Re: [PATCH v9 06/11] dma-buf: provide phys_vec to scatter-gather mapping routine

[Pranjal Shrivastava <praan@google.com>]

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