[PATCH] Generic infrastructure for acls

The patches solve the following problem: We want to grant access to devices
based on who is logged in from where, etc.  This includes switching back and
forth between multiple user sessions, etc.

Using ACLs to define device access for logged-in users gives us all the
flexibility we need in order to fully solve the problem.

Device special files nowadays usually live on tmpfs, hence tmpfs ACLs.

Different distros have come up with solutions that solve the problem to
different degrees: SUSE uses a resource manager which tracks login sessions
and sets ACLs on device inodes as appropriate.  RedHat uses pam_console, which
changes the primary file ownership to the logged-in user.  Others use a set of
groups that users must be in in order to be granted the appropriate accesses.

The freedesktop.org project plans to implement a combination of a
console-tracker and a HAL-device-list based solution to grant access to
devices to users, and more distros will likely follow this approach.

These patches have first been posted here on 2 February 2005, and again
on 8 January 2006. We have been shipping them in SLES9 and SLES10 with
no problems reported.  The previous submission is archived here:

   http://lkml.org/lkml/2006/1/8/229
   http://lkml.org/lkml/2006/1/8/230
   http://lkml.org/lkml/2006/1/8/231

This patch:

Add some infrastructure for access control lists on in-memory
filesystems such as tmpfs.

Signed-off-by: Andreas Gruenbacher <agruen@suse.de>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>