Re: [PATCH STABLE] xfs: trim IO to found COW exent limit

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Sep 23, 2020 at 05:35:44PM -0500, Eric Sandeen wrote:
> A bug existed in the XFS reflink code between v5.1 and v5.5 in which
> the mapping for a COW IO was not trimmed to the mapping of the COW
> extent that was found.  This resulted in a too-short copy, and
> corruption of other files which shared the original extent.
> 
> (This happened only when extent size hints were set, which bypasses
> delalloc and led to this code path.)
> 
> This was (inadvertently) fixed upstream with
> 
> 36adcbace24e "xfs: fill out the srcmap in iomap_begin"
> 
> and related patches which moved lots of this functionality to
> the iomap subsystem.
> 
> Hence, this is a -stable only patch, targeted to fix this
> corruption vector without other major code changes.
> 
> Fixes: 78f0cc9d55cb ("xfs: don't use delalloc extents for COW on files with extsize hints")
> Cc: <stable@vger.kernel.org> # 5.4.x
> Signed-off-by: Eric Sandeen <sandeen@redhat.com>

Looks good to me,
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

(Note: Someone please fix the typo "exent" in the subject line.)

--D

> ---
> 
> I've tested this with a targeted reproducer (in next email) as well as
> with xfstests.
> 
> Stable folk, not sure how to send a "stable only" patch, or if that's even
> valid.  Assuming you're willing to accept it, I would still like to have
> some formal Reviewed-by's from the xfs developer community before it gets
> merged.
> 
> Big thanks to Darrick & Dave for letting me whine about this bug and
> offering suggestions for testing and ultimately, a patch to test.
> 
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 06b9e0aacf54..3289d0f4bb03 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -1002,9 +1002,15 @@ xfs_file_iomap_begin(
>  		 * I/O, which must be block aligned, we need to report the
>  		 * newly allocated address.  If the data fork has a hole, copy
>  		 * the COW fork mapping to avoid allocating to the data fork.
> +		 *
> +		 * Otherwise, ensure that the imap range does not extend past
> +		 * the range allocated/found in cmap.
>  		 */
>  		if (directio || imap.br_startblock == HOLESTARTBLOCK)
>  			imap = cmap;
> +		else
> +			xfs_trim_extent(&imap, cmap.br_startoff,
> +					cmap.br_blockcount);
>  
>  		end_fsb = imap.br_startoff + imap.br_blockcount;
>  		length = XFS_FSB_TO_B(mp, end_fsb) - offset;
>