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Tue, 21 Apr 2026 15:23:52 -0700 (PDT) Date: Tue, 21 Apr 2026 15:23:51 -0700 From: Stanislav Fomichev To: Jason Xing Cc: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, bjorn@kernel.org, magnus.karlsson@intel.com, maciej.fijalkowski@intel.com, jonathan.lemon@gmail.com, sdf@fomichev.me, ast@kernel.org, daniel@iogearbox.net, hawk@kernel.org, john.fastabend@gmail.com, bpf@vger.kernel.org, netdev@vger.kernel.org, Jason Xing Subject: Re: [PATCH net v2 8/8] xsk: fix u64 descriptor address truncation on 32-bit architectures Message-ID: References: <20260420082805.14844-1-kerneljasonxing@gmail.com> <20260420082805.14844-9-kerneljasonxing@gmail.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On 04/21, Jason Xing wrote: > On Tue, Apr 21, 2026 at 3:49 AM Stanislav Fomichev wrote: > > > > On 04/20, Jason Xing wrote: > > > From: Jason Xing > > > > > > In copy mode TX, xsk_skb_destructor_set_addr() stores the 64-bit > > > descriptor address into skb_shinfo(skb)->destructor_arg (void *) via a > > > uintptr_t cast: > > > > > > skb_shinfo(skb)->destructor_arg = (void *)((uintptr_t)addr | 0x1UL); > > > > > > On 32-bit architectures uintptr_t is 32 bits, so the upper 32 bits of > > > the descriptor address are silently dropped. In XDP_ZEROCOPY unaligned > > > mode the chunk offset is encoded in bits 48-63 of the descriptor > > > address (XSK_UNALIGNED_BUF_OFFSET_SHIFT = 48), meaning the offset is > > > lost entirely. The completion queue then returns a truncated address to > > > userspace, making buffer recycling impossible. > > > > > > Fix this by handling the 32-bit case directly in > > > xsk_skb_destructor_set_addr(): when !CONFIG_64BIT, allocate an xsk_addrs > > > struct (the same path already used for multi-descriptor SKBs) to store > > > the full u64 address. > > > > Is it easier to make XSK `depends on 64BIT` to avoid dealing with that? Does > > Of course, it would be super easy. Actually the initial version looks > like this. One line of coder is simply enough. > > > anybody seriously run af_xdp on 32 bit systems? > > But my worry as you guess is if there exists a 32 bit system? I doubt > it. That's why I put some effort into adding the compatibility code to > cover the case. Good news is that it doesn't add any side effects of > the 64 bit system since they are protected under IS_ENABLED condition. If someone complains, we can follow up? af_xdp is all u64 everywhere, including uapi, I doubt someone is using it on 32 bit systems. We don't test this part on 32 bit systems on nipa either..