[PATCH v2] null_blk: set dma alignment to logical block size

[PATCH v2] null_blk: set dma alignment to logical block size

[Hans Holmberg]

This driver assumes that bio vectors are memory aligned to the logical
block size, so set the queue limit to reflect that.

Unless we set up the limit based on the logical block size, we will go
out of page bounds in copy_to_nullb / copy_from_nullb.

Fixes: bf8d08532bc1 ("iomap: add support for dma aligned direct-io")
Fixes: b1a000d3b8ec ("block: relax direct io memory alignment")
Signed-off-by: Hans Holmberg <hans.holmberg@wdc.com>
---

Changes in v2:
* Added fixes tags from Christoph
* Added reviewed-bys from Keith and Christoph

v1: https://lore.kernel.org/all/20251029133956.19554-1-hans.holmberg@wdc.com/


 drivers/block/null_blk/main.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
index f982027e8c85..0ee55f889cfd 100644
--- a/drivers/block/null_blk/main.c
+++ b/drivers/block/null_blk/main.c
@@ -1949,6 +1949,7 @@ static int null_add_dev(struct nullb_device *dev)
 		.logical_block_size	= dev->blocksize,
 		.physical_block_size	= dev->blocksize,
 		.max_hw_sectors		= dev->max_sectors,
+		.dma_alignment		= dev->blocksize - 1,
 	};
 
 	struct nullb *nullb;
-- 
2.34.1

Re: [PATCH v2] null_blk: set dma alignment to logical block size

[Christoph Hellwig]

On Fri, Oct 31, 2025 at 10:02:09AM +0100, Hans Holmberg wrote:
> This driver assumes that bio vectors are memory aligned to the logical
> block size, so set the queue limit to reflect that.
> 
> Unless we set up the limit based on the logical block size, we will go
> out of page bounds in copy_to_nullb / copy_from_nullb.

Please also add something like:

Apparently this wasn't noticed so far because none of the tests generate
such buffers, but since 851c4c96db00 ("xfs: implement XFS_IOC_DIOINFO in
terms of vfs_getattr"), xfstests generates unaligned I/O, which now lead
to memory corruption when using null_blk.

> Fixes: bf8d08532bc1 ("iomap: add support for dma aligned direct-io")
> Fixes: b1a000d3b8ec ("block: relax direct io memory alignment")
> Signed-off-by: Hans Holmberg <hans.holmberg@wdc.com>
> ---
> 
> Changes in v2:
> * Added fixes tags from Christoph
> * Added reviewed-bys from Keith and Christoph

I don't actually see the Reviewed-by tags above..