PID_NAMESPACES
Section: Linux Programmer's Manual (7)
Updated: 2017-05-03
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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 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)).
A child process that is orphaned within the namespace will be reparented
to this process rather than
init(1)
(unless one of the ancestors of the child
in the same PID namespace employed the
prctl(2)
PR_SET_CHILD_SUBREAPER
command to mark itself as the reaper of orphaned descendant processes).
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 will fail with the error
ENOMEM;
it is not possible to create a new processes 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)
will 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 details.
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).
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.
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 useful for introspection purposes,
when a process wants to discover its PID in other namespaces.
Miscellaneous
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
Namespaces are a Linux-specific feature.
EXAMPLE
See
user_namespaces(7).
SEE ALSO
clone(2),
setns(2),
unshare(2),
proc(5),
capabilities(7),
credentials(7),
namespaces(7),
user_namespaces(7),
switch_root(8)
COLOPHON
This page is part of release 4.13 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
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- DESCRIPTION
-
- The namespace init process
-
- Nesting PID namespaces
-
- setns(2) and unshare(2) semantics
-
- Compatibility of CLONE_NEWPID with other CLONE_* flags
-
- /proc and PID namespaces
-
- Miscellaneous
-
- CONFORMING TO
-
- EXAMPLE
-
- SEE ALSO
-
- COLOPHON
-
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