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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4620b3ed0d2sm18936916f8f.22.2026.06.17.14.30.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 17 Jun 2026 14:30:57 -0700 (PDT) Date: Wed, 17 Jun 2026 22:30:56 +0100 From: David Laight To: Johannes Berg Cc: Andy Shevchenko , linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org, linux-wireless@vger.kernel.org, Kees Cook , "Gustavo A. R. Silva" Subject: Re: [PATCH v1 1/2] overflow: Allow to sum a few arguments at once Message-ID: <20260617223056.754bfcb8@pumpkin> In-Reply-To: <1e656f5798a9f2f36daa00aba60d2196b2456335.camel@sipsolutions.net> References: <20260617112250.2791461-1-andriy.shevchenko@linux.intel.com> <20260617112250.2791461-2-andriy.shevchenko@linux.intel.com> <1e656f5798a9f2f36daa00aba60d2196b2456335.camel@sipsolutions.net> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-wireless@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 Wed, 17 Jun 2026 14:56:09 +0200 Johannes Berg wrote: > On Wed, 2026-06-17 at 13:12 +0200, Andy Shevchenko wrote: > > Convert size_add() to take variadic argument, so we can simplify users > > with using a macro only once. > > > +#define __size_add3(addend1, addend2, addend3, addend4, ...) \ > > + __size_add(__size_add2(addend1, addend2, addend3), addend4) > > +#define __size_add4(addend1, addend2, addend3, addend4, addend5, ...) \ > > + __size_add(__size_add3(addend1, addend2, addend3, addend4), addend5) > > I guess it's not going to really matter, but it would generate fewer > calls to have something more like > > #define __size_add3(a1, a2, a3, a4) \ > size_add(size_add(a1, a2), size_add(a3, a4)) > #define __size_add4(a1, a2, a3, a4, a5) \ > size_add(size_add(a1, a2), size_add(a3, a4, a5)) > > as a binary tree, rather than only cutting one off every time. Not sure > that results in hugely different code though - maybe fewer overflow > checks? The binary tree stands a chance of executing less slowly because the leaf adds can be executed in parallel. Excluding the saturation checks (wtf is it called size_add() not saturating_add() ?) (a + b) + (c + d) will usually execute faster than ((a + b) + c) + d because the (a + b) and (c + d) can execute at the same time; unfortunately gcc will always generate the latter. David > > Although your version make it really completely equivalent to the > nl80211.c code, clearly it doesn't matter if all the values are "good", > and I believe the overflow behaviour means it doesn't matter for the > overflow case either? > > johannes >