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

[T Pratham]

On 20/03/25 10:28, T Pratham wrote:
> On 20/03/25 07:16, Andrew Morton wrote:
>> On Wed, 19 Mar 2025 16:44:38 +0530 T Pratham <t-pratham@xxxxxx> 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.
> Hi,
>
> I am using this function in a crypto driver that I'm currently
> developing (not yet sent to mailing list). During testing, it was
> observed that the output scatterlists (except the first one) contained
> incorrect garbage data.
>
> I narrowed this issue down to the call of sg_split(). Upon debugging
> inside this function, I found that this resetting of offset is the cause
> of the problem, causing the subsequent scatterlists to point to
> incorrect memory locations in a page. By removing this code, I am
> obtaining expected data in all the split output scatterlists. Thus, this
> was indeed causing observable runtime effects!
>
> Regards
> T Pratham <t-pratham@xxxxxx>

Hi Andrew,

Do you need the above details to be incorporated into the commit message
and be resent? Kindly let me know.

Regards
T Pratham <t-pratham@xxxxxx>

>>> ...
>>>
>>> --- 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