICENSE_END
|
|
.\"
|
|
.\"
|
|
.TH PID_NAMESPACES 7 2020-11-01 "Linux" "Linux Programmer's Manual"
|
|
.SH NAME
|
|
pid_namespaces \- overview of Linux PID namespaces
|
|
.SH DESCRIPTION
|
|
For an overview of namespaces, see
|
|
.BR namespaces (7).
|
|
.PP
|
|
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.
|
|
.PP
|
|
PIDs in a new PID namespace start at 1,
|
|
somewhat like a standalone system, and calls to
|
|
.BR fork (2),
|
|
.BR vfork (2),
|
|
or
|
|
.BR clone (2)
|
|
will produce processes with PIDs that are unique within the namespace.
|
|
.PP
|
|
Use of PID namespaces requires a kernel that is configured with the
|
|
.B CONFIG_PID_NS
|
|
option.
|
|
.\"
|
|
.\" ============================================================
|
|
.\"
|
|
.SS The namespace "init" process
|
|
The first process created in a new namespace
|
|
(i.e., the process created using
|
|
.BR clone (2)
|
|
with the
|
|
.BR CLONE_NEWPID
|
|
flag, or the first child created by a process after a call to
|
|
.BR unshare (2)
|
|
using the
|
|
.BR CLONE_NEWPID
|
|
flag) has the PID 1, and is the "init" process for the namespace (see
|
|
.BR 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).
|
|
.PP
|
|
If the "init" process of a PID namespace terminates,
|
|
the kernel terminates all of the processes in the namespace via a
|
|
.BR 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
|
|
.BR fork (2)
|
|
into this PID namespace fail with the error
|
|
.BR 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
|
|
.I /proc/[pid]/ns/pid
|
|
file corresponding to a process that was in a namespace to
|
|
.BR setns (2)
|
|
into that namespace after the "init" process has terminated.
|
|
Another possible scenario can occur after a call to
|
|
.BR unshare (2):
|
|
if the first child subsequently created by a
|
|
.BR fork (2)
|
|
terminates, then subsequent calls to
|
|
.BR fork (2)
|
|
fail with
|
|
.BR ENOMEM .
|
|
.PP
|
|
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.
|
|
.PP
|
|
Likewise, a process in an ancestor namespace
|
|
can\(emsubject to the usual permission checks described in
|
|
.BR kill (2)\(emsend
|
|
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
|
|
.I siginfo_t
|
|
.I si_pid
|
|
field described in
|
|
.BR sigaction (2)
|
|
will be zero.)
|
|
.B SIGKILL
|
|
or
|
|
.B 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).
|
|
.PP
|
|
Starting with Linux 3.4, the
|
|
.BR reboot (2)
|
|
system call causes a signal to be sent to the namespace "init" process.
|
|
See
|
|
.BR reboot (2)
|
|
for more details.
|
|
.\"
|
|
.\" ============================================================
|
|
.\"
|
|
.SS 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
|
|
.BR clone (2)
|
|
or
|
|
.BR unshare (2).
|
|
PID namespaces thus form a tree,
|
|
with all namespaces ultimately tracing their ancestry to the root namespace.
|
|
Since Linux 3.7,
|
|
.\" commit f2302505775fd13ba93f034206f1e2a587017929
|
|
.\" The kernel constant MAX_PID_NS_LEVEL
|
|
the kernel limits the maximum nesting depth for PID namespaces to 32.
|
|
.PP
|
|
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
|
|
.BR kill (2),
|
|
set nice values with
|
|
.BR setpriority (2),
|
|
etc.) only processes contained in its own PID namespace
|
|
and in descendants of that namespace.
|
|
.PP
|
|
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
|
|
.BR getpid (2)
|
|
always returns the PID associated with the namespace in which
|
|
the process was created.
|
|
.PP
|
|
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
|
|
.BR init (1)
|
|
process with PID 1) is necessarily in another namespace.
|
|
Likewise, the direct children of a process that uses
|
|
.BR setns (2)
|
|
to cause its children to join a PID namespace are in a different
|
|
PID namespace from the caller of
|
|
.BR setns (2).
|
|
Calls to
|
|
.BR getppid (2)
|
|
for such processes return 0.
|
|
.PP
|
|
While processes may freely descend into child PID namespaces
|
|
(e.g., using
|
|
.BR 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.
|
|
.PP
|
|
The
|
|
.BR NS_GET_PARENT
|
|
.BR ioctl (2)
|
|
operation can be used to discover the parental relationship
|
|
between PID namespaces; see
|
|
.BR ioctl_ns (2).
|
|
.\"
|
|
.\" ============================================================
|
|
.\"
|
|
.SS setns(2) and unshare(2) semantics
|
|
Calls to
|
|
.BR setns (2)
|
|
that specify a PID namespace file descriptor
|
|
and calls to
|
|
.BR unshare (2)
|
|
with the
|
|
.BR 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
|
|
.IR /proc/[pid]/ns/pid_for_children
|
|
file, as described in
|
|
.BR 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
|
|
.BR getpid ()),
|
|
which would break many applications and libraries.
|
|
.PP
|
|
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.
|
|
.PP
|
|
A process may call
|
|
.BR unshare (2)
|
|
with the
|
|
.B CLONE_NEWPID
|
|
flag only once.
|
|
After it has performed this operation, its
|
|
.IR /proc/PID/ns/pid_for_children
|
|
symbolic link will be empty until the first child is created in the namespace.
|
|
.\"
|
|
.\" ============================================================
|
|
.\"
|
|
.SS 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
|
|
.BR prctl (2)
|
|
.B PR_SET_CHILD_SUBREAPER
|
|
command to mark itself as the reaper of orphaned descendant processes).
|
|
Note that because of the
|
|
.BR setns (2)
|
|
and
|
|
.BR unshare (2)
|
|
semantics described above, this may be the "init" process in the PID
|
|
namespace that is the
|
|
.I parent
|
|
of the child's PID namespace,
|
|
rather than the "init" process in the child's own PID namespace.
|
|
.\" Furthermore, by definition, the parent of the "init" process
|
|
.\" of a PID namespace resides in the parent PID namespace.
|
|
.\"
|
|
.\" ============================================================
|
|
.\"
|
|
.SS Compatibility of CLONE_NEWPID with other CLONE_* flags
|
|
In current versions of Linux,
|
|
.BR CLONE_NEWPID
|
|
can't be combined with
|
|
.BR 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 process in the
|
|
.BR 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
|
|
.I siginfo_t
|
|
type in
|
|
.BR sigaction (2)).
|
|
Since this is computed when a signal is enqueued,
|
|
a signal queue shared by processes in multiple PID namespaces
|
|
would defeat that.
|
|
.PP
|
|
.\" Note these restrictions were all introduced in
|
|
.\" 8382fcac1b813ad0a4e68a838fc7ae93fa39eda0
|
|
.\" when CLONE_NEWPID|CLONE_VM was disallowed
|
|
In earlier versions of Linux,
|
|
.BR CLONE_NEWPID
|
|
was additionally disallowed (failing with the error
|
|
.BR EINVAL )
|
|
in combination with
|
|
.BR CLONE_SIGHAND
|
|
.\" (restriction lifted in faf00da544045fdc1454f3b9e6d7f65c841de302)
|
|
(before Linux 4.3) as well as
|
|
.\" (restriction lifted in e79f525e99b04390ca4d2366309545a836c03bf1)
|
|
.BR CLONE_VM
|
|
(before Linux 3.12).
|
|
The changes that lifted these restrictions have also been ported to
|
|
earlier stable kernels.
|
|
.\"
|
|
.\" ============================================================
|
|
.\"
|
|
.SS /proc and PID namespaces
|
|
A
|
|
.I /proc
|
|
filesystem shows (in the
|
|
.I /proc/[pid]
|
|
directories) only processes visible in the PID namespace
|
|
of the process that performed the mount, even if the
|
|
.I /proc
|
|
filesystem is viewed from processes in other namespaces.
|
|
.PP
|
|
After creating a new PID namespace,
|
|
it is useful for the child to change its root directory
|
|
and mount a new procfs instance at
|
|
.I /proc
|
|
so that tools such as
|
|
.BR ps (1)
|
|
work correctly.
|
|
If a new mount namespace is simultaneously created by including
|
|
.BR CLONE_NEWNS
|
|
in the
|
|
.IR flags
|
|
argument of
|
|
.BR clone (2)
|
|
or
|
|
.BR unshare (2),
|
|
then it isn't necessary to change the root directory:
|
|
a new procfs instance can be mounted directly over
|
|
.IR /proc .
|
|
.PP
|
|
From a shell, the command to mount
|
|
.I /proc
|
|
is:
|
|
.PP
|
|
.in +4n
|
|
.EX
|
|
$ mount \-t proc proc /proc
|
|
.EE
|
|
.in
|
|
.PP
|
|
Calling
|
|
.BR readlink (2)
|
|
on the path
|
|
.I /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.
|
|
.\"
|
|
.\" ============================================================
|
|
.\"
|
|
.SS /proc files
|
|
.TP
|
|
.BR /proc/sys/kernel/ns_last_pid " (since Linux 3.3)"
|
|
.\" commit b8f566b04d3cddd192cfd2418ae6d54ac6353792
|
|
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 lowest unallocated PID
|
|
that is greater than this value,
|
|
and when this file is subsequently read it will show that PID.
|
|
.IP
|
|
This file is writable by a process that has the
|
|
.B CAP_SYS_ADMIN
|
|
or (since Linux 5.9)
|
|
.B CAP_CHECKPOINT_RESTORE
|
|
capability inside the user namespace that owns the PID namespace.
|
|
.\" This ability is necessary to support checkpoint restore in user-space
|
|
This makes it possible to determine the PID that is allocated
|
|
to the next process that is created inside this PID namespace.
|
|
.\"
|
|
.\" ============================================================
|
|
.\"
|
|
.SS Miscellaneous
|
|
When a process ID is passed over a UNIX domain socket to a
|
|
process in a different PID namespace (see the description of
|
|
.B SCM_CREDENTIALS
|
|
in
|
|
.BR unix (7)),
|
|
it is translated into the corresponding PID value in
|
|
the receiving process's PID namespace.
|
|
.SH CONFORMING TO
|
|
Namespaces are a Linux-specific feature.
|
|
.SH EXAMPLES
|
|
See
|
|
.BR user_namespaces (7).
|
|
.SH SEE ALSO
|
|
.BR clone (2),
|
|
.BR reboot (2),
|
|
.BR setns (2),
|
|
.BR unshare (2),
|
|
.BR proc (5),
|
|
.BR capabilities (7),
|
|
.BR credentials (7),
|
|
.BR mount_namespaces (7),
|
|
.BR namespaces (7),
|
|
.BR user_namespaces (7),
|
|
.BR switch_root (8)
|