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

[Jason Gunthorpe <jgg@nvidia.com>]

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:
> > 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;

Yeah, and i should not be signed.

> > > 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