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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 447CDC2D0DB for ; Wed, 22 Jan 2020 14:47:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 193612465B for ; Wed, 22 Jan 2020 14:47:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1579704469; bh=iGhKZwakJs42H/3vv6PL/lPNZYOMCVLzVyr9jWnb1Ws=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=giENbveBWKFbZrVJe3Ujw/mAPVqh+XsdmaWJlqBWU38Dg69KahPV33yPJANTdinVx sH/tN3cYP7AgpnMtKFJ9T1GQGD6wd1nSPSsSJAC3RPXNRQQmr1bTscc2P0BM+2y9PI ntetBD8cRSiacfCiIWTzuayQUCYJ8fD33Xb0B+8Q= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727893AbgAVOrs (ORCPT ); Wed, 22 Jan 2020 09:47:48 -0500 Received: from mail.kernel.org ([198.145.29.99]:33900 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725802AbgAVOrs (ORCPT ); Wed, 22 Jan 2020 09:47:48 -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 AB3D921835; Wed, 22 Jan 2020 14:47:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1579704467; bh=iGhKZwakJs42H/3vv6PL/lPNZYOMCVLzVyr9jWnb1Ws=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=gQ0JHEXzVtt1qhRCk7weZHxDeTz1iI4eishO7/If3CCQJJcwBErXGhMrn5kGcUASH 0jw9DGc+GKyc6OcKhnHxpQVW28nXOl28Fl7GbJaCuk6jlvcXD1mVkFgyUOlOxCgyBc sNJd8qXtB8jihGAArNSchUHslZq1fMViU8NxU3Qc= Date: Wed, 22 Jan 2020 14:47:42 +0000 From: Will Deacon To: Josh Poimboeuf Cc: Nick Desaulniers , Julien Thierry , Nathan Chancellor , LKML , Linux ARM , Peter Zijlstra , raphael.gault@arm.com, Catalin Marinas , clang-built-linux Subject: Re: [RFC v5 00/57] objtool: Add support for arm64 Message-ID: <20200122144741.GA16622@willie-the-truck> References: <20200109160300.26150-1-jthierry@redhat.com> <20200112084258.GA44004@ubuntu-x2-xlarge-x86> <20200121103101.GE11154@willie-the-truck> <20200121180632.GA13592@willie-the-truck> <20200121183009.253yh6aehvnvxoew@treble> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200121183009.253yh6aehvnvxoew@treble> 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, Jan 21, 2020 at 12:30:09PM -0600, Josh Poimboeuf wrote: > On Tue, Jan 21, 2020 at 06:06:34PM +0000, Will Deacon wrote: > > On Tue, Jan 21, 2020 at 09:08:29AM -0800, Nick Desaulniers wrote: > > > On Tue, Jan 21, 2020 at 2:31 AM Will Deacon wrote: > > > > > > > > On Mon, Jan 13, 2020 at 07:57:48AM +0000, Julien Thierry wrote: > > > > > On 1/12/20 8:42 AM, Nathan Chancellor wrote: > > > > > > The 0day bot reported a couple of issues with clang with this series; > > > > > > the full report is available here (clang reports are only sent to our > > > > > > mailing lists for manual triage for the time being): > > > > > > > > > > > > https://groups.google.com/d/msg/clang-built-linux/MJbl_xPxawg/mWjgDgZgBwAJ > > > > > > > > > > > > > > > > Thanks, I'll have a look at those. > > > > > > > > > > > The first obvious issue is that this series appears to depend on a GCC > > > > > > plugin? I'll be quite honest, objtool and everything it does is rather > > > > > > over my head but I see this warning during configuration (allyesconfig): > > > > > > > > > > > > WARNING: unmet direct dependencies detected for GCC_PLUGIN_SWITCH_TABLES > > > > > > Depends on [n]: GCC_PLUGINS [=n] && ARM64 [=y] > > > > > > Selected by [y]: > > > > > > - ARM64 [=y] && STACK_VALIDATION [=y] > > > > > > > > > > > > Followed by the actual error: > > > > > > > > > > > > error: unable to load plugin > > > > > > './scripts/gcc-plugins/arm64_switch_table_detection_plugin.so': > > > > > > './scripts/gcc-plugins/arm64_switch_table_detection_plugin.so: cannot > > > > > > open shared object file: No such file or directory' > > > > > > > > > > > > If this plugin is absolutely necessary and can't be implemented in > > > > > > another way so that clang can be used, seems like STACK_VALIDATION > > > > > > should only be selected on ARM64 when CONFIG_CC_IS_GCC is not zero. > > > > > > > > > > > > > > > > So currently the plugin is necessary for proper validation. One option can > > > > > be to just let objtool output false positives on files containing jump > > > > > tables when the plugin cannot be used. But overall I guess it makes more > > > > > sense to disable stack validation for non-gcc builds, for now. > > > > > > > > Alternatively, could we add '-fno-jump-tables' to the KBUILD_CFLAGS if > > > > STACK_VALIDATION is selected but we're not using GCC? Is that sufficient > > > > to prevent generation of these things? > > > > > > Surely we wouldn't want to replace jump tables with long chains of > > > comparisons just because objtool couldn't validate jump tables without > > > a GCC plugin for aarch64 for some reason, right? objtool validation > > > is valuable, but tying runtime performance to a GCC plugin used for > > > validation seems bad. > > > > I'm only suggesting it if STACK_VALIDATION is selected. It's off by default, > > and lives in Kconfig.debug. I'd prefer that to "cross your fingers are do > > nothing differently", which is what the other option seems to be. > > I don't know what the right answer is here, but keep in mind that > objtool is on by default for x86, so don't be surprised if that > eventually happens to arch64 too. > > Short term it might be ok to disable jump tables with objtool enabled, > or to disable objtool when clang is in use, but long term we'll need to > figure out a better solution. Oh, absolutely. No objection from me fixing this properly in the long term. I just don't want to be in a situation where STACK_VALIDATION is silently ignored in the meantime. Will