2020-05-04 13:00:53

by Marek Szyprowski

Subject: [PATCH v2 09/21] drm: panfrost: fix sg_table nents vs. orig_nents misuse

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.

Signed-off-by: Marek Szyprowski <[email protected]>
---
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;
}
--
1.9.1

2020-05-04 13:29:56

by Steven Price

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 <[email protected]>

>
> Signed-off-by: Marek Szyprowski <[email protected]>
> ---
> 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;
> }
>