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=-10.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,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 12439C43603 for ; Thu, 12 Dec 2019 10:46:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D8E50206DA for ; Thu, 12 Dec 2019 10:46:27 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="Orp9wN4g" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728822AbfLLKq0 (ORCPT ); Thu, 12 Dec 2019 05:46:26 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:47884 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728302AbfLLKq0 (ORCPT ); Thu, 12 Dec 2019 05:46:26 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender: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=m0DNBgYu8LCe79BqvAy4WOPe50stwFHhuZ+SzerBWbI=; b=Orp9wN4g59rbDfKqcXXswzjw6 OSBOx+4SN3Dw5rH0t0Jt5LIasJxe5ysb9t9GMNq2bmA/OOZgUOVOOiTPkEyYbsjBORUVI/qpaoPXi EkilUHTRS9V385BTjMbA8Mc+pU+4eJeP9WKold3fPmg0XjZIkH/n7/ulLYgzbdoeJq09oeh9hOViL t0KwxNyViNw2Kocxumatd+rAPPip0KrOPfX215ZVQ68wXzy97ww3VIjQ/96C3Mu9kn+xg4gBqEKBZ 070DG34knK2bZxfHqQUHGtnUeZiIces++8YEX5+0LR4ano1iAsoVcw1wPLWVk5XekTsGmkOBL4l7m PMZKxpIpQ==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1ifLz4-00070y-RD; Thu, 12 Dec 2019 10:46:15 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 2B74F305DD6; Thu, 12 Dec 2019 11:44:50 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 7793320121961; Thu, 12 Dec 2019 11:46:10 +0100 (CET) Date: Thu, 12 Dec 2019 11:46:10 +0100 From: Peter Zijlstra To: Will Deacon Cc: Michael Ellerman , Linus Torvalds , dja@axtens.net, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, christophe.leroy@c-s.fr, linux-arch@vger.kernel.org, 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: <20191212104610.GW2827@hirez.programming.kicks-ass.net> References: <87blslei5o.fsf@mpe.ellerman.id.au> <20191206131650.GM2827@hirez.programming.kicks-ass.net> <875zimp0ay.fsf@mpe.ellerman.id.au> <20191212080105.GV2844@hirez.programming.kicks-ass.net> <20191212100756.GA11317@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191212100756.GA11317@willie-the-truck> 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 Thu, Dec 12, 2019 at 10:07:56AM +0000, Will Deacon wrote: > > So your proposed change _should_ be fine. Will, I'm assuming you never > > saw this on your ARGH64 builds when you did this code ? > > I did see it, but (a) looking at the code out-of-line makes it look a lot > worse than it actually is (so the ext4 example is really helpful -- thanks > Michael!) and (b) I chalked it up to a crappy compiler. > > However, see this comment from Arnd on my READ_ONCE series from the other > day: > > https://lore.kernel.org/lkml/CAK8P3a0f=WvSQSBQ4t0FmEkcFE_mC3oARxaeTviTSkSa-D2qhg@mail.gmail.com > > In which case, I'm thinking that we should be doing better in READ_ONCE() > for non-buggy compilers which would also keep the KCSAN folks happy for this > code (and would help with [1] too). So something like this then? Although I suppose that should be moved into compiler-gcc.h and then guarded by #ifndef READ_ONCE or so. --- diff --git a/include/linux/compiler.h b/include/linux/compiler.h index ad8c76144a3c..8326e2cf28b4 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -179,20 +179,8 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val, #include #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(); \ - } \ -}) +#include +#include #ifdef CONFIG_KASAN /* @@ -222,6 +210,22 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val, #define __no_sanitize_or_inline __always_inline #endif +#ifdef GCC_VERSION < 40800 + +#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(); \ + } \ +}) + static __no_kcsan_or_inline void __read_once_size(const volatile void *p, void *res, int size) { @@ -274,9 +278,6 @@ void __write_once_size(volatile void *p, void *res, int size) * with an explicit memory barrier or atomic instruction that provides the * required ordering. */ -#include -#include - #define __READ_ONCE(x, check) \ ({ \ union { typeof(x) __val; char __c[1]; } __u; \ @@ -295,6 +296,23 @@ void __write_once_size(volatile void *p, void *res, int size) */ #define READ_ONCE_NOCHECK(x) __READ_ONCE(x, 0) +#else /* GCC_VERSION < 40800 */ + +#define READ_ONCE_NOCHECK(x) \ +({ \ + typeof(x) __x = *(volatile typeof(x))&(x); \ + smp_read_barrier_depends(); \ + __x; +}) + +#define READ_ONCE(x) \ +({ \ + kcsan_check_atomic_read(&(x), sizeof(x)); \ + READ_ONCE_NOCHECK(x); \ +}) + +#endif /* GCC_VERSION < 40800 */ + static __no_kasan_or_inline unsigned long read_word_at_a_time(const void *addr) {