Messages in this thread Patch in this message |  | | Date | Wed, 26 Nov 2025 13:12:40 +0000 | | From | Pranjal Shrivastava <> | | Subject | Re: [PATCH v9 06/11] dma-buf: provide phys_vec to scatter-gather mapping routine |
| |
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 ?
--- 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; /* * 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; + sg_dma_address(sgl) = addr; sg_dma_len(sgl) = len; + + addr += len; + length -= len; sgl = sg_next(sgl); } Thanks, Praan
|  |