[PATCH 2/2] dmaengine: pxa_dma: Annotate struct pxad_desc_sw with __counted_by

[Christophe JAILLET]

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).

To do so, the code needs a little shuffling related to how hw_desc is used
and nb_desc incremented.

The one by one increment is needed for the error handling path, calling
pxad_free_desc(), to work correctly.

So, add a new intermediate variable, desc, to store the result of the
dma_pool_alloc() call.

Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
---
This patch is part of a work done in parallel of what is currently worked
on by Kees Cook.

My patches are only related to corner cases that do NOT match the
semantic of his Coccinelle script[1].

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci
---
 drivers/dma/pxa_dma.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/drivers/dma/pxa_dma.c b/drivers/dma/pxa_dma.c
index 94cef2905940..c6e2862896e3 100644
--- a/drivers/dma/pxa_dma.c
+++ b/drivers/dma/pxa_dma.c
@@ -91,7 +91,8 @@ struct pxad_desc_sw {
 	bool			cyclic;
 	struct dma_pool		*desc_pool;	/* Channel's used allocator */
 
-	struct pxad_desc_hw	*hw_desc[];	/* DMA coherent descriptors */
+	struct pxad_desc_hw	*hw_desc[] __counted_by(nb_desc);
+						/* DMA coherent descriptors */
 };
 
 struct pxad_phy {
@@ -739,6 +740,7 @@ pxad_alloc_desc(struct pxad_chan *chan, unsigned int nb_hw_desc)
 {
 	struct pxad_desc_sw *sw_desc;
 	dma_addr_t dma;
+	void *desc;
 	int i;
 
 	sw_desc = kzalloc(struct_size(sw_desc, hw_desc, nb_hw_desc),
@@ -748,20 +750,21 @@ pxad_alloc_desc(struct pxad_chan *chan, unsigned int nb_hw_desc)
 	sw_desc->desc_pool = chan->desc_pool;
 
 	for (i = 0; i < nb_hw_desc; i++) {
-		sw_desc->hw_desc[i] = dma_pool_alloc(sw_desc->desc_pool,
-						     GFP_NOWAIT, &dma);
-		if (!sw_desc->hw_desc[i]) {
+		desc = dma_pool_alloc(sw_desc->desc_pool, GFP_NOWAIT, &dma);
+		if (!desc) {
 			dev_err(&chan->vc.chan.dev->device,
 				"%s(): Couldn't allocate the %dth hw_desc from dma_pool %p\n",
 				__func__, i, sw_desc->desc_pool);
 			goto err;
 		}
 
+		sw_desc->nb_desc++;
+		sw_desc->hw_desc[i] = desc;
+
 		if (i == 0)
 			sw_desc->first = dma;
 		else
 			sw_desc->hw_desc[i - 1]->ddadr = dma;
-		sw_desc->nb_desc++;
 	}
 
 	return sw_desc;
-- 
2.34.1