The namespaces compatibility list doc

People discuss how the namespaces are working/going-to-work together.

Ted Ts'o proposed to create some document that describes what problems user
may have when he/she creates some new namespace, but keeps others shared.
I liked this idea, so here's the initial version of such a document with
the problems I currently have in mind and can describe somewhat audibly -
the "namespaces compatibility list".

The Documentation/namespaces/ directory is about to contain more docs about
the namespaces stuff.

Thanks to Cedirc for notes and spell checks on the doc, to Daniel for
additional info about IPC and User namespaces interaction and to Randy, who
alluded me to using a spell checker before sending the documentation :)
Signed-off-by: NPavel Emelyanov <xemul@openvz.org>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Daniel Lezcano <dlezcano@fr.ibm.com>
Cc: Theodore Tso <tytso@mit.edu>
Cc: Cedric Le Goater <clg@fr.ibm.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/00-INDEX

@@ -262,6 +262,8 @@ mtrr.txt
 	- how to use PPro Memory Type Range Registers to increase performance.
 mutex-design.txt
 	- info on the generic mutex subsystem.
+namespaces/
+	- directory with various information about namespaces
 nbd.txt
 	- info on a TCP implementation of a network block device.
 netlabel/

Documentation/namespaces/compatibility-list.txt

+	Namespaces compatibility list
+
+This document contains the information about the problems user
+may have when creating tasks living in different namespaces.
+
+Here's the summary. This matrix shows the known problems, that
+occur when tasks share some namespace (the columns) while living
+in different other namespaces (the rows):
+
+	UTS	IPC	VFS	PID	User	Net
+UTS	 X
+IPC		 X	 1
+VFS			 X
+PID		 1	 1	 X
+User		 2	 2		 X
+Net						 X
+
+1. Both the IPC and the PID namespaces provide IDs to address
+   object inside the kernel. E.g. semaphore with IPCID or
+   process group with pid.
+
+   In both cases, tasks shouldn't try exposing this ID to some
+   other task living in a different namespace via a shared filesystem
+   or IPC shmem/message. The fact is that this ID is only valid
+   within the namespace it was obtained in and may refer to some
+   other object in another namespace.
+
+2. Intentionally, two equal user IDs in different user namespaces
+   should not be equal from the VFS point of view. In other
+   words, user 10 in one user namespace shouldn't have the same
+   access permissions to files, belonging to user 10 in another
+   namespace.
+
+   The same is true for the IPC namespaces being shared - two users
+   from different user namespaces should not access the same IPC objects
+   even having equal UIDs.
+
+   But currently this is not so.
+