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Received: from shell.armlinux.org.uk ([fd8f:7570:feb6:1:5054:ff:fe00:4ec]:46046) by pandora.armlinux.org.uk with esmtpsa (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1m3GV5-00060D-8l; Tue, 13 Jul 2021 12:22:55 +0100 Received: from linux by shell.armlinux.org.uk with local (Exim 4.92) (envelope-from ) id 1m3GV3-0000Ew-8A; Tue, 13 Jul 2021 12:22:53 +0100 Date: Tue, 13 Jul 2021 12:22:53 +0100 From: "Russell King (Oracle)" To: David Laight Cc: Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= , Salah Triki , "fabrice.gasnier@foss.st.com" , "thierry.reding@gmail.com" , "lee.jones@linaro.org" , "mcoquelin.stm32@gmail.com" , "alexandre.torgue@foss.st.com" , "linux-pwm@vger.kernel.org" , "linux-stm32@st-md-mailman.stormreply.com" , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH] divide by 3*sizeof(u32) when computing array_size Message-ID: <20210713112253.GH22278@shell.armlinux.org.uk> References: <20210712231910.GA1831270@pc> <20210713063053.qqttzxlopvpnadj3@pengutronix.de> <20210713091954.GG22278@shell.armlinux.org.uk> <012ccfea2a564274bd9d2e1cfc130873@AcuMS.aculab.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <012ccfea2a564274bd9d2e1cfc130873@AcuMS.aculab.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210713_042302_943802_81F34680 X-CRM114-Status: GOOD ( 19.40 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Jul 13, 2021 at 11:07:00AM +0000, David Laight wrote: > From: Russell King > > Sent: 13 July 2021 10:20 > .... > > I would also note that the code relies on there being no padding in > > struct stm32_breakinput - it should be noted that a strict > > interpretation of the C standard allows padding to be added anywhere > > to a structure - at the start, end or between members. > > I'm pretty certain I remember that padding before the first member > isn't allowed. You may be right there. > In any case the kernel generally assumes there is no extra padding. > (eg for structures that map hardware registers.) That's incorrect. Places where we care either generally end up with __packed or are carefully layed out to ensure members are naturally aligned to reduce the likelyhood of it. 32-bit OABI ARM has been particularly "fun" in this respect. > For big structures it is worth adding a compile-time check of > the structure size - but not really for three u32. Sorry, structure size has absolutely nothing to do with whether it's a good idea to have a compile-time check. The deciding factor is whether the code relies on some property such as it being a certain size. Such as in this exact case. If you grep for "BUILD_BUG_ON.*sizeof" in fs/ for example, this illustrates the point rather well. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last! _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel