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Subject: Re: [PATCH] scsi: advansys: Prefer struct_size over open coded
 arithmetic
To:     Len Baker <len.baker@gmx.com>,
        Matthew Wilcox <willy@infradead.org>,
        Hannes Reinecke <hare@suse.com>,
        "James E.J. Bottomley" <jejb@linux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>
Cc:     "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        linux-hardening@vger.kernel.org, linux-scsi@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20210918111805.15471-1-len.baker@gmx.com>
From:   "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Message-ID: <422fbbe3-6f22-b16d-7af2-23efeb6dd125@embeddedor.com>
Date:   Mon, 20 Sep 2021 18:57:06 -0500
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On 9/18/21 06:18, Len Baker wrote:
> As noted in the "Deprecated Interfaces, Language Features, Attributes,
> and Conventions" documentation [1], size calculations (especially
> multiplication) should not be performed in memory allocator (or similar)
> function arguments due to the risk of them overflowing. This could lead
> to values wrapping around and a smaller allocation being made than the
> caller was expecting. Using those allocations could lead to linear
> overflows of heap memory and other misbehaviors.
> 
> So, refactor the code a bit to use the struct_size() helper instead of
> the argument "size + count * size" in the kzalloc() function.
> 
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments
> 
> Signed-off-by: Len Baker <len.baker@gmx.com>
> ---
>  drivers/scsi/advansys.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/scsi/advansys.c b/drivers/scsi/advansys.c
> index ffb391967573..fe882502e7bf 100644
> --- a/drivers/scsi/advansys.c
> +++ b/drivers/scsi/advansys.c
> @@ -7465,6 +7465,7 @@ static int asc_build_req(struct asc_board *boardp, struct scsi_cmnd *scp,
>  		return ASC_BUSY;
>  	} else if (use_sg > 0) {
>  		int sgcnt;
> +		size_t size;

I don't think a new variable is needed.

>  		struct scatterlist *slp;
>  		struct asc_sg_head *asc_sg_head;
> 
> @@ -7477,8 +7478,8 @@ static int asc_build_req(struct asc_board *boardp, struct scsi_cmnd *scp,
>  			return ASC_ERROR;
>  		}
> 
> -		asc_sg_head = kzalloc(sizeof(asc_scsi_q->sg_head) +
> -			use_sg * sizeof(struct asc_sg_list), GFP_ATOMIC);
> +		size = struct_size(asc_scsi_q->sg_head, sg_list, use_sg);
> +		asc_sg_head = kzalloc(size, GFP_ATOMIC);

You can go with this:

diff --git a/drivers/scsi/advansys.c b/drivers/scsi/advansys.c
index ffb391967573..e341b3372482 100644
--- a/drivers/scsi/advansys.c
+++ b/drivers/scsi/advansys.c
@@ -7477,8 +7477,8 @@ static int asc_build_req(struct asc_board *boardp, struct scsi_cmnd *scp,
                        return ASC_ERROR;
                }

-               asc_sg_head = kzalloc(sizeof(asc_scsi_q->sg_head) +
-                       use_sg * sizeof(struct asc_sg_list), GFP_ATOMIC);
+               asc_sg_head = kzalloc(struct_size(asc_sg_head, sg_list, use_sg),
+                                     GFP_ATOMIC);
                if (!asc_sg_head) {
                        scsi_dma_unmap(scp);
                        set_host_byte(scp, DID_SOFT_ERROR);

It perfectly fits withing the 80 columns, which by the way is deprecated
but still good-to-have.

Also, if you are finding these instances with the help of Coccinelle, it'd be nice
if you mention that in the changelog text. :)

Thanks!
--
Gustavo

>  		if (!asc_sg_head) {
>  			scsi_dma_unmap(scp);
>  			set_host_byte(scp, DID_SOFT_ERROR);
> --
> 2.25.1
>