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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-42cb7fd9b45sm16124281f8f.43.2025.11.22.03.31.40 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 22 Nov 2025 03:31:41 -0800 (PST) Date: Sat, 22 Nov 2025 11:31:39 +0000 From: David Laight To: Jens Axboe Cc: linux-kernel@vger.kernel.org, io-uring@vger.kernel.org Subject: Re: [PATCH 04/44] io_uring/net: Change some dubious min_t() Message-ID: <20251122113139.2462035d@pumpkin> In-Reply-To: <312fe285-3915-4108-bc49-3357977d644d@kernel.dk> References: <20251119224140.8616-1-david.laight.linux@gmail.com> <20251119224140.8616-5-david.laight.linux@gmail.com> <3202c47d-532d-4c74-aff9-992ec1d9cbeb@kernel.dk> <20251120154817.0160eeac@pumpkin> <312fe285-3915-4108-bc49-3357977d644d@kernel.dk> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Thu, 20 Nov 2025 08:53:56 -0700 Jens Axboe wrote: > On 11/20/25 8:48 AM, David Laight wrote: > > On Thu, 20 Nov 2025 07:48:58 -0700 > > Jens Axboe wrote: > > > >> On 11/19/25 3:41 PM, david.laight.linux@gmail.com wrote: > >>> From: David Laight > >>> > >>> Since iov_len is 'unsigned long' it is possible that the cast > >>> to 'int' will change the value of min_t(int, iov[nbufs].iov_len, ret). > >>> Use a plain min() and change the loop bottom to while (ret > 0) so that > >>> the compiler knows 'ret' is always positive. > >>> > >>> Also change min_t(int, sel->val, sr->mshot_total_len) to a simple min() > >>> since sel->val is also long and subject to possible trunctation. > >>> > >>> It might be that other checks stop these being problems, but they are > >>> picked up by some compile-time tests for min_t() truncating values. > >> > >> Fails with clang-21: > >> > >> io_uring/net.c:855:26: error: call to '__compiletime_assert_2006' declared with 'error' attribute: min(sel->val, sr->mshot_total_len) signedness error > >> 855 | sr->mshot_total_len -= min(sel->val, sr->mshot_total_len); > > > > I'll take a look, I normally use gcc but there must be something > > subtle going on. Actually which architecture? I only tested x86-64. > > This is x86-64. > It is related to the test a few lines higher: if (sel->val > 0 && sr->flags & IORING_RECV_MSHOT_LIM) { 'sel->val' is ssize_t, gcc is tracking that test so statically_true(sel->val >= 0) is 'true' and the signed variable can be compared against the 'unsigned' 'sr->mshot_total_len'. It seems clang isn't as clever. Perhaps it reloads sel->val from memory - so loses the result of the comparison. Even swapping the order of the two tests might fix it. As might caching sel->val in a local (even a signed one). The comment in the header file for io_br_sel.val doesn't seem to include the case where 'val' is a length! The simple fix is to use umin() since all the values are clearly non-negative. David