On Wed, Nov 26, 2025 at 08:08:24AM -0800, Alex Mastro wrote:

On Wed, Nov 26, 2025 at 01:12:40PM +0000, Pranjal Shrivastava wrote:

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On Tue, Nov 25, 2025 at 04:18:03PM -0800, Alex Mastro wrote:

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On Thu, Nov 20, 2025 at 11:28:25AM +0200, Leon Romanovsky wrote:

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+static struct scatterlist *fill_sg_entry(struct scatterlist *sgl, size_t length,

• 			 dma_addr_t addr)
  

+{

• unsigned int len, nents;
• int i;
• nents = DIV_ROUND_UP(length, UINT_MAX);
• for (i = 0; i < nents; i++) {

• len = min_t(size_t, length, UINT_MAX);
  

• length -= len;
  

• /*
  

•  * DMABUF abuses scatterlist to create a scatterlist
  

•  * that does not have any CPU list, only the DMA list.
  

•  * Always set the page related values to NULL to ensure
  

•  * importers can't use it. The phys_addr based DMA API
  

•  * does not require the CPU list for mapping or unmapping.
  

•  */
  

• sg_set_page(sgl, NULL, 0, 0);
  

• sg_dma_address(sgl) = addr + i * UINT_MAX;
  

(i * UINT_MAX) happens in 32-bit before being promoted to dma_addr_t for addition with addr. Overflows for i >=2 when length >= 8 GiB. Needs a cast:

sg_dma_address(sgl) = addr + (dma_addr_t)i * UINT_MAX;

Yeah, and i should not be signed.

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Discovered this while debugging why dma-buf import was failing for an 8 GiB dma-buf using my earlier toy program [1]. It was surfaced by ib_umem_find_best_pgsz() returning 0 due to malformed scatterlist, which bubbles up as an EINVAL.

Thanks a lot for testing & reporting this!

However, I believe the casting approach is a little fragile (and potentially prone to issues depending on how dma_addr_t is sized on different platforms). Thus, approaching this with accumulation seems better as it avoids the multiplication logic entirely, maybe something like the following (untested) diff ?

If the function input range is well-formed, then all values in [addr..addr+length) must be expressible by dma_addr_t, so I don't think overflow after casting is possible as long as nents is valid.

It is probably not perfect, but validate_dmabuf_input() limits length to a valid size_t

The signature is:

bool dma_iova_try_alloc(struct device *dev, struct dma_iova_state *state, phys_addr_t phys, size_t size)

And that function should fail if size is too large. I think it mostly does, but it looks like there are a few little misses:

iova_align(iovad, size + iova_off), return ALIGN(size, iovad->granule);

etc are all unchecked math that could overflow.

That said, `nents = DIV_ROUND_UP(length, UINT_MAX)` is simply broken on any system where size_t is 32b. I don't know if that's a practical consideration for these code paths though.

Yeah, that's a good point.

Casting to u64 will trigger 64 bit device errors on 32 bit too.

// DIV_ROUND_UP that is safe at the type limits nents = size / UINT_MAX; if (size % UINT_MAX) nents++;

Compiler should turn the % into bit math.

Jason