Re: [f2fs-dev] [PATCH v17 13/21] xfs: add XFS_BMAPI_UNWRITTEN to unmap unwritten extents in __xfs_bunmapi()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] xfs: __xfs_bunmapi data corruption on realtime fsverity files
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commit 97b9c0f9cb7278bb7ab6643a57cb0d0f1ac3924b
Author: Andrey Albershteyn via Linux-f2fs-devel <linux-f2fs-devel@lists.sourceforge.net>

xfs: add XFS_BMAPI_UNWRITTEN to unmap unwritten extents in __xfs_bunmapi()

The __xfs_bunmapi() is used to remove all extents past EOF. Add
XFS_BMAPI_UNWRITTEN to remove only unwritten extents, while leaving
written ones in place. This will be used in following patch to clean up
unwritten extents on fsverity inodes.

> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index 19ca2043c2a8a..49b093ef0321f 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -5277,6 +5277,10 @@ __xfs_bunmapi(
>  		if (del.br_startoff + del.br_blockcount > end + 1)
>  			del.br_blockcount = end + 1 - del.br_startoff;
>  
> +		if ((flags & XFS_BMAPI_UNWRITTEN) &&
> +		    del.br_state != XFS_EXT_UNWRITTEN)
> +			goto skip;
> +

[Severity: High]
Does this check sufficiently protect written extents when operating on
realtime files?

If an unwritten extent is unaligned within a realtime block, the later
logic in __xfs_bunmapi() will fetch the contiguous written left neighbor
and unconditionally convert its tail into an unwritten extent:

fs/xfs/libxfs/xfs_bmap.c:__xfs_bunmapi() {
    ...
		mod = xfs_rtb_to_rtxoff(mp, del.br_startblock);
		if (mod) {
    ...
			} else if (del.br_state == XFS_EXT_UNWRITTEN) {
				struct xfs_bmbt_irec	prev;
				xfs_fileoff_t		unwrite_start;

				/*
				 * This one is already unwritten.
				 * It must have a written left neighbor.
				 * Unwrite the killed part of that one and
				 * try again.
				 */
				if (!xfs_iext_prev_extent(ifp, &icur, &prev))
					ASSERT(0);
				ASSERT(prev.br_state == XFS_EXT_NORM);
    ...
				prev.br_state = XFS_EXT_UNWRITTEN;
				error = xfs_bmap_add_extent_unwritten_real(tp,
						ip, whichfork, &icur, &cur,
						&prev, &logflags);

Could this unwrite the left neighbor even when XFS_BMAPI_UNWRITTEN is set,
destroying the adjacent written data (such as an fsverity Merkle tree)?

>  		if (!isrt || (flags & XFS_BMAPI_REMAP))
>  			goto delete;
>

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Sashiko AI review · https://sashiko.dev/#/patchset/20261002223705.2175542-1-aalbersh@kernel.org?part=13