From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754097AbcAVQmz (ORCPT ); Fri, 22 Jan 2016 11:42:55 -0500 Received: from mx2.parallels.com ([199.115.105.18]:48390 "EHLO mx2.parallels.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752311AbcAVQmy (ORCPT ); Fri, 22 Jan 2016 11:42:54 -0500 Subject: Re: UBSAN: run-time undefined behavior sanity checker To: Dave Jones , Linux Kernel Mailing List References: <20160121205717.AF61F661293@gitolite.kernel.org> <20160122051539.GA1326@codemonkey.org.uk> From: Andrey Ryabinin Message-ID: <56A25C4B.2000204@virtuozzo.com> Date: Fri, 22 Jan 2016 19:43:55 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.0 MIME-Version: 1.0 In-Reply-To: <20160122051539.GA1326@codemonkey.org.uk> Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: US-EXCH.sw.swsoft.com (10.255.249.47) To US-EXCH.sw.swsoft.com (10.255.249.47) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/22/2016 08:15 AM, Dave Jones wrote: > On Thu, Jan 21, 2016 at 08:57:17PM +0000, Linux Kernel wrote: > > Web: https://git.kernel.org/torvalds/c/c6d308534aef6c99904bf5862066360ae067abc4 > > Commit: c6d308534aef6c99904bf5862066360ae067abc4 > > Parent: 68920c973254c5b71a684645c5f6f82d6732c5d6 > > Refname: refs/heads/master > > Author: Andrey Ryabinin > > AuthorDate: Wed Jan 20 15:00:55 2016 -0800 > > Committer: Linus Torvalds > > CommitDate: Wed Jan 20 17:09:18 2016 -0800 > > > > UBSAN: run-time undefined behavior sanity checker > > > > UBSAN uses compile-time instrumentation to catch undefined behavior > > (UB). Compiler inserts code that perform certain kinds of checks before > > operations that could cause UB. If check fails (i.e. UB detected) > > __ubsan_handle_* function called to print error message. > > > > So the most of the work is done by compiler. This patch just implements > > ubsan handlers printing errors. > > > > GCC has this capability since 4.9.x [1] (see -fsanitize=undefined > > option and its suboptions). > > However GCC 5.x has more checkers implemented [2]. > > Article [3] has a bit more details about UBSAN in the GCC. > > If I enable this and CONFIG_UBSAN_ALIGNMENT, the kernel doesn't boot, > and hangs really early (pretty much as soon as I hit return in grub) > far too early for serial console or even tty output. > > Compiler is debian unstable's 5.3.1 20160114 > > I don't know if this is worth chasing down, I chose to just disable it, > but figured I'd post in case other people stumble across the same issue. > Likely caused by unaligned access in very early code, which ends up in too early printk() call. You could try to disable instrumentation (UBSAN_SANITIZE := n) in early code. Be aware that CONFIG_UBSAN_ALIGNMENT causes a *lot* of spam in dmesg. Since x86 supports unaligned accesses, the significant amount of that spam just a false-positive reports.