From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754201AbdKGRzn (ORCPT ); Tue, 7 Nov 2017 12:55:43 -0500 Received: from mx1.redhat.com ([209.132.183.28]:51478 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753892AbdKGRzl (ORCPT ); Tue, 7 Nov 2017 12:55:41 -0500 DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com 668797E380 Authentication-Results: ext-mx04.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com Authentication-Results: ext-mx04.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=jpoimboe@redhat.com Date: Tue, 7 Nov 2017 11:55:39 -0600 From: Josh Poimboeuf To: Mikulas Patocka Cc: Andy Lutomirski , Borislav Petkov , Brian Gerst , Denys Vlasenko , "H. Peter Anvin" , Jiri Slaby , Peter Zijlstra , Thomas Gleixner , live-patching@vger.kernel.org, linux-kernel@vger.kernel.org, Ingo Molnar Subject: Re: [PATCH] objtool: fix build of 64-bit kernel with 32-bit userspace Message-ID: <20171107175539.zkbzaapmok2b7hbb@treble> References: 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) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.28]); Tue, 07 Nov 2017 17:55:41 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org [ Adding Ingo to cc because I believe it was his suggestion to hide the guess unwinder behind CONFIG_EXPERT. ] On Mon, Nov 06, 2017 at 06:27:53PM -0500, Mikulas Patocka wrote: > This patch fixes building of 64-bit kernel on 32-bit userspace (I tested > it on RHEL-6-i686 and Debian-Sid-x32). Thanks, I'll review the patch. > There are still some more bugs - when building 64-bit kernel on 32-bit > Debian 7 distribution, objtool corrupts object files so that the linker > doesn't recognize them (ld doesn't recognize the format of the file at all > and gold prints a lot of errors). Hm, we should figure out what's going on there... > Another problem - when building 32-bit kernel, why do you force frame > pointers on and why did you hide CONFIG_GUESS_UNWINDER behind > CONFIG_EXPERT? Frame pointers increase code size and register pressure, > most users don't need precise stacktraces, so CONFIG_GUESS_UNWINDER should > be default for non-expert users just like it was before. > > Is there some technical reason why do you want to avoid > CONFIG_GUESS_UNWINDER? The technical reason for avoiding the guess unwinder is that it's sketchy: it gives false positive results. Not only for oopses, but for all the other users of the unwinder: /proc//stack, perf, lockdep, etc. So it's a correctness issue. I agree with you that the frame pointer unwinder has drawbacks, but if somebody cares about those drawbacks, I would consider that person an "expert" ;-) -- Josh