linux-kernel - Re: [PATCH] lib: scatterlist: Fix sg_split_phys to preserve original scatterlist offsets

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Message-Id: <20250319184605.809fc9ce3b169478102b9313@linux-foundation.org>
Date: Wed, 19 Mar 2025 18:46:05 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: T Pratham <t-pratham@...com>
Cc: Jens Axboe <axboe@...nel.dk>, Robert Jarzmik <robert.jarzmik@...e.fr>,
 Kamlesh Gurudasani <kamlesh@...com>, Vignesh Raghavendra <vigneshr@...com>,
 Praneeth Bajjuri <praneeth@...com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lib: scatterlist: Fix sg_split_phys to preserve
 original scatterlist offsets

On Wed, 19 Mar 2025 16:44:38 +0530 T Pratham <t-pratham@...com> wrote:

> The split_sg_phys function was incorrectly setting the offsets of all
> scatterlist entries (except the first) to 0. Only the first scatterlist
> entry's offset and length needs to be modified to account for the skip.
> Setting the rest entries' offsets to 0 could lead to incorrect data
> access.
> 
> This patch removes the offending code, ensuring that the page offsets
> in the input scatterlist are preserved in the output scatterlist.

Is this merely from code inspection, or is this issues known to have
observable runtime effects?

If the latter, please provide a complete description.

>
> ...
>
> --- a/lib/sg_split.c
> +++ b/lib/sg_split.c
> @@ -88,8 +88,6 @@ static void sg_split_phys(struct sg_splitter *splitters, const int nb_splits)
>  			if (!j) {
>  				out_sg->offset += split->skip_sg0;
>  				out_sg->length -= split->skip_sg0;
> -			} else {
> -				out_sg->offset = 0;
>  			}
>  			sg_dma_address(out_sg) = 0;
>  			sg_dma_len(out_sg) = 0;
> -- 
> 2.34.1