[Linaro-mm-sig] Re: [PATCH 2/2] dma-buf: improve sg_table debugging hack

-----Original Message----- From: dri-devel dri-devel-bounces@lists.freedesktop.org On Behalf Of Christian König Sent: Monday, October 6, 2025 9:47 AM To: sumit.semwal@linaro.org; linux-media@vger.kernel.org; dri- devel@lists.freedesktop.org; linaro-mm-sig@lists.linaro.org; simona.vetter@ffwll.ch Subject: [PATCH 2/2] dma-buf: improve sg_table debugging hack

Instead of just mangling the page link create a copy of the sg_table but only copy over the DMA addresses and not the pages.

This made the issue obvious. If I abuse this now how will I know I am doing the wrong thing?

Still quite a hack but this at least allows the exporter to properly keeps it's sg_table intact.

This is important for example for the system DMA-heap which needs it's sg_table to sync CPU writes with device accesses.

So the mangling actually breaks a usage? I thought that the usage was incorrect...is the dma-heap using this incorrectly?

Signed-off-by: Christian König christian.koenig@amd.com

drivers/dma-buf/dma-buf.c | 68 +++++++++++++++++++++++++++++++-----

1 file changed, 54 insertions(+), 14 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index 2305bb2cc1f1..1fe5781d8862 100644 --- a/drivers/dma-buf/dma-buf.c +++ b/drivers/dma-buf/dma-buf.c @@ -828,21 +828,59 @@ void dma_buf_put(struct dma_buf *dmabuf) } EXPORT_SYMBOL_NS_GPL(dma_buf_put, "DMA_BUF");

-static void mangle_sg_table(struct sg_table *sg_table) +static int dma_buf_mangle_sg_table(struct sg_table **sg_table) { -#ifdef CONFIG_DMABUF_DEBUG

• int i;
• struct scatterlist *sg;
• /* To catch abuse of the underlying struct page by importers mix

• * up the bits, but take care to preserve the low SG_ bits to
  

• * not corrupt the sgt. The mixing is undone on unmap
  

• * before passing the sgt back to the exporter.
  

• struct sg_table *to, *from = *sg_table;
• struct scatterlist *to_sg, *from_sg;
• int i, ret;
• if (!IS_ENABLED(CONFIG_DMABUF_DEBUG))

• return 0;
  

• /*

• * To catch abuse of the underlying struct page by importers copy the
  

• * sg_table without copying the page_link and give only the copy back
  

to

• * the importer.
  

  */

• for_each_sgtable_sg(sg_table, sg, i)

• sg->page_link ^= ~0xffUL;
  

-#endif

• to = kzalloc(sizeof(*to), GFP_KERNEL);

• if (!to)

• return -ENOMEM;
  
  

• ret = sg_alloc_table(to, from->nents, GFP_KERNEL);

• if (ret)

• goto free_to;
  

• to_sg = to->sgl;

• for_each_sgtable_dma_sg(from, from_sg, i) {

• sg_set_page(to_sg, NULL, 0, 0);
  

•            sg_dma_address(to_sg) = sg_dma_address(from_sg);
  

•            sg_dma_len(to_sg) = sg_dma_len(from_sg);
  

Is the indentation correct here?

M

• to_sg = sg_next(to_sg);
  

• }
• /*

• * Store the original sg_table in the first page_link of the copy so
  

• * that we can revert everything back again on unmap.
  

• */
  

• to->sgl[0].page_link = (unsigned long)from;
• *sg_table = to;
• return 0;

+free_to:

• kfree(to);
• return ret;

+}

+static void dma_buf_demangle_sg_table(struct sg_table **sg_table) +{

• struct sg_table *copy = *sg_table;
• if (!IS_ENABLED(CONFIG_DMABUF_DEBUG))

• return;
  

• *sg_table = (void *)copy->sgl[0].page_link;
• sg_free_table(copy);
• kfree(copy);

}

static inline bool @@ -1139,7 +1177,9 @@ struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach, if (ret < 0) goto error_unmap; }

• mangle_sg_table(sg_table);

• ret = dma_buf_mangle_sg_table(&sg_table);

• if (ret)

• goto error_unmap;
  
  

  if (IS_ENABLED(CONFIG_DMA_API_DEBUG)) { struct scatterlist *sg;

@@ -1220,7 +1260,7 @@ void dma_buf_unmap_attachment(struct dma_buf_attachment *attach,

dma_resv_assert_held(attach->dmabuf->resv);

• mangle_sg_table(sg_table);

• dma_buf_demangle_sg_table(&sg_table); attach->dmabuf->ops->unmap_dma_buf(attach, sg_table, direction);

  if (dma_buf_pin_on_map(attach))

-- 2.43.0