[PATCH 12/21] dmaengine: stm32-dma: Annotate struct stm32_dma_desc with __counted_by

[Kees Cook]

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 stm32_dma_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@xxxxxxxxxx>
Cc: Maxime Coquelin <mcoquelin.stm32@xxxxxxxxx>
Cc: Alexandre Torgue <alexandre.torgue@xxxxxxxxxxx>
Cc: dmaengine@xxxxxxxxxxxxxxx
Cc: linux-stm32@xxxxxxxxxxxxxxxxxxxxxxxxxxxx
Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 drivers/dma/stm32-dma.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/drivers/dma/stm32-dma.c b/drivers/dma/stm32-dma.c
index 5c36811aa134..a732b3807b11 100644
--- a/drivers/dma/stm32-dma.c
+++ b/drivers/dma/stm32-dma.c
@@ -191,7 +191,7 @@ struct stm32_dma_desc {
 	struct virt_dma_desc vdesc;
 	bool cyclic;
 	u32 num_sgs;
-	struct stm32_dma_sg_req sg_req[];
+	struct stm32_dma_sg_req sg_req[] __counted_by(num_sgs);
 };
 
 /**
@@ -1105,6 +1105,7 @@ static struct dma_async_tx_descriptor *stm32_dma_prep_slave_sg(
 	desc = kzalloc(struct_size(desc, sg_req, sg_len), GFP_NOWAIT);
 	if (!desc)
 		return NULL;
+	desc->num_sgs = sg_len;
 
 	/* Set peripheral flow controller */
 	if (chan->dma_sconfig.device_fc)
@@ -1141,8 +1142,6 @@ static struct dma_async_tx_descriptor *stm32_dma_prep_slave_sg(
 			desc->sg_req[i].chan_reg.dma_sm1ar += sg_dma_len(sg);
 		desc->sg_req[i].chan_reg.dma_sndtr = nb_data_items;
 	}
-
-	desc->num_sgs = sg_len;
 	desc->cyclic = false;
 
 	return vchan_tx_prep(&chan->vchan, &desc->vdesc, flags);
@@ -1216,6 +1215,7 @@ static struct dma_async_tx_descriptor *stm32_dma_prep_dma_cyclic(
 	desc = kzalloc(struct_size(desc, sg_req, num_periods), GFP_NOWAIT);
 	if (!desc)
 		return NULL;
+	desc->num_sgs = num_periods;
 
 	for (i = 0; i < num_periods; i++) {
 		desc->sg_req[i].len = period_len;
@@ -1232,8 +1232,6 @@ static struct dma_async_tx_descriptor *stm32_dma_prep_dma_cyclic(
 		if (!chan->trig_mdma)
 			buf_addr += period_len;
 	}
-
-	desc->num_sgs = num_periods;
 	desc->cyclic = true;
 
 	return vchan_tx_prep(&chan->vchan, &desc->vdesc, flags);
@@ -1254,6 +1252,7 @@ static struct dma_async_tx_descriptor *stm32_dma_prep_dma_memcpy(
 	desc = kzalloc(struct_size(desc, sg_req, num_sgs), GFP_NOWAIT);
 	if (!desc)
 		return NULL;
+	desc->num_sgs = num_sgs;
 
 	threshold = chan->threshold;
 
@@ -1283,8 +1282,6 @@ static struct dma_async_tx_descriptor *stm32_dma_prep_dma_memcpy(
 		desc->sg_req[i].chan_reg.dma_sndtr = xfer_count;
 		desc->sg_req[i].len = xfer_count;
 	}
-
-	desc->num_sgs = num_sgs;
 	desc->cyclic = false;
 
 	return vchan_tx_prep(&chan->vchan, &desc->vdesc, flags);
-- 
2.34.1