linux-kernel - Re: [PATCH v2 09/21] drm: panfrost: fix sg_table nents vs. orig

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Date:   Mon, 4 May 2020 14:27:56 +0100
From:   Steven Price <steven.price@....com>
To:     Marek Szyprowski <m.szyprowski@...sung.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     Christoph Hellwig <hch@....de>,
        Robin Murphy <Robin.Murphy@....com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>, Rob Herring <robh@...nel.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>
Subject: Re: [PATCH v2 09/21] drm: panfrost: fix sg_table nents vs. orig_nents
 misuse

On 04/05/2020 13:53, Marek Szyprowski wrote:
> The Documentation/DMA-API-HOWTO.txt states that dma_map_sg returns the
> numer of the created entries in the DMA address space. However the
> subsequent calls to dma_sync_sg_for_{device,cpu} and dma_unmap_sg must be
> called with the original number of entries passed to dma_map_sg. The
> sg_table->nents in turn holds the result of the dma_map_sg call as stated
> in include/linux/scatterlist.h. Adapt the code to obey those rules.

I find this commit message a bit confusing, but AFAICT the problem with 
the Panfrost code is really in mmu_map_sg() where we don't have the 
return value from dma_map_sg() and the for_each_sg() loop could (in 
theory) run off the end of the list.

The fix seems correct - store the return where it's meant to be (nents) 
and make sure when unmapping we use the original (orig_nents). So you 
might also consider adding:

Fixes: f3ba91228e8e ("drm/panfrost: Add initial panfrost driver")

Even better would be the wrappers you mention in the cover letter! ;)

Reviewed-by: Steven Price <steven.price@....com>

> 
> Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
> ---
> For more information, see '[PATCH v2 00/21] DRM: fix struct sg_table nents
> vs. orig_nents misuse' thread: https://lkml.org/lkml/2020/5/4/373
> ---
>   drivers/gpu/drm/panfrost/panfrost_gem.c | 3 ++-
>   drivers/gpu/drm/panfrost/panfrost_mmu.c | 4 +++-
>   2 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panfrost/panfrost_gem.c b/drivers/gpu/drm/panfrost/panfrost_gem.c
> index 17b654e..22fec7c 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_gem.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_gem.c
> @@ -42,7 +42,8 @@ static void panfrost_gem_free_object(struct drm_gem_object *obj)
>   		for (i = 0; i < n_sgt; i++) {
>   			if (bo->sgts[i].sgl) {
>   				dma_unmap_sg(pfdev->dev, bo->sgts[i].sgl,
> -					     bo->sgts[i].nents, DMA_BIDIRECTIONAL);
> +					     bo->sgts[i].orig_nents,
> +					     DMA_BIDIRECTIONAL);
>   				sg_free_table(&bo->sgts[i]);
>   			}
>   		}
> diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> index ed28aeb..2d9b1f9 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
> @@ -517,7 +517,9 @@ static int panfrost_mmu_map_fault_addr(struct panfrost_device *pfdev, int as,
>   	if (ret)
>   		goto err_pages;
>   
> -	if (!dma_map_sg(pfdev->dev, sgt->sgl, sgt->nents, DMA_BIDIRECTIONAL)) {
> +	sgt->nents = dma_map_sg(pfdev->dev, sgt->sgl, sgt->orig_nents,
> +				DMA_BIDIRECTIONAL);
> +	if (!sgt->nents) {
>   		ret = -EINVAL;
>   		goto err_map;
>   	}
>