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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 57D09C433EF for ; Wed, 1 Dec 2021 18:35:11 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id EC3796B0081; Wed, 1 Dec 2021 13:29:14 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id E4F296B0082; Wed, 1 Dec 2021 13:29:14 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id CC6776B0083; Wed, 1 Dec 2021 13:29:14 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0001.hostedemail.com [216.40.44.1]) by kanga.kvack.org (Postfix) with ESMTP id BA0B06B0081 for ; Wed, 1 Dec 2021 13:29:14 -0500 (EST) Received: from smtpin15.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 80D9182499B9 for ; Wed, 1 Dec 2021 18:29:04 +0000 (UTC) X-FDA: 78870062208.15.6232BAB Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf26.hostedemail.com (Postfix) with ESMTP id 94B3420019C4 for ; Wed, 1 Dec 2021 18:29:02 +0000 (UTC) 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 4AAE01477; Wed, 1 Dec 2021 10:29:03 -0800 (PST) Received: from FVFF77S0Q05N (unknown [10.57.65.205]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 6AFB73F766; Wed, 1 Dec 2021 10:29:00 -0800 (PST) Date: Wed, 1 Dec 2021 18:28:57 +0000 From: Mark Rutland To: Marco Elver Cc: Andrew Morton , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H. Peter Anvin" , linux-kernel@vger.kernel.org, linux-mm@kvack.org, kasan-dev@googlegroups.com, Peter Zijlstra , Nick Desaulniers , Nathan Chancellor , Dmitry Vyukov , Andrey Konovalov , Catalin Marinas , Will Deacon , linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH] kcov: fix generic Kconfig dependencies if ARCH_WANTS_NO_INSTR Message-ID: References: <20211201152604.3984495-1-elver@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 94B3420019C4 X-Stat-Signature: nw8511nmnooznmjof58qttn5btqymxog Authentication-Results: imf26.hostedemail.com; dkim=none; spf=pass (imf26.hostedemail.com: domain of mark.rutland@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=mark.rutland@arm.com; dmarc=pass (policy=none) header.from=arm.com X-HE-Tag: 1638383342-402389 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Dec 01, 2021 at 07:16:25PM +0100, Marco Elver wrote: > On Wed, 1 Dec 2021 at 18:46, Mark Rutland wrote: > [...] > > > > Currently we mostly get away with disabling KCOV for while compilation units, > > > > so maybe it's worth waiting for the GCC 12.0 release, and restricting things > > > > once that's out? > > > > > > An alternative would be to express 'select ARCH_WANTS_NO_INSTR' more > > > precisely, say with an override or something. Because as-is, > > > ARCH_WANTS_NO_INSTR then doesn't quite reflect reality on arm64 > > > (yet?). > > > > It's more of a pragmatic thing -- ARCH_WANTS_NO_INSTR does reflect reality, and > > we do *want* to enforce that strictly, it's just that we're just struck between > > a rock and a hard place where until GCC 12 is released we either: > > > > a) Strictly enforce noinstr, and be sure there aren't any bugs from unexpected > > instrumentation, but we can't test GCC-built kernels under Syzkaller due to > > the lack of KCOV. > > > > b) Don't strictly enforce noinstr, and have the same latent bugs as today (of > > unknown severity), but we can test GCC-built kernels under Syzkaller. > > > > ... and since this (currently only affects KCOV, which people only practically > > enable for Syzkaller, I think it's ok to wait until GCC 12 is out, so that we > > can have the benefit of Sykaller in the mean time, and subsequrntly got for > > option (a) and say those people need to use GCC 12+ (and clang 13+). > > > > > But it does look simpler to wait, so I'm fine with that. I leave it to you. > > > > FWIW, for my purposes I'm happy to take this immediately and to have to apply a > > local patch to my fuzzing branches until GCC 12 is out, but I assume we'd want > > the upstream testing to work in the mean time without requiring additional > > patches. > > Agree, it's not an ideal situation. :-/ > > syzkaller would still work, just not as efficiently. Not sure what's > worse, less efficient fuzzing, or chance of random crashes. In fact, > on syzbot we already had to disable it: > https://github.com/google/syzkaller/blob/61f862782082c777ba335aa4b4b08d4f74d7d86e/dashboard/config/linux/bits/base.yml#L110 > https://lore.kernel.org/linux-arm-kernel/20210119130010.GA2338@C02TD0UTHF1T.local/T/#m78fdfcc41ae831f91c93ad5dabe63f7ccfb482f0 > > So if we ran into issues with KCOV on syzbot for arm64, I'm sure it's > not just us. I can't quite see what the reasons for the crashes are, > but ruling out noinstr vs. KCOV would be a first step. > > So I'm inclined to suggest we take this patch now and not wait for GCC > 12, given we're already crashing with KCOV and therefore have KCOV > disabled on arm64 syzbot. > > I'm still fine waiting, but just wanted to point out you can fuzz > without KCOV. Preferences? If it's not used by Syzbot, that's good enough for me -- I can apply local hacks to run with KCOV if I want to in the mean time, and I can debug my own mess if I have to. So FWIW, for taking that now: Acked-by: Mark Rutland Thanks, Mark.