xfs: don't drop errno values when we fail to ficlone the entire range

xfs: don't drop errno values when we fail to ficlone the entire range

Way back when we first implemented FICLONE for XFS, life was simple --
either the the entire remapping completed, or something happened and we
had to return an errno explaining what happened.  Neither of those
ioctls support returning partial results, so it's all or nothing.

Then things got complicated when copy_file_range came along, because it
actually can return the number of bytes copied, so commit 3f68c1f562f1e4
tried to make it so that we could return a partial result if the
REMAP_FILE_CAN_SHORTEN flag is set.  This is also how FIDEDUPERANGE can
indicate that the kernel performed a partial deduplication.

Unfortunately, the logic is wrong if an error stops the remapping and
CAN_SHORTEN is not set.  Because those callers cannot return partial
results, it is an error for ->remap_file_range to return a positive
quantity that is less than the @len passed in.  Implementations really
should be returning a negative errno in this case, because that's what
btrfs (which introduced FICLONE{,RANGE}) did.

Therefore, ->remap_range implementations cannot silently drop an errno
that they might have when the number of bytes remapped is less than the
number of bytes requested and CAN_SHORTEN is not set.

Found by running generic/562 on a 64k fsblock filesystem and wondering
why it reported corrupt files.

Cc: <stable@vger.kernel.org> # v4.20
Fixes: 3fc9f5e409319e ("xfs: remove xfs_reflink_remap_range")
Really-Fixes: 3f68c1f562f1e4 ("xfs: support returning partial reflink results")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>