For review: pid_namespaces(7) man page

Michael Kerrisk (man-pages)
2014-08-20T23:38:22Z

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Hello Eric et al.

Here is the current draft of the pid_namespaces(7) man page, which
described PID namespaces. The rendered version is below, and the
source is attached.

Review comments/suggestions for improvements / bug fixes welcome.

Cheers,

Michael

==

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 sus‐
pending/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 stand‐
alone 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_GET_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 pro‐
cesses 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 names‐
pace 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 sig‐
nal 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 associ‐
ated with those signals (respectively, terminating and stopping
the process).

Starting with Linux 3.4, the reboot(2) system 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 ances‐
try 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 kill(2), set nice val‐
ues 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 out‐
side 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 chil‐
dren 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.

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. 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
CLONE_NEWPID can't be combined with some other CLONE_* flags:

* CLONE_THREAD requires being in the same PID namespace in order
that 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.

* CLONE_SIGHAND requires being in the same PID namespace; other‐
wise 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)). A signal
queue shared by processes in multiple PID namespaces will
defeat that.

* CLONE_VM requires all of the threads to be in the same PID
namespace, because, from the point of view of a core dump, if
two processes share the same address space they are threads
and will be core dumped together. When a core dump is writ‐
ten, the PID of each thread is written into the core dump.
Writing the process IDs could not meaningfully succeed if some
of the process IDs were in a parent PID namespace.

To summarize: there is a technical requirement for each of
CLONE_THREAD, CLONE_SIGHAND, and CLONE_VM to share a PID names‐
pace. (Note furthermore that in clone(2) requires CLONE_VM to be
specified if CLONE_THREAD or CLONE_SIGHAND is specified.) Thus,
call sequences such as the following will fail (with the error
EINVAL):

unshare(CLONE_NEWPID);
clone(..., CLONE_VM, ...); /* Fails */

setns(fd, CLONE_NEWPID);
clone(..., CLONE_VM, ...); /* Fails */

clone(..., CLONE_VM, ...);
setns(fd, CLONE_NEWPID); /* Fails */

clone(..., CLONE_VM, ...);
unshare(CLONE_NEWPID); /* Fails */

/proc and PID namespaces
A /proc filesystem shows (in the /proc/PID directories) only pro‐
cesses 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 nec‐
essary 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 corre‐
sponding 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), credentials(7), capabil‐
ities(7), user_namespaces(7), switch_root(8)

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

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