scsi: advansys: Prefer struct_size() over open-coded arithmetic

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.

Use the struct_size() helper to do the arithmetic instead of the argument
"size + count * size" in the kzalloc() function.

This code was detected with the help of Coccinelle and audited and fixed
manually.

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments

Link: https://lore.kernel.org/r/20210925114205.11377-1-len.baker@gmx.comReviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Len Baker <len.baker@gmx.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

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);