On Wed, Nov 26, 2025 at 01:12:40PM +0000, Pranjal Shrivastava wrote:
On Tue, Nov 25, 2025 at 04:18:03PM -0800, Alex Mastro wrote:
On Thu, Nov 20, 2025 at 11:28:25AM +0200, Leon Romanovsky wrote:
+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;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.
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.
--- a/drivers/dma-buf/dma-buf-mapping.c +++ b/drivers/dma-buf/dma-buf-mapping.c @@ -252,14 +252,14 @@ static struct scatterlist *fill_sg_entry(struct scatterlist *sgl, size_t length, nents = DIV_ROUND_UP(length, UINT_MAX); for (i = 0; i < nents; i++) { len = min_t(size_t, length, UINT_MAX);
length -= len;*/ sg_set_page(sgl, NULL, 0, 0);
- 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_dma_address(sgl) = addr + i * UINT_MAX;
sg_dma_address(sgl) = addr;addr += len;length -= len;Thanks, Praan