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NAME | DESCRIPTION | CONFORMING TO | EXAMPLE | SEE ALSO | COLOPHON |
PID_NAMESPACES(7) Linux Programmer's Manual PID_NAMESPACES(7)
pid_namespaces - overview of Linux PID namespaces
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).
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 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.
Namespaces are a Linux-specific feature.
See user_namespaces(7).
clone(2), setns(2), unshare(2), proc(5), capabilities(7),
credentials(7), namespaces(7), user_namespaces(7), switch_root(8)
This page is part of release 4.12 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/.
Linux 2017-05-03 PID_NAMESPACES(7)
Pages that refer to this page: nsenter(1), unshare(1), clone(2), fork(2), getpid(2), ioctl_ns(2), reboot(2), setns(2), unshare(2), credentials(7), namespaces(7), user_namespaces(7)