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mount.2: Add description of namespace 

Describe per-process namespaces, including discussion
of clone() and unshare CLONE_NEWNS, and /proc/PID/mounts.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2008-10-06 14:07:03 +02:00
parent 852ae8c875
commit 0a4d78bbe1
1 changed files with 43 additions and 0 deletions
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@ -34,6 +34,8 @@
.\" Enhanced descriptions of MS_MOVE, MS_BIND, and MS_REMOUNT
.\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" 2005-05-18, mtk, Added MNT_EXPIRE, plus a few other tidy-ups.
.\" 2008-10-06, mtk: move umount*() material into separate umount.2 page.
.\" 2008-10-06, mtk: Add discussion of namespaces.
.\"
.TH MOUNT 2 2008-10-06 "Linux" "Linux Programmer's Manual"
.SH NAME
@ -373,6 +375,47 @@ would fail with
Since Linux 2.4 the set-user-ID and set-group-ID bits are
just silently ignored in this case.
.\" The change is in patch-2.4.0-prerelease.
.SS Per-process Namespaces
Starting with kernel 2.4.19, Linux provides per-process namespaces.
A namespace is the set of file system mounts that are visible to a process.
Namespaces can be (and usually are) shared between multiple processes,
and changes to the namespace (i.e., mounts and unmounts) by one process
are visible to all other processes sharing the same namespace.
(The pre-2.4.19 situation can be considered as one in which there was
a single namespace was shared by every process on the system.)
A child process created by
.BR fork (2)
shares its parent's namespace;
the namespace is preserved across an
.BR execve (2).
A process can obtain a private namespace if:
it was created using the
.BR clone ()
.BR CLONE_NEWNS
flag,
in which case its new namespace is initialized to be a
.I copy
of the namespace of the process that called
.BR clone ();
or it calls
.BR unshare (2)
with the
.BR CLONE_NEWNS
flag,
which causes the caller's namespace to obtain a private copy
of the namespace that it was previously sharing with other processes,
so that future mounts and unmounts by the caller are invisible
to other processes (except child processes that the caller
subsequently creates) and vice versa.
The Linux-specific
.I /proc/PID/self
file exposes the list of mount points in the namespace of the process with
the specified ID; see
.BR proc (5)
for details.
.SH "SEE ALSO"
.BR umount (2),
.BR path_resolution (7),