[PATCH V2] xfs: be more forgiving of a v4 secondary sb w/ junk in v5 fields

[Ben Myers]

Hey Eric,

On Mon, Sep 09, 2013 at 03:33:29PM -0500, Eric Sandeen wrote:
> Today, if xfs_sb_read_verify encounters a v4 superblock
> with junk past v4 fields which includes data in sb_crc,
> it will be treated as a failing checksum and a significant
> corruption.
> 
> There are known prior bugs which leave junk at the end
> of the V4 superblock; we don't need to actually fail the
> verification in this case if other checks pan out ok.
> 
> So if this is a secondary superblock, and the primary
> superblock doesn't indicate that this is a V5 filesystem,
> don't treat this as an actual checksum failure.
> 
> We should probably check the garbage condition as
> we do in xfs_repair, and possibly warn about it
> or self-heal, but that's a different scope of work.
> 
> Stable folks: This can go back to v3.10, which is what
> introduced the sb CRC checking that is tripped up by old,
> stale, incorrect V4 superblocks w/ unzeroed bits.
> 
> Cc: stable at vger.kernel.org
> Signed-off-by: Eric Sandeen <sandeen at redhat.com>

Applied this one.  Thanks.

-Ben