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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,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 13549C433E0 for ; Mon, 6 Jul 2020 16:00:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E70A7206DF for ; Mon, 6 Jul 2020 16:00:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729429AbgGFQAa (ORCPT ); Mon, 6 Jul 2020 12:00:30 -0400 Received: from foss.arm.com ([217.140.110.172]:52050 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729297AbgGFQA3 (ORCPT ); Mon, 6 Jul 2020 12:00:29 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 659CB1FB; Mon, 6 Jul 2020 09:00:28 -0700 (PDT) Received: from arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A61A23F68F; Mon, 6 Jul 2020 09:00:25 -0700 (PDT) Date: Mon, 6 Jul 2020 17:00:23 +0100 From: Dave Martin To: Will Deacon Cc: Mark Rutland , "Michael S. Tsirkin" , Peter Zijlstra , Catalin Marinas , Jason Wang , virtualization@lists.linux-foundation.org, Arnd Bergmann , Alan Stern , Sami Tolvanen , Matt Turner , kernel-team@android.com, Marco Elver , Kees Cook , "Paul E. McKenney" , Boqun Feng , Josh Triplett , Ivan Kokshaysky , linux-arm-kernel@lists.infradead.org, Richard Henderson , Nick Desaulniers , linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org Subject: Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y Message-ID: <20200706160023.GB10992@arm.com> References: <20200630173734.14057-1-will@kernel.org> <20200630173734.14057-19-will@kernel.org> <20200701170722.4rte5ssnmrn2uqzg@bakewell.cambridge.arm.com> <20200702072301.GA15963@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200702072301.GA15963@willie-the-truck> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 02, 2020 at 08:23:02AM +0100, Will Deacon wrote: > On Wed, Jul 01, 2020 at 06:07:25PM +0100, Dave P Martin wrote: > > On Tue, Jun 30, 2020 at 06:37:34PM +0100, Will Deacon wrote: > > > When building with LTO, there is an increased risk of the compiler > > > converting an address dependency headed by a READ_ONCE() invocation > > > into a control dependency and consequently allowing for harmful > > > reordering by the CPU. > > > > > > Ensure that such transformations are harmless by overriding the generic > > > READ_ONCE() definition with one that provides acquire semantics when > > > building with LTO. > > > > > > Signed-off-by: Will Deacon > > > --- > > > arch/arm64/include/asm/rwonce.h | 63 +++++++++++++++++++++++++++++++ > > > arch/arm64/kernel/vdso/Makefile | 2 +- > > > arch/arm64/kernel/vdso32/Makefile | 2 +- > > > 3 files changed, 65 insertions(+), 2 deletions(-) > > > create mode 100644 arch/arm64/include/asm/rwonce.h > > > > > > diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h > > > new file mode 100644 > > > index 000000000000..515e360b01a1 > > > --- /dev/null > > > +++ b/arch/arm64/include/asm/rwonce.h > > > @@ -0,0 +1,63 @@ > > > +/* SPDX-License-Identifier: GPL-2.0 */ > > > +/* > > > + * Copyright (C) 2020 Google LLC. > > > + */ > > > +#ifndef __ASM_RWONCE_H > > > +#define __ASM_RWONCE_H > > > + > > > +#ifdef CONFIG_CLANG_LTO > > > > Don't we have a generic option for LTO that's not specific to Clang. > > /me looks at the LTO series some more > > Oh yeah, there's CONFIG_LTO which is selected by CONFIG_LTO_CLANG, which is > the non-typoed version of the above. I can switch this to CONFIG_LTO. > > > Also, can you illustrate code that can only be unsafe with Clang LTO? > > I don't have a concrete example, but it's an ongoing concern over on the LTO > thread [1], so I cooked this to show one way we could deal with it. The main > concern is that the whole-program optimisations enabled by LTO may allow the > compiler to enumerate possible values for a pointer at link time and replace > an address dependency between two loads with a control dependency instead, > defeating the dependency ordering within the CPU. Why can't that happen without LTO? > We likely won't realise if/when this goes wrong, other than impossible to > debug, subtle breakage that crops up seemingly randomly. Ideally, we'd be > able to detect this sort of thing happening at build time, and perhaps > even prevent it with compiler options or annotations, but none of that is > close to being available and I'm keen to progress the LTO patches in the > meantime because they are a requirement for CFI. My concern was not so much why LTO makes things dangerous, as why !LTO makes things safe... Cheers ---Dave From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Martin Subject: Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y Date: Mon, 6 Jul 2020 17:00:23 +0100 Message-ID: <20200706160023.GB10992@arm.com> References: <20200630173734.14057-1-will@kernel.org> <20200630173734.14057-19-will@kernel.org> <20200701170722.4rte5ssnmrn2uqzg@bakewell.cambridge.arm.com> <20200702072301.GA15963@willie-the-truck> Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <20200702072301.GA15963@willie-the-truck> Sender: linux-alpha-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Will Deacon Cc: Mark Rutland , "Michael S. Tsirkin" , Peter Zijlstra , Catalin Marinas , Jason Wang , virtualization@lists.linux-foundation.org, Arnd Bergmann , Alan Stern , Sami Tolvanen , Matt Turner , kernel-team@android.com, Marco Elver , Kees Cook , "Paul E. McKenney" , Boqun Feng , Josh Triplett , Ivan Kokshaysky , linux-arm-kernel@lists.infradead.org, Richard Henderson , Nick Desaulniers On Thu, Jul 02, 2020 at 08:23:02AM +0100, Will Deacon wrote: > On Wed, Jul 01, 2020 at 06:07:25PM +0100, Dave P Martin wrote: > > On Tue, Jun 30, 2020 at 06:37:34PM +0100, Will Deacon wrote: > > > When building with LTO, there is an increased risk of the compiler > > > converting an address dependency headed by a READ_ONCE() invocation > > > into a control dependency and consequently allowing for harmful > > > reordering by the CPU. > > > > > > Ensure that such transformations are harmless by overriding the generic > > > READ_ONCE() definition with one that provides acquire semantics when > > > building with LTO. > > > > > > Signed-off-by: Will Deacon > > > --- > > > arch/arm64/include/asm/rwonce.h | 63 +++++++++++++++++++++++++++++++ > > > arch/arm64/kernel/vdso/Makefile | 2 +- > > > arch/arm64/kernel/vdso32/Makefile | 2 +- > > > 3 files changed, 65 insertions(+), 2 deletions(-) > > > create mode 100644 arch/arm64/include/asm/rwonce.h > > > > > > diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h > > > new file mode 100644 > > > index 000000000000..515e360b01a1 > > > --- /dev/null > > > +++ b/arch/arm64/include/asm/rwonce.h > > > @@ -0,0 +1,63 @@ > > > +/* SPDX-License-Identifier: GPL-2.0 */ > > > +/* > > > + * Copyright (C) 2020 Google LLC. > > > + */ > > > +#ifndef __ASM_RWONCE_H > > > +#define __ASM_RWONCE_H > > > + > > > +#ifdef CONFIG_CLANG_LTO > > > > Don't we have a generic option for LTO that's not specific to Clang. > > /me looks at the LTO series some more > > Oh yeah, there's CONFIG_LTO which is selected by CONFIG_LTO_CLANG, which is > the non-typoed version of the above. I can switch this to CONFIG_LTO. > > > Also, can you illustrate code that can only be unsafe with Clang LTO? > > I don't have a concrete example, but it's an ongoing concern over on the LTO > thread [1], so I cooked this to show one way we could deal with it. The main > concern is that the whole-program optimisations enabled by LTO may allow the > compiler to enumerate possible values for a pointer at link time and replace > an address dependency between two loads with a control dependency instead, > defeating the dependency ordering within the CPU. Why can't that happen without LTO? > We likely won't realise if/when this goes wrong, other than impossible to > debug, subtle breakage that crops up seemingly randomly. Ideally, we'd be > able to detect this sort of thing happening at build time, and perhaps > even prevent it with compiler options or annotations, but none of that is > close to being available and I'm keen to progress the LTO patches in the > meantime because they are a requirement for CFI. My concern was not so much why LTO makes things dangerous, as why !LTO makes things safe... Cheers ---Dave 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=-8.3 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,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 1E590C433DF for ; Mon, 6 Jul 2020 16:01:57 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (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 D9AE7206DF for ; Mon, 6 Jul 2020 16:01:56 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="wQ3KEU5Y" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D9AE7206DF Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Transfer-Encoding: Content-Type:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References:Message-ID: Subject:To:From:Date:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=FB7MIUw1FN3JAn9NGriSswA5a9hisT2SgN68EXEhMK4=; b=wQ3KEU5Yt7d0a72RpgIzrtwCS dNQxDvxTff1u11jDY01g1H5HHcoIbWH7aTUir28dPmLAYI2V6hIvfGvZz7Fe1s1FU6oVT0hCBm5aO LRGIwOvxKWG19bCuFGUn4ZwWXdDEThFQxU8++gImLFvBoHGmmgd9T6PMdr38YorrHWVzv8J10xr8I SdyPfPgxGizreEDYxE5tarAZmo7y+2yUH0Nef13Qf4bh9ltHfWgRrcqvfxT5R7t81yXwz1xIcqF9Z k9rjaJSJ8JwppYqoMwOou06ElzSux9pz/Iep2fZTjfwW57YGIoBnfAf5W/WCJIj59ieL6vllyjShs 24eI+m6AQ==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jsTXk-0005QV-GB; Mon, 06 Jul 2020 16:00:32 +0000 Received: from foss.arm.com ([217.140.110.172]) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jsTXh-0005PI-B2 for linux-arm-kernel@lists.infradead.org; Mon, 06 Jul 2020 16:00:30 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 659CB1FB; Mon, 6 Jul 2020 09:00:28 -0700 (PDT) Received: from arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A61A23F68F; Mon, 6 Jul 2020 09:00:25 -0700 (PDT) Date: Mon, 6 Jul 2020 17:00:23 +0100 From: Dave Martin To: Will Deacon Subject: Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y Message-ID: <20200706160023.GB10992@arm.com> References: <20200630173734.14057-1-will@kernel.org> <20200630173734.14057-19-will@kernel.org> <20200701170722.4rte5ssnmrn2uqzg@bakewell.cambridge.arm.com> <20200702072301.GA15963@willie-the-truck> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20200702072301.GA15963@willie-the-truck> User-Agent: Mutt/1.5.23 (2014-03-12) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200706_120029_501106_9DA4D4D5 X-CRM114-Status: GOOD ( 32.01 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Mark Rutland , "Michael S. Tsirkin" , Peter Zijlstra , Catalin Marinas , Jason Wang , virtualization@lists.linux-foundation.org, Arnd Bergmann , Alan Stern , Sami Tolvanen , Matt Turner , kernel-team@android.com, Marco Elver , Kees Cook , "Paul E. McKenney" , Boqun Feng , Josh Triplett , Ivan Kokshaysky , linux-arm-kernel@lists.infradead.org, Richard Henderson , Nick Desaulniers , linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org 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 Thu, Jul 02, 2020 at 08:23:02AM +0100, Will Deacon wrote: > On Wed, Jul 01, 2020 at 06:07:25PM +0100, Dave P Martin wrote: > > On Tue, Jun 30, 2020 at 06:37:34PM +0100, Will Deacon wrote: > > > When building with LTO, there is an increased risk of the compiler > > > converting an address dependency headed by a READ_ONCE() invocation > > > into a control dependency and consequently allowing for harmful > > > reordering by the CPU. > > > > > > Ensure that such transformations are harmless by overriding the generic > > > READ_ONCE() definition with one that provides acquire semantics when > > > building with LTO. > > > > > > Signed-off-by: Will Deacon > > > --- > > > arch/arm64/include/asm/rwonce.h | 63 +++++++++++++++++++++++++++++++ > > > arch/arm64/kernel/vdso/Makefile | 2 +- > > > arch/arm64/kernel/vdso32/Makefile | 2 +- > > > 3 files changed, 65 insertions(+), 2 deletions(-) > > > create mode 100644 arch/arm64/include/asm/rwonce.h > > > > > > diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h > > > new file mode 100644 > > > index 000000000000..515e360b01a1 > > > --- /dev/null > > > +++ b/arch/arm64/include/asm/rwonce.h > > > @@ -0,0 +1,63 @@ > > > +/* SPDX-License-Identifier: GPL-2.0 */ > > > +/* > > > + * Copyright (C) 2020 Google LLC. > > > + */ > > > +#ifndef __ASM_RWONCE_H > > > +#define __ASM_RWONCE_H > > > + > > > +#ifdef CONFIG_CLANG_LTO > > > > Don't we have a generic option for LTO that's not specific to Clang. > > /me looks at the LTO series some more > > Oh yeah, there's CONFIG_LTO which is selected by CONFIG_LTO_CLANG, which is > the non-typoed version of the above. I can switch this to CONFIG_LTO. > > > Also, can you illustrate code that can only be unsafe with Clang LTO? > > I don't have a concrete example, but it's an ongoing concern over on the LTO > thread [1], so I cooked this to show one way we could deal with it. The main > concern is that the whole-program optimisations enabled by LTO may allow the > compiler to enumerate possible values for a pointer at link time and replace > an address dependency between two loads with a control dependency instead, > defeating the dependency ordering within the CPU. Why can't that happen without LTO? > We likely won't realise if/when this goes wrong, other than impossible to > debug, subtle breakage that crops up seemingly randomly. Ideally, we'd be > able to detect this sort of thing happening at build time, and perhaps > even prevent it with compiler options or annotations, but none of that is > close to being available and I'm keen to progress the LTO patches in the > meantime because they are a requirement for CFI. My concern was not so much why LTO makes things dangerous, as why !LTO makes things safe... Cheers ---Dave _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel