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=-7.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=unavailable 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 8CA8AC43603 for ; Tue, 17 Dec 2019 17:09:38 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 0745A21582 for ; Tue, 17 Dec 2019 17:09:37 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="MCZA19g8" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0745A21582 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from lists.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 47cl4H5yt3zDqRp for ; Wed, 18 Dec 2019 04:09:35 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=kernel.org (client-ip=198.145.29.99; helo=mail.kernel.org; envelope-from=will@kernel.org; receiver=) Authentication-Results: lists.ozlabs.org; dmarc=pass (p=none dis=none) header.from=kernel.org Authentication-Results: lists.ozlabs.org; dkim=pass (1024-bit key; unprotected) header.d=kernel.org header.i=@kernel.org header.b="MCZA19g8"; dkim-atps=neutral Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 47cl1r0Sj2zDqQq for ; Wed, 18 Dec 2019 04:07:27 +1100 (AEDT) Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 10F5D21582; Tue, 17 Dec 2019 17:07:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1576602445; bh=9qLjCTJpYDZjDSgYG+Ze2rFSDFpRuJkwVOsNbCXu0yY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=MCZA19g8+N5u/xEbIwE1OkPJs7Q9S+e4Y81lid+WvA3+cRX4qe8OuIKisihkv0c/l 11VbnaWSg9gj0yX2tA2M/5poVsgWBQNWKvgdJIz/uubigq7HhGPmSVAoAbgku2i5Ye YBVeE6w6e3k1AR2O/IkXLuxvyZNJbHTKwRVO/jMc= Date: Tue, 17 Dec 2019 17:07:19 +0000 From: Will Deacon To: Linus Torvalds Subject: Re: READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops)) Message-ID: <20191217170719.GA869@willie-the-truck> References: <20191212100756.GA11317@willie-the-truck> <20191212104610.GW2827@hirez.programming.kicks-ass.net> <20191212180634.GA19020@willie-the-truck> <20191212193401.GB19020@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch , Arnd Bergmann , Peter Zijlstra , Linux Kernel Mailing List , Christian Borntraeger , Mark Rutland , linuxppc-dev@lists.ozlabs.org, dja@axtens.net Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Thu, Dec 12, 2019 at 12:49:52PM -0800, Linus Torvalds wrote: > On Thu, Dec 12, 2019 at 11:34 AM Will Deacon wrote: > > > > The root of my concern in all of this, and what started me looking at it in > > the first place, is the interaction with 'typeof()'. Inheriting 'volatile' > > for a pointer means that local variables in macros declared using typeof() > > suddenly start generating *hideous* code, particularly when pointless stack > > spills get stackprotector all excited. > > Yeah, removing volatile can be a bit annoying. > > For the particular case of the bitops, though, it's not an issue. > Since you know the type there, you can just cast it. > > And if we had the rule that READ_ONCE() was an arithmetic type, you could do > > typeof(0+(*p)) __var; > > since you might as well get the integer promotion anyway (on the > non-volatile result). > > But that doesn't work with structures or unions, of course. > > I'm not entirely sure we have READ_ONCE() with a struct. I do know we > have it with 64-bit entities on 32-bit machines, but that's ok with > the "0+" trick. Other than the two trivial examples Arnd and I spotted, it looks like we're in for some fun with the page-table types such as pud_t but that /should/ be fixable with enough effort. However, I'm really banging my head against the compiler trying to get your trick above to work for pointer types when the pointed-to-type is not defined. As a very cut down example (I pulled this back out of the preprocessor and cleaned it up a bit): struct dentry { struct inode *d_inode; }; static inline struct inode *d_inode_rcu(struct dentry *dentry) { return ({ typeof(0 + dentry->d_inode) __x = (*(volatile typeof(dentry->d_inode) *)&(dentry->d_inode)); (typeof(dentry->d_inode))__x; }); } Trying to compile this results in: | In function 'd_inode_rcu': | error: invalid use of undefined type 'struct inode' whereas it compiles fine if you remove the '0 +' from the first typeof. What am I missing? Perhaps the compiler wants the size information of 'struct inode' before it will contemplate the arithmetic, but if so then I don't think we can use this trick after all. Hmm. Will