Re: [PATCH v3] dmaengine: loongson1-apb-dma: avoid using iterator variable after list_for_each_entry()

[Frank Li]

On Thu, Sep 17, 2026 at 04:30:25PM +0000, Mahad Ibrahim wrote:
> 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
>
> Compile test only; Hardware testing by Keguang Zhang.
>
> Signed-off-by: Mahad Ibrahim <mahad.ibrahim.dev@xxxxxxxxx>
> Reviewed-by: Keguang Zhang <keguang.zhang@xxxxxxxxx>
> Tested-by: Keguang Zhang <keguang.zhang@xxxxxxxxx> # on LS1B & LS1C
> ---

Reviewed-by: Frank Li <Frank.Li@xxxxxxx>

>
> v2:
> - encapsulate residue calculation and dev_dbg inside the
>   list_for_each_entry() macro. Treat non-matching LLI as an unexpected
>   case.
> - link: https://lore.kernel.org/all/20260729143247.6111-1-mahad.ibrahim.dev@xxxxxxxxx/
>
>
> v3:
> - Collect tags from Keguang Zhang.
> - Repost patch in a new thread.
> - Change chan2dev to the newer dmaengine_chan_dev.
>
>
>  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 46b4bfef45e2..e62ab26c8327 100644
> --- a/drivers/dma/loongson/loongson1-apb-dma.c
> +++ b/drivers/dma/loongson/loongson1-apb-dma.c
> @@ -441,22 +441,27 @@ 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(dmaengine_chan_dev(dchan), "current lli_phys=%pad",
> -				&lli->phys);
> +				dev_dbg(dmaengine_chan_dev(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(dmaengine_chan_dev(dchan),
> +				 "unable to locate current lli.\n");
>  		}
>  	}
>
> -	dma_set_residue(state, bytes);
> -
>  	return status;
>  }
>
> --
> 2.54.0
>