From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Cyrus-Session-Id: sloti22d1t05-2179907-1518719323-2-6276034439939934327 X-Sieve: CMU Sieve 3.0 X-Spam-known-sender: no ("Email failed DMARC policy for domain") X-Spam-score: 0.0 X-Spam-hits: BAYES_00 -1.9, HEADER_FROM_DIFFERENT_DOMAINS 0.001, ME_NOAUTH 0.01, RCVD_IN_DNSWL_HI -5, T_RP_MATCHES_RCVD -0.01, LANGUAGES en, BAYES_USED global, SA_VERSION 3.4.0 X-Spam-source: IP='209.132.180.67', Host='vger.kernel.org', Country='US', FromHeader='com', MailFrom='org' X-Spam-charsets: plain='utf-8' X-IgnoreVacation: yes ("Email failed DMARC policy for domain") X-Resolved-to: greg@kroah.com X-Delivered-to: greg@kroah.com X-Mail-from: stable-owner@vger.kernel.org ARC-Seal: i=1; a=rsa-sha256; cv=none; d=messagingengine.com; s=arctest; t=1518719323; b=gqrYFu/lQ+o3Z/9uNJwwppfAmByQLQWmAbkTPxFZQ6goWqD 1vmat9E7cN5BQfwBBO3kmqfxTe3ub7xoQ0I24QFZd3/xRQWIuTpC5AsW+J6dB1bT z80NskdrvNr2Q6wwIiNvausCLUY8MZ+FW/30nt+vZcgKgNutjIJHkV1LpACm4GTZ mNtmLtwvpABAsf7wCeVoEyXDTYVvOR9DnppE6YYip4UnqZRjWGszU4Heb9W29MZJ Wu8rajQUuLr1c1ofGhk2ILikJyfJYm7ofoMzmAyVVs8uYi9YaUtk07cd94DULGpd qXpkJRn5uvdJNPdPhqkS5AbsuzeXRwIYWoAZAmA== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d= messagingengine.com; h=date:from:to:cc:subject:message-id :references:mime-version:content-type:in-reply-to:sender :list-id; s=arctest; t=1518719323; bh=hXGja0F5cEOedIDdss7xyBKOrx 1HLtk8FV6iWiSgKFA=; b=cFjj4vqmLjlPMip6TONeLbO77f1H09nGnqow3yFdIp zuG+bw+yfqChIzvksHSnjVCQ0zJaaLTypQahHFS8o+ctLEbTo3zQ/NPL09t7Scic vXNIbTg7hbgy+XTkDgozwCCxi7VM4amdnC9MiKK39hlK6RmT6ZsrUP46LCkSl4UA hXQTExwDn+wSq4mOz+l2dqCDlL7UpLdE42rVrCf8F2ht3ZEvsXsHREzCASTZjXxy 2HrbeuCPEZ3vbQVDjgR0dtE0fKHTPw5QKWk0+WdkBrxVoWPGLmCH0dJoErR7KImT zWxEMQrL9Nlfr/mTrI56uETw+yFdr44juwebxkn0NNSg== ARC-Authentication-Results: i=1; mx5.messagingengine.com; arc=none (no signatures found); dkim=none (no signatures found); dmarc=fail (p=none,has-list-id=yes,d=none) header.from=redhat.com; iprev=pass policy.iprev=209.132.180.67 (vger.kernel.org); spf=none smtp.mailfrom=stable-owner@vger.kernel.org smtp.helo=vger.kernel.org; x-aligned-from=fail; x-ptr=pass x-ptr-helo=vger.kernel.org x-ptr-lookup=vger.kernel.org; x-return-mx=pass smtp.domain=vger.kernel.org smtp.result=pass smtp_org.domain=kernel.org smtp_org.result=pass smtp_is_org_domain=no header.domain=redhat.com header.result=pass header_is_org_domain=yes Authentication-Results: mx5.messagingengine.com; arc=none (no signatures found); dkim=none (no signatures found); dmarc=fail (p=none,has-list-id=yes,d=none) header.from=redhat.com; iprev=pass policy.iprev=209.132.180.67 (vger.kernel.org); spf=none smtp.mailfrom=stable-owner@vger.kernel.org smtp.helo=vger.kernel.org; x-aligned-from=fail; x-ptr=pass x-ptr-helo=vger.kernel.org x-ptr-lookup=vger.kernel.org; x-return-mx=pass smtp.domain=vger.kernel.org smtp.result=pass smtp_org.domain=kernel.org smtp_org.result=pass smtp_is_org_domain=no header.domain=redhat.com header.result=pass header_is_org_domain=yes Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1163178AbeBOPZ0 (ORCPT ); Thu, 15 Feb 2018 10:25:26 -0500 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:49024 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1162134AbeBOPZX (ORCPT ); Thu, 15 Feb 2018 10:25:23 -0500 Date: Thu, 15 Feb 2018 09:25:21 -0600 From: Josh Poimboeuf To: Arnd Bergmann Cc: Linux Kernel Mailing List , stable , Peter Zijlstra , Kees Cook Subject: Re: objtool warnings on 4.14-stable/gcc-7.3.0 Message-ID: <20180215152521.eoz6mhgw22ekyaev@treble> References: <20180214222412.arg77l6vjpkje6pj@treble> <20180214224525.esxiqt7yjjjpzyz4@treble> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.6.0.1 (2016-04-01) Sender: stable-owner@vger.kernel.org X-Mailing-List: stable@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: On Thu, Feb 15, 2018 at 04:01:57PM +0100, Arnd Bergmann wrote: > On Wed, Feb 14, 2018 at 11:45 PM, Josh Poimboeuf wrote: > > On Wed, Feb 14, 2018 at 04:24:12PM -0600, Josh Poimboeuf wrote: > >> On Wed, Feb 14, 2018 at 04:11:15PM +0100, Arnd Bergmann wrote: > >> > Hi Josh, > >> > > >> > I recently did some randconfig testing with a plain 4.14-stable kernel > >> > and gcc-7.3.0, and came across three distinct objtool warnings: > >> > > >> > drivers/misc/lkdtm_bugs.o: warning: objtool: > >> > lkdtm_CORRUPT_LIST_ADD()+0x15: return with modified stack frame > > > > While this is probably an objtool bug, the code is very odd: > > > > 00000000000001a8 : > > 1a8: e8 00 00 00 00 callq 1ad > > 1a9: R_X86_64_PC32 __fentry__-0x4 > > 1ad: 55 push %rbp > > 1ae: 48 89 e5 mov %rsp,%rbp > > 1b1: 48 83 e4 f0 and $0xfffffffffffffff0,%rsp > > 1b5: 48 83 ec 20 sub $0x20,%rsp > > 1b9: 48 89 ec mov %rbp,%rsp > > 1bc: 5d pop %rbp > > 1bd: c3 retq > > > > The function just allocates/aligns its stack space and then returns. It > > seems like GCC was too smart for its own good here, as the function > > doesn't test what it's supposed to. > > AFAIU, there is an optimization step in gcc that eliminates basic blocks > that contain an unconditional NULL pointer dereference, based on the > assumption that it's undefined behavior, and if we ever get here, it is > free to drop not only code after but also before it as long as it doesn't > have any side-effects. Ok, I expected something like that. GCC "undefined behavior" strikes again. Kees, I suppose you'll need to obfuscate the code to stay one step ahead of GCC. While this may be an objtool bug, I might not fix it because it served a useful purpose here in finding GCC crap. > I would have expected an actual NULL pointer dereference to remain > in the function though, or at least another trapping instruction. > > > Can you share the config for this one? > > https://pastebin.com/qFV6SPWP Would be interesting to analyze that config to understand what options are causing GCC to do that. I don't see this "optimization" with my config. -- Josh