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

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Date:   Thu, 25 Mar 2021 23:34:27 -0400
From:   "Martin K. Petersen" <martin.petersen@...cle.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     "Martin K. Petersen" <martin.petersen@...cle.com>,
        "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

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
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.

That said, this whole function makes my head hurt!

-- 
Martin K. Petersen	Oracle Linux Engineering