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[34.142.210.164]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-29b5b105e4csm194769895ad.2.2025.11.26.05.12.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 26 Nov 2025 05:12:49 -0800 (PST) Date: Wed, 26 Nov 2025 13:12:40 +0000 From: Pranjal Shrivastava To: Alex Mastro Cc: Leon Romanovsky , Bjorn Helgaas , Logan Gunthorpe , Jens Axboe , Robin Murphy , Joerg Roedel , Will Deacon , Marek Szyprowski , Jason Gunthorpe , Andrew Morton , Jonathan Corbet , Sumit Semwal , Christian =?iso-8859-1?Q?K=F6nig?= , Kees Cook , "Gustavo A. R. Silva" , Ankit Agrawal , Yishai Hadas , Shameer Kolothum , Kevin Tian , Alex Williamson , Krishnakant Jaju , Matt Ochs , linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, iommu@lists.linux.dev, linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, kvm@vger.kernel.org, linux-hardening@vger.kernel.org, Nicolin Chen , Jason Gunthorpe Subject: Re: [PATCH v9 06/11] dma-buf: provide phys_vec to scatter-gather mapping routine Message-ID: References: <20251120-dmabuf-vfio-v9-0-d7f71607f371@nvidia.com> <20251120-dmabuf-vfio-v9-6-d7f71607f371@nvidia.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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