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=-10.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 30704C43603 for ; Thu, 19 Dec 2019 12:11:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D4B7B227BF for ; Thu, 19 Dec 2019 12:11:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1576757483; bh=RE2T76IU/lncwRrDRAlG+ju/T9yNzhZZa+kkSWXB+hk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=mv0AsvcX3s/Dqw5nOrX1mqs2bdgD/VIlGp7pFrsPTfgwSVQWj4xGAuIzrbUg8DTTI zs2XIURNFJoltcuOVjDyzzPqiE2tOsfVzeCo4qVdyWM9rIYtSFiVP6Qec1r2Tv87ez 6cB9PvywXaQ/6vdDHeiEinaBZNDBHem6iXE6xh2g= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726828AbfLSMLW (ORCPT ); Thu, 19 Dec 2019 07:11:22 -0500 Received: from mail.kernel.org ([198.145.29.99]:39744 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726668AbfLSMLV (ORCPT ); Thu, 19 Dec 2019 07:11:21 -0500 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 1BEFF20716; Thu, 19 Dec 2019 12:11:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1576757481; bh=RE2T76IU/lncwRrDRAlG+ju/T9yNzhZZa+kkSWXB+hk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=0ixnxT3uqPi6FVe/deKUePFfnKx9P1IGOKn1P09zsr5cJF5tc+Lb/RinNJ6zRexCz d4w/YFMVv6noAYAo12vcpw9/xsADKJmW1JkRrwXaOXPOxbKCpRgXB/UF6b9ftaH2wF dIhF3//DAkyn3jnXauqEMxnnraV8I9v9EaclRVHs= Date: Thu, 19 Dec 2019 12:11:16 +0000 From: Will Deacon To: Linus Torvalds Cc: Peter Zijlstra , Michael Ellerman , Daniel Axtens , Linux Kernel Mailing List , linuxppc-dev , Christophe Leroy , linux-arch , Mark Rutland , Segher Boessenkool , Arnd Bergmann , Christian Borntraeger 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: <20191219121115.GB32361@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> <20191217170719.GA869@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) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Dec 17, 2019 at 10:32:35AM -0800, Linus Torvalds wrote: > On Tue, Dec 17, 2019 at 10:04 AM Linus Torvalds > wrote: > > > > Let me think about it. > > How about we just get rid of the union entirely, and just use > 'unsigned long' or 'unsigned long long' depending on the size. > > Something like the attached patch - it still requires that it be an > arithmetic type, but now because of the final cast. > > But it might still be a cast to a volatile type, of course. Then the > result will be volatile, but at least now READ_ONCE() won't be taking > the address of a volatile variable on the stack - does that at least > fix some of the horrible code generation. Hmm? Sounds like it according to mpe, but I'll confirm too for arm64. > This is untested, because I obviously still have the cases of > structures (page table entries) being accessed once.. > > Linus > include/linux/compiler.h | 33 +++++++++++++++++---------------- > 1 file changed, 17 insertions(+), 16 deletions(-) > > diff --git a/include/linux/compiler.h b/include/linux/compiler.h > index 5e88e7e33abe..8b4282194f16 100644 > --- a/include/linux/compiler.h > +++ b/include/linux/compiler.h > @@ -179,18 +179,18 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val, > > #include > > -#define __READ_ONCE_SIZE \ > -({ \ > - switch (size) { \ > - case 1: *(__u8 *)res = *(volatile __u8 *)p; break; \ > - case 2: *(__u16 *)res = *(volatile __u16 *)p; break; \ > - case 4: *(__u32 *)res = *(volatile __u32 *)p; break; \ > - case 8: *(__u64 *)res = *(volatile __u64 *)p; break; \ > - default: \ > - barrier(); \ > - __builtin_memcpy((void *)res, (const void *)p, size); \ > - barrier(); \ > - } \ > +/* "unsigned long" or "unsigned long long" - make it fit in a register if possible */ > +#define __READ_ONCE_TYPE(size) \ > + __typeof__(__builtin_choose_expr(size > sizeof(0UL), 0ULL, 0UL)) Ha, I wondered when '__builtin_choose_expr()' would make an appearance in this thread! Nice trick. I'll try integrating this with what I have and see what I run into next. Back down the rabbit hole... Will