[PATCH v2] dmaengine: loongson1-apb-dma: avoid using iterator variable after list_for_each_entry()

[Mahad Ibrahim]

ls1x_dma_tx_status() locates the descriptor actively being processed by
walking the LLI list and comparing the hardware reported next descriptor
pointer against each element's next-descriptor pointer.

A list_for_each_entry macro is used in the comparison phase. Which at
the end of the loop leaves the lli pointer at the currently executing LLI.
However this also subsequently runs for a non-match lli, in which it
points at the head. This causes a type confusion bug which treats the
head, which is a ls1x_dma_desc, as a ls1x_dma_lli object. Additionally it
goes forwards and prints garbage via the dev_dbg.

Fix the type confusion bug by only allowing matched LLI descriptor chains
to print the current LLI and residue calculation, as failing to match
should be treated as an unexpected condition.

Found by the following Coccinelle check:

  scripts/coccinelle/iterators/use_after_iter.cocci

  drivers/dma/loongson/loongson1-apb-dma.c:461:6-9: ERROR: invalid
  reference to the index variable of the iterator on line 450

I did not see a bug upstream detailing this error, nor do I have the
hardware to confirm this bug or error, all this is from a pure code
examination.

As I do not possess the hardware, I cannot test the patch. Compile tested
only with mips64-linux-gnu-gcc.

Signed-off-by: Mahad Ibrahim <mahad.ibrahim.dev@xxxxxxxxx>
---

v2:
- encapsulate residue calculation and dev_dbg inside the
  list_for_each_entry() macro. Treat non-matching LLI as an unexpected
  case.


 drivers/dma/loongson/loongson1-apb-dma.c | 27 ++++++++++++++----------
 1 file changed, 16 insertions(+), 11 deletions(-)

diff --git a/drivers/dma/loongson/loongson1-apb-dma.c b/drivers/dma/loongson/loongson1-apb-dma.c
index 89786cbd20ab..8658d5377795 100644
--- a/drivers/dma/loongson/loongson1-apb-dma.c
+++ b/drivers/dma/loongson/loongson1-apb-dma.c
@@ -446,22 +446,27 @@ static enum dma_status ls1x_dma_tx_status(struct dma_chan *dchan,
 
 			/* locate the current lli */
 			next_phys = chan->curr_lli->hw[LS1X_DMADESC_NEXT];
-			list_for_each_entry(lli, &desc->lli_list, node)
-				if (lli->hw[LS1X_DMADESC_NEXT] == next_phys)
-					break;
+			list_for_each_entry(lli, &desc->lli_list, node) {
+				if (lli->hw[LS1X_DMADESC_NEXT] != next_phys)
+					continue;
 
-			dev_dbg(chan2dev(dchan), "current lli_phys=%pad",
-				&lli->phys);
+				dev_dbg(chan2dev(dchan), "current lli_phys=%pad\n",
+					&lli->phys);
 
-			/* count the residues */
-			list_for_each_entry_from(lli, &desc->lli_list, node)
-				bytes += lli->hw[LS1X_DMADESC_LENGTH] *
-					 chan->bus_width;
+				/* count the residues */
+				list_for_each_entry_from(lli, &desc->lli_list, node)
+					bytes += lli->hw[LS1X_DMADESC_LENGTH] *
+						 chan->bus_width;
+
+				dma_set_residue(state, bytes);
+				return status;
+			}
+
+			dev_warn(chan2dev(dchan),
+				 "unable to locate current lli.\n");
 		}
 	}
 
-	dma_set_residue(state, bytes);
-
 	return status;
 }
 
-- 
2.54.0