Re: [PATCH v7 09/11] scsi: scsi_debug: Refuse a short WRITE ATOMIC (16) before writing it

[John Garry <john.garry@linux.dev>]

On 9/25/26 08:17, Niklas Cassel wrote:
> A write whose data-out buffer is shorter than its transfer length is
> short. An ordinary write transfers what the buffer holds and reports the
> rest as a residual, but an atomic write cannot be short: SBC-6 r02
> (T10/BSR INCITS 587), 4.28.1, requires each atomic write operation to
> write either all of its data or none of it, and 4.28.2 requires one that
> cannot complete to leave the LBAs that it specifies unaltered.
> 
> resp_atomic_write() does fail a short WRITE ATOMIC (16) with DID_ERROR,
> but only after do_device_access() has returned, and do_device_access()
> copies one logical block at a time, so by then the blocks that the
> buffer did hold have been written. An eight block WRITE ATOMIC (16) with
> a buffer of four fails having overwritten the first four.
> 
> Check the length of the buffer before writing anything, and fail the
> command with the same DID_ERROR as before.
> 
> Assisted-by: LLM
> Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
> Fixes: 84f3a3c01d70 ("scsi: scsi_debug: Atomic write support")
> Signed-off-by: Niklas Cassel <cassel@kernel.org>
> ---
> Tested with:
> 
>    modprobe scsi_debug sector_size=512 physblk_exp=3 dev_size_mb=128 \
>        atomic_wr=1 lbpu=1
> 
> issuing a WRITE ATOMIC (16) of eight blocks through SG_IO with a buffer
> of four blocks of 0xaa. It fails with DID_ERROR before and after this
> patch, but before it the first four blocks read back as 0xaa afterwards,
> and after it all eight read back as they were. A WRITE ATOMIC (16) with
> a full buffer writes all eight blocks, as before.
> 

What is the command which you use?

I am just wondering if this check should go into sd_setup_atomic_cmnd() 
or somewhere else higher up. I mean, this check is not really specific 
to scsi_debug, right?

> Changes since v6: the comment is reworded, as Damien suggested.
> ---
>   drivers/scsi/scsi_debug.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c
> index f59c4f11c851..c0bcf8155fc8 100644
> --- a/drivers/scsi/scsi_debug.c
> +++ b/drivers/scsi/scsi_debug.c
> @@ -6247,6 +6247,10 @@ static int resp_atomic_write(struct scsi_cmnd *scp,
>   		}
>   	}
>   
> +	/* Short atomic writes are not allowed. */
> +	if (scsi_bufflen(scp) < len * sdebug_sector_size)
> +		return DID_ERROR << 16;
> +
>   	ret = do_device_access(sip, scp, 0, lba, len, 0, true, true);
>   	if (unlikely(ret == -1))
>   		return DID_ERROR << 16;