Eric et al,
Eventually, there will be more namespace man pages, but let us start
now with one for PID namespaces. The attached page aims to provide a
fairly complete overview of PID namespaces.
Eric, various pieces of the page are shifted out of other pages
(clone(2), setns(2), etc.) and are derived from comments you've
emailed me off list, so you are (jointly) in the copyright of the
page. I've chosen the common license for man-pages; let me know if you
have any objections to that license.
I'm looking for review comments (corrections, improvements, additions,
etc.) on this page. I've provided it in two forms inline below, and
reviewers can comment comment on whichever form they are most
comfortable with:
1) The rendered page as plain text
2) The *roff source (also attached); rendering that source will enable
readers to see proper formatting for the page.
Note that the namespaces(7) page referred to in this page is not yet
finished; I'll send it out for review at a future time.
Thanks,
Michael
==========
PID_NAMESPACES(7) Linux Programmer's Manual PID_NAMESPACES(7)
NAME
pid_namespaces - overview of Linux PID namespaces
DESCRIPTION
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 migrate 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 names‐
pace.
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)). Children that are orphaned within
the namespace will be reparented to this process rather than
init(1).
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 names‐
pace. In this case, a subsequent fork(2) into this PID names‐
pace (e.g., from a process that has done a setns(2) into the
namespace using an open file descriptor for a
/proc/[pid]/ns/pid file corresponding to a process that was in
the namespace) will fail with the error ENOMEM; it is not pos‐
sible to create a new processes in a PID namespace whose "init"
process has terminated.
Only signals for which the "init" process has established a
signal handler can be sent to the "init" process by other mem‐
bers 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).
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.
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, "visi‐
ble" 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 values with setpriority(2), etc.) only pro‐
cesses 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 names‐
pace 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 ini‐
tial 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 names‐
pace 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 chil‐
dren subsequently created by the caller to be placed in a dif‐
ferent PID namespace from the caller. These calls do not, how‐
ever, 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 between PID
namespaces: the parent of a process is either in the same
namespace or resides in the immediate parent PID namespace.
Every thread in a process must be in the same PID namespace.
For this reason, the two following call sequences will fail:
unshare(CLONE_NEWPID);
clone(..., CLONE_VM, ...); /* Fails */
setns(fd, CLONE_NEWPID);
clone(..., CLONE_VM, ...); /* Fails */
Because the above unshare(2) and setns(2) calls only change the
PID namespace for created children, the clone(2) calls neces‐
sarily put the new thread in a different PID namespace from the
calling thread.
Miscellaneous
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.)
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).
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.
CONFORMING TO
Namespaces are a Linux-specific feature.
SEE ALSO
unshare(1), clone(2), setns(2), unshare(2), proc(5), creden‐
tials(7), capabilities(7), user_namespaces(7), switch_root(8)
Linux 2013-01-14 PID_NAMESPACES(7)
=========== *roff source ==========
$ cat pid_namespaces.7
.\" Copyright (c) 2013 by Michael Kerrisk <mtk.manpages@xxxxxxxxx>
.\" and Copyright (c) 2012 by Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
.\"
.\" 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.
.\"
.\"
.TH PID_NAMESPACES 7 2013-01-14 "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).
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 migrate 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
.BR fork (2),
.BR vfork (2),
or
.BR 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
.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)).
Children that are orphaned within the namespace will be reparented
to this process rather than
.BR init (1).
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 (e.g., from a process that has done a
.BR setns (2)
into the namespace using an open file descriptor for a
.I /proc/[pid]/ns/pid
file corresponding to a process that was in the namespace)
will fail with the error
.BR ENOMEM ;
it is not possible to create a new processes in a PID namespace whose "init"
process has terminated.
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\(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).
.\"
.\" ============================================================
.\"
.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.
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
.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.
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.
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.
.\"
.\" ============================================================
.\"
.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.
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.
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 between PID namespaces:
the parent of a process is either in the same namespace
or resides in the immediate parent PID namespace.
Every thread in a process must be in the same PID namespace.
For this reason, the two following call sequences will fail:
.nf
unshare(CLONE_NEWPID);
clone(..., CLONE_VM, ...); /* Fails */
setns(fd, CLONE_NEWPID);
clone(..., CLONE_VM, ...); /* Fails */
.fi
Because the above
.BR unshare (2)
and
.BR setns (2)
calls only change the PID namespace for created children, the
.BR clone (2)
calls necessarily put the new thread in a different PID namespace from
the calling thread.
.\"
.\" ============================================================
.\"
.SS Miscellaneous
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.
.\" mount -t proc proc /proc
(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 .)
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).
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 SEE ALSO
.BR unshare (1),
.BR clone (2),
.BR setns (2),
.BR unshare (2),
.BR proc (5),
.BR credentials (7),
.BR capabilities (7),
.BR user_namespaces (7),
.BR switch_root (8)
Attachment:
pid_namespaces.7
Description: Binary data
