.\" Copyright (c) 2013 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" and Copyright (c) 2012 by Eric W. Biederman <ebiederm@xmission.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\"
.TH pid_namespaces 7 2023-03-30 "Linux man-pages 6.05.01"
.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
.B CLONE_NEWPID
flag, or the first child created by a process after a call to
.BR unshare (2)
using the
.B 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
.B 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
.IR /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\[em]subject to the usual permission checks described in
.BR kill (2)\[em]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
.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
.B 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
.B 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,
.B 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,
.B CLONE_NEWPID
was additionally disallowed (failing with the error
.BR EINVAL )
in combination with
.B CLONE_SIGHAND
.\" (restriction lifted in faf00da544045fdc1454f3b9e6d7f65c841de302)
(before Linux 4.3) as well as
.\" (restriction lifted in e79f525e99b04390ca4d2366309545a836c03bf1)
.B 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
.IR /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
.B  CLONE_NEWNS
in the
.I 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 STANDARDS
Linux.
.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)