Re: [PATCH v5 11/29] md: add helper to split bios at reshape offset

Thank you for your contribution! Sashiko AI review found 1 potential issue(=
s) to consider:
- [Critical] Torn read of 64-bit reshape_position on 32-bit architectures l=
eads to silent data corruption.
--

--- Patch [11]: [PATCH v5 11/29] md: add helper to split bios at reshape of=
fset ---
commit 7816e7809a77baa291a2c8013717413f881d0cdc
Author: Yu Kuai <[email protected]>

md: add helper to split bios at reshape offset

This commit adds mddev_bio_split_at_reshape_offset() to allow md
personalities to share reshape-offset bio splitting. It snapshots
mddev->reshape_position to determine if the bio crosses the reshape
boundary and limits the front bio accordingly.

> diff --git a/drivers/md/md.c b/drivers/md/md.c
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
[ ... ]
> @@ -9388,6 +9388,45 @@ void md_submit_discard_bio(struct mddev *mddev, st=
ruct md_rdev *rdev,
> +struct bio *mddev_bio_split_at_reshape_offset(struct mddev *mddev,
> +					      struct bio *bio,
> +					      unsigned int *max_sectors,
> +					      struct bio_set *bs)
> +{
> +	sector_t boundary;
> +	sector_t start;
> +	sector_t end;
> +	unsigned int split_sectors;
> +
> +	split_sectors =3D bio_sectors(bio);
> +	if (max_sectors && *max_sectors && *max_sectors < split_sectors)
> +		split_sectors =3D *max_sectors;
> +
> +	if (!test_bit(MD_RECOVERY_RESHAPE, &mddev->recovery))
> +		goto split;
> +
> +	boundary =3D READ_ONCE(mddev->reshape_position);

[Severity: Critical]
Is it safe to use READ_ONCE() on a 64-bit value here?

On 32-bit architectures, sector_t is 64-bit, so READ_ONCE() will generate t=
wo
non-atomic 32-bit memory load instructions. If the reshape thread updates
mddev->reshape_position concurrently, this could result in a torn read.

A torn read would make boundary completely incorrect, causing split_sectors
to be miscalculated. Because the reshape boundary separates two different
physical data layouts, could sending IO based on a torn boundary write data
to the wrong physical disk locations and cause data corruption?

> +	start =3D bio->bi_iter.bi_sector;
> +	end =3D bio_end_sector(bio);
> +	if (start >=3D boundary || end <=3D boundary)
> +		goto split;
> +
> +	if (boundary - start < split_sectors)
> +		split_sectors =3D boundary - start;
> +
> +split:
[ ... ]

--=20
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