From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C7B95C11F66 for ; Tue, 13 Jul 2021 11:23:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B2465611C0 for ; Tue, 13 Jul 2021 11:23:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235891AbhGMLZz (ORCPT ); Tue, 13 Jul 2021 07:25:55 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58100 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235709AbhGMLZx (ORCPT ); Tue, 13 Jul 2021 07:25:53 -0400 Received: from pandora.armlinux.org.uk (pandora.armlinux.org.uk [IPv6:2001:4d48:ad52:32c8:5054:ff:fe00:142]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0E493C0613DD; Tue, 13 Jul 2021 04:23:03 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=armlinux.org.uk; s=pandora-2019; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=yGhDFnZM0A65W5PQXub87bUt7hi0BbiF5BkXDcA/Zr4=; b=ffLTrSEP1oIazt9MQBn1/8er4 3sgMpXXzNFjn9Tdwdueb36QlruzqqCcSURPnHeqedRvfFPWhkTHYdYjjG4fvWr24TDV3ZFZnD7Vqb 6zUhqHqoYcrwDJGYnSZGJEfEIPtXjGRiYqf6qn+ULprCyZCttHZ0+87T4GbudVKnv/URIkSiVuXsP qCm2q6xIAIof9Wc45z860/XHYcV2wK1xBl3lBTz8N6rV2i90D+fjEcaAOJkmffMvNQ2AZIyLtgsfp EvE/2PWJx5eau5kMIjcVs9L5SlE0lWfY4I6n9gTkwU5GVX+8uXaNBX3kL3L5xuitIuy8Q5zXnHOYz fWMql0T3Q==; 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-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <012ccfea2a564274bd9d2e1cfc130873@AcuMS.aculab.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: Russell King (Oracle) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.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!