dmaengine: axi-dmac: Annotate struct axi_dmac_desc with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct axi_dmac_desc.
Additionally, since the element count member must be set before accessing
the annotated flexible array member, move its initialization earlier.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Vinod Koul <vkoul@kernel.org>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: dmaengine@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230817235859.49846-3-keescook@chromium.orgSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/dma-axi-dmac.c

@@ -117,7 +117,7 @@ struct axi_dmac_desc {
 	unsigned int num_submitted;
 	unsigned int num_completed;
 	unsigned int num_sgs;
-	struct axi_dmac_sg sg[];
+	struct axi_dmac_sg sg[] __counted_by(num_sgs);
 };
 
 struct axi_dmac_chan {
@@ -484,12 +484,11 @@ static struct axi_dmac_desc *axi_dmac_alloc_desc(unsigned int num_sgs)
 	desc = kzalloc(struct_size(desc, sg, num_sgs), GFP_NOWAIT);
 	if (!desc)
 		return NULL;
+	desc->num_sgs = num_sgs;
 
 	for (i = 0; i < num_sgs; i++)
 		desc->sg[i].id = AXI_DMAC_SG_UNUSED;
 
-	desc->num_sgs = num_sgs;
-
 	return desc;
 }