[PATCH 0/4, V2] xfs: validate inode numbers in file handles correctly

[Dave Chinner <david@fromorbit.com>]

This series closes a recently discovered problem in XFS filehandle conversion.
On systems where inodes are dynamically deleted, XFS does not adequately verify
the inode numbers in the filehandles, which results in reading stale inodes
from disk and potentially returning them as valid files. Because these unlinked
inodes were never zeroed out when the chunk was deallocated, some inodes in the
chunk can still appear to have to data extents attached to them. This can lead
to stale data exposure, exposure of active data and potentially overwriting of
active data if the stale extents referenced in the unlinked inodes have been
re-allocated.

Both NFS filehandles and local filehandles provided through libhandle have this
same problem. libhandle requires root permissions to use the interface, so it
is not exposing information that you can't get more easily with other means
(e.g. xfs_db or reading directly form the block device), so there isn't really
an issue here.

For NFS, we may incorrectly accept stale file handles for unlinked inodes after
a server reboot if the unlinked inodes have not been overwritten leading to the
above issues being triggered if multiple NFS clients are accessing the same
files.

Christoph's make-bulkstat-coherent patch is the basis for this series as
bulkstat can also expose unlinked inodes and information about them back to
userspace as it makes the same assumptions about inode lookups as the file
handle interfaces.

As a result, the first two patches of the series make up the real bug fix. The
last two patches make it clear we are lookuping up untrusted inode numbers and
clear away a shortcut that these interfaces used that we do not want used any
more.  Hence for backports to other kernels, only the first two patches are
necessary.

The test program that demonstrates the issue via the open_by_handle interface
can be found here:

http://oss.sgi.com/archives/xfs/2010-06/msg00191.html

Version 2:
- removed useless ip->i_imap.im_blkno initialisation in xfs_iread()
- reworked a comment refering to bulkstat when it should refer to untrusted
  inodes.
- removed typedefs from xfs_imap_lookup()
- killed useless error logging from xfs_imap_lookup()
- rearranged the logic flow of xfs_imap_lookup() to remove the gotos.


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