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Thu, 23 Apr 2026 12:14:59 +0000 Message-ID: Date: Thu, 23 Apr 2026 14:14:52 +0200 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net v3 8/8] xsk: don't support AF_XDP on 32-bit architectures To: Jason Xing , Maciej Fijalkowski CC: , , , , , , , , , , , , , , Jason Xing References: <20260422033650.68457-1-kerneljasonxing@gmail.com> <20260422033650.68457-9-kerneljasonxing@gmail.com> <2e0e0e18-cb75-4638-9a12-5906de6a8308@intel.com> Content-Language: en-US From: Alexander Lobakin In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-ClientProxiedBy: VI1PR06CA0124.eurprd06.prod.outlook.com (2603:10a6:803:a0::17) To DS0PR11MB8718.namprd11.prod.outlook.com (2603:10b6:8:1b9::20) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS0PR11MB8718:EE_|PH8PR11MB9878:EE_ X-MS-Office365-Filtering-Correlation-Id: 34c5f5ad-299a-4f0e-fc69-08dea131f05b X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|7416014|376014|56012099003|18002099003|22082099003; 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>>>>> >>>>> 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. >>>> >>>> What if we relax the restriction a bit? For example, refuse to configure >>> >>> As to the bug itself, yes, It only affects the unaligned mode. >>> >>> I wonder if we can support this after someone requires us to support >>> 32-bit arch and use it in the real world, then we can use the previous >>> patch to complete the full support (which doesn't harm the path on >>> 64-bit arch). >>> >>> The code looks like this based on your suggestion. Just for the record. >>> diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c >>> index 58da2f4f4397..03417b04592f 100644 >>> --- a/net/xdp/xdp_umem.c >>> +++ b/net/xdp/xdp_umem.c >>> @@ -177,6 +177,9 @@ static int xdp_umem_reg(struct xdp_umem *umem, >>> struct xdp_umem_reg *mr) >>> if (mr->flags & ~XDP_UMEM_FLAGS_VALID) >>> return -EINVAL; >>> + if (!IS_ENABLED(CONFIG_64BIT) && unaligned_chunks) >>> + return -EOPNOTSUPP; >>> + >>> if (!unaligned_chunks && !is_power_of_2(chunk_size)) >>> return -EINVAL; >>> >>> Actually I'm fine with either of them. Right now I'm not so sure which >>> direction this patch should take :) >> >> Abstracting from discussion, that is not a patch we would want against >> -net. We should not forbid 32bit on stable kernels, maybe there is someone >> in the 'basement' using 32bit xsk on stable kernel. Plus the fixes tag vs >> patch's content looks weird ;) > > Sorry, I don't follow this. Any specific suggestions on how to move forward? Maciej just says that if we go the first route (forbid 32-bit arches), then this commit should not go to the stable/LTS kernels. But > > Now we have three approaches w/o the fixes tag: > 1. completely forbid 32-bit arch > 2. only forbid unaligned mode on 32-bit arch > 3. completely fix/modify the allocation logic of the first frag on > 32-bit (see the v2-0008 patch) > > I'm wondering how to proceed with that? I've just reread the v2 thread (I realized I missed it earlier) and to me, the v2-0008 (route 3) solution looks good. It introduces a bit of overhead, but only for 32-bit arches, where the XSk performance will never be push-it-to-the-limit anyway (esp. given that this is skb mode). So I'd personally vote for number 3. 47 insertions(+), 7 deletions(-) is not a big deal. And for 64-bit arches, there won't be any object code changes at all (unconditional `return 0` and checking the return value of xsk_skb_init_misc() will be optimized out by the compiler). Number 2 is also okay-ish, if we're sure that addr is always 32-bit on 32-bit arches in the aligned mode. But given that you already has a proper fix (n. 3) which allows to not forbid anything at all, I believe it's more preferred to fix rather than disable. + as mentioned above, if you go route number 3, the series could be sent to -net and the fix would eventually hit the stable/LTS trees, but I don't think we can disable/forbid something in the stable kernels. > > Thanks, > Jason Thanks, Olek