dmaengine: moxart-dma: Annotate struct moxart_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 moxart_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: 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/20230817235859.49846-6-keescook@chromium.orgSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/moxart-dma.c

@@ -124,7 +124,7 @@ struct moxart_desc {
 	unsigned int			dma_cycles;
 	struct virt_dma_desc		vd;
 	uint8_t				es;
-	struct moxart_sg		sg[];
+	struct moxart_sg		sg[] __counted_by(sglen);
 };
 
 struct moxart_chan {
@@ -309,6 +309,7 @@ static struct dma_async_tx_descriptor *moxart_prep_slave_sg(
 	d = kzalloc(struct_size(d, sg, sg_len), GFP_ATOMIC);
 	if (!d)
 		return NULL;
+	d->sglen = sg_len;
 
 	d->dma_dir = dir;
 	d->dev_addr = dev_addr;
@@ -319,8 +320,6 @@ static struct dma_async_tx_descriptor *moxart_prep_slave_sg(
 		d->sg[i].len = sg_dma_len(sgent);
 	}
 
-	d->sglen = sg_len;
-
 	ch->error = 0;
 
 	return vchan_tx_prep(&ch->vc, &d->vd, tx_flags);