[PATCH, RFC] fix attr fit checking for filesystems which have lost their attr2

[Eric Sandeen <sandeen@sandeen.net>]

Regarding the F8 corruption...  I have a pretty narrow testcase
now, and it turns out this was a bit of a perfect storm.

First, F8 shipped 2.6.23 which had the problem with sb_features2
padding out on 64-bit boxes, but this was ok because userspace
and kernelspace both did this, and it was properly mounting &
running as attr2.

However, hch came along in 2.6.24 and did some endian annotation 
for the superblock and in the process:
> A new helper xfs_sb_from_disk handles the other (read)
> direction and doesn't need the slightly hacky table-driven approach
> because we only ever read the full sb from disk.

However, this resulted in kernelspace behaving differently,
and now *missing* the attr2 flag in sb_features2, (actually
sb_bad_features2) so we mounted as if we had attr1.  Which 
is really supposed to be ok, IIRC, except in 
xfs_attr_shortform_bytesfit we return the default
fork offset value from the superblock, even if di_forkoff
is *already* set.  In the error case I had, di_forkoff was set
to 15 (from previously being attr2...) but we returned 14
(the mp default) and I think this is where things started
going wrong; I think this caused us to write an attr on top
of the extent data.

My understanding of this is that if di_forkoff is non-zero,
we should always be using it for space calculations, regardless
of whether we are mounted with attr2 or not...

and with that, how's this look, to be honest I haven't run it
through QA yet...

I'm not certain if xfs_bmap_compute_maxlevels() may lead
to similar problems....

------------------

always use di_forkoff for when checking for attr space

In the case where we mount a filesystem which was previously
using the attr2 format as attr1, returning the default
mp->m_attroffset instead of the per-inode di_forkoff for
inline attribute fit calculations may result in corruption,
if the existing "attr2" formatted attribute is already taking
more space than the default.

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
---


Index: linux-2.6-git/fs/xfs/xfs_attr_leaf.c
===================================================================
--- linux-2.6-git.orig/fs/xfs/xfs_attr_leaf.c
+++ linux-2.6-git/fs/xfs/xfs_attr_leaf.c
@@ -166,7 +166,7 @@ xfs_attr_shortform_bytesfit(xfs_inode_t 
 
 	if (!(mp->m_flags & XFS_MOUNT_ATTR2)) {
 		if (bytes <= XFS_IFORK_ASIZE(dp))
-			return mp->m_attroffset >> 3;
+			return dp->i_d.di_forkoff;
 		return 0;
 	}