proc.5, namespaces.7: Move /proc/[pid]/mountstats from proc.5 to namespaces.7

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.TH NAMESPACES 7 2013-01-14 "Linux" "Linux Programmer's Manual"
.SH NAME
namespaces \- overview of Linux namespaces
.SH DESCRIPTION
A namespace wraps a global system resource in an abstraction that
makes it appear to the processes within the namespace that they
have their own isolated instance of the global resource.
Changes to the global resource are visible to other processes
that are members of the namespace, but are invisible to other processes.
One use of namespaces is to implement containers.

This page describes the various namespaces and the associated
.I /proc
files, and summarizes the APIs for working with namespaces.

.SS The namespaces API

As well as various
.I /proc
files described below,
the namespaces API comprises the following system calls:

.TP
.BR clone (2)
The
.BR clone (2)
system call creates a new process.
If the
.I flags
argument of the call specifies one or more of the
.B CLONE_NEW*
flags listed below, then new namespaces are created for each flag,
and the child process is made a member of those namespaces.
(This system call also implements a number of features
unrelated to namespaces.)

.TP
.BR setns (2)
The
.BR setns (2)
system call allows the calling process to join an existing namespace.
The namespace to join is specified via a file descriptor that refers to
one of the
.IR /proc/[pid]/ns
files described below.

.TP
.BR unshare (2)
The
.BR unshare (2)
system call moves the calling process to a new namespace.
If the
.I flags
argument of the call specifies one or more of the
.B CLONE_NEW*
flags listed below, then new namespaces are created for each flag,
and the calling process is made a member of those namespaces.
(This system call also implements a number of features
unrelated to namespaces.)

Leaving aside the other effects of the
.BR clone (2)
system call, the following call:

    clone(..., CLONE_NEWXXX, ...);

is equivalent in namespace terms to:

    if (fork() == 0)            /* if child */
        unshare(CLONE_NEWXXX);

.SS IPC namespaces (CLONE_NEWIPC)

IPC namespaces isolate certain IPC resources,
namely, System V IPC objects (see
.BR svipc (7))
and (since Linux 2.6.30) POSIX message queues (see
.BR mq_overview (7).
Each IPC namespace has its own set of System V IPC identifiers and
its own POSIX message queue file system.

.SS Network namespaces (CLONE_NEWNET)

Network namespaces provide isolation of the system resources associated
with networking: network devices, IP addresses, IP routing tables,
.I /proc/net
directory,
.I /sys/class/net 
directory, port numbers, and so on.

.SS Mount namespaces (CLONE_NEWNS)

Mount namespaces isolate the set of file system mount points,
meaning that processes in different mount namespaces can
have different views of the file system hierarchy.
The set of mounts in a mount namespace is modified using
.BR mount (2)
and
.BR umount (2).

The
.IR /proc/[pid]/mounts
file (present since Linux 2.4.19)
lists all the file systems currently mounted in the
process's mount namespace.
The format of this file is documented in
.BR fstab (5).
Since kernel version 2.6.15, this file is pollable:
after opening the file for reading, a change in this file
(i.e., a file system mount or unmount) causes
.BR select (2)
to mark the file descriptor as readable, and
.BR poll (2)
and
.BR epoll_wait (2)
mark the file as having an error condition.

The
.IR /proc/[pid]/mountstats
file (present since Linux 2.6.17)
exports information (statistics, configuration information)
about the mount points in the process's mount namespace.
This file is only readable by the owner of the process.
Lines in this file have the form:
.RS
.in 12
.nf

device /dev/sda7 mounted on /home with fstype ext3 [statistics]
(       1      )            ( 2 )             (3 ) (4)
.fi
.in

The fields in each line are:
.TP 5
(1)
The name of the mounted device
(or "nodevice" if there is no corresponding device).
.TP
(2)
The mount point within the file system tree.
.TP
(3)
The file system type.
.TP
(4)
Optional statistics and configuration information.
Currently (as at Linux 2.6.26), only NFS file systems export
information via this field.
.RE

.SS PID namespaces (CLONE_NEWPID)

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 hosts
while the processes inside the container maintain the same PIDs.
Each PID namespace has its own init (PID 1, see
.BR init (1)),
the "ancestor of all processes" that
manages various system initialization tasks and
reaps orphaned child processes when they terminate.

From the point of view of a particular PID namespace instance,
a process has two PIDs: the PID inside the namespace,
and the PID outside the namespace on the host system.
PID namespaces can be nested:
a process will have one PID for each of the layers of the hierarchy
starting from the PID namespace in which it resides
through to the root PID namespace.
A process can see (e.g., send signals with
.BR kill(2))
only processes contained in its own PID namespace
and the namespaces nested below that PID namespace.

.SS User namespaces (CLONE_NEWUSER)

User namespaces isolate the user and group ID number spaces.
In other words, a process's user and group IDs can be different
inside and outside a user namespace.
A process can have a normal unprivileged user ID outside a user namespace
while at the same time having a user ID of 0 inside the namespace;
in other words,
the process has full privileges for operations inside the user namespace,
but is unprivileged for operations outside the namespace.

Starting in Linux 3.8, unprivileged processes can create user namespaces.

.SS UTS namespaces (CLONE_NEWUTS)

UTS namespaces provide isolation of two system identifiers:
the hostname and the NIS domain name.
These identifiers are set using
.BR sethostname (2)
and
.BR setdomainname (2),
and can be retrieved using
.BR uname (2),
.BR gethostname (2),
and
.BR getdomainname (2).

.SH CONFORMING TO
Namespaces are a Linux-specific feature.
.SH SEE ALSO
.BR readlink (1),
.BR clone (2),
.BR setns (2),
.BR unshare (2),
.BR proc (5),
.BR credentials (7),
.BR capabilities (7)