linux-kernel - Re: [PATCH v9 06/11] dma-buf: provide phys

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Message-ID: <aScl+LCPN2TiN7Pd@devgpu015.cco6.facebook.com>
Date: Wed, 26 Nov 2025 08:08:24 -0800
From: Alex Mastro <amastro@...com>
To: Pranjal Shrivastava <praan@...gle.com>
CC: Leon Romanovsky <leon@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
        Logan Gunthorpe <logang@...tatee.com>, Jens Axboe <axboe@...nel.dk>,
        Robin
 Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>,
        Will Deacon
	<will@...nel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Jason
 Gunthorpe <jgg@...pe.ca>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan
 Corbet <corbet@....net>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian
 König <christian.koenig@....com>,
        Kees Cook
	<kees@...nel.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Ankit
 Agrawal <ankita@...dia.com>, Yishai Hadas <yishaih@...dia.com>,
        Shameer
 Kolothum <skolothumtho@...dia.com>,
        Kevin Tian <kevin.tian@...el.com>, Alex
 Williamson <alex@...zbot.org>,
        Krishnakant Jaju <kjaju@...dia.com>, Matt Ochs
	<mochs@...dia.com>,
        <linux-pci@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-block@...r.kernel.org>, <iommu@...ts.linux.dev>,
        <linux-mm@...ck.org>, <linux-doc@...r.kernel.org>,
        <linux-media@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
        <linaro-mm-sig@...ts.linaro.org>, <kvm@...r.kernel.org>,
        <linux-hardening@...r.kernel.org>, Nicolin Chen <nicolinc@...dia.com>,
        Jason
 Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH v9 06/11] dma-buf: provide phys_vec to scatter-gather
 mapping routine

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;
>  		/*
>  		 * 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