linux-kernel - Re: [PATCH][next] scsi: aacraid: Replace one-element array with flexible-array member

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Date:   Thu, 25 Mar 2021 22:07:16 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     "Martin K. Petersen" <martin.petersen@...cle.com>
Cc:     "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Adaptec OEM Raid Solutions <aacraid@...rosemi.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] scsi: aacraid: Replace one-element array with
 flexible-array member

Hi Martin,


On 3/25/21 22:34, Martin K. Petersen wrote:
> 
> Gustavo,
> 
>> Precisely this sort of confusion is one of the things we want to avoid
>> by using flexible-array members instead of one-element arrays.
> 
> Ah, you're right!
> 
> Now that I look at it again I also don't think that was the issue that
> originally caused concern.
> 
> @@ -4020,7 +4020,8 @@ static int aac_convert_sgraw2(struct aac_raw_io2 *rio2, int pages, int nseg, int
>  		}
>  	}
>  	sge[pos] = rio2->sge[nseg-1];
> -	memcpy(&rio2->sge[1], &sge[1], (nseg_new-1)*sizeof(struct sge_ieee1212));
> +	memcpy(&rio2->sge[1], &sge[1],
> +	       flex_array_size(rio2, sge, nseg_new - 1));
>  
>  	kfree(sge);
>  	rio2->sgeCnt = cpu_to_le32(nseg_new);
> 
> I find it counter-intuitive to use the type of the destination array to
> size the amount of source data to copy. "Are source and destination same

The destination and source arrays are of the same type. :)

drivers/scsi/aacraid/aachba.c:
3999         struct sge_ieee1212 *sge;

> type? Does flex_array_size() do the right thing given the ->sge[1]
> destination offset?". It wasn't immediately obvious. To me, "copy this
> many scatterlist entries" in the original is much more readable.

Yeah; it does the right thing because flex_array_size() doesn't know about
offsets. It just calculates the amount of bytes to be copied based on the
type of the object passed as second argument and a "count" passed as third
argument. So, in this case, the "count" is "nseg_new - 1", which in some
way is already taking care of that sge[1] offset.

--
Gustavo