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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, 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 942C4C7618B for ; Wed, 24 Jul 2019 14:05:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7731322CD0 for ; Wed, 24 Jul 2019 14:05:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728185AbfGXOFm (ORCPT ); Wed, 24 Jul 2019 10:05:42 -0400 Received: from mx1.redhat.com ([209.132.183.28]:37983 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726276AbfGXOFm (ORCPT ); Wed, 24 Jul 2019 10:05:42 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 9B1A0308FBA6; Wed, 24 Jul 2019 14:05:41 +0000 (UTC) Received: from treble (ovpn-122-90.rdu2.redhat.com [10.10.122.90]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 6FE6B600C4; Wed, 24 Jul 2019 14:05:40 +0000 (UTC) Date: Wed, 24 Jul 2019 09:05:33 -0500 From: Josh Poimboeuf To: Peter Zijlstra Cc: Thomas Gleixner , LKML , Nick Desaulniers , Nathan Chancellor , clang-built-linux , x86@kernel.org, Arnd Bergmann , Sedat Dilek , Linus Torvalds Subject: Re: x86 - clang / objtool status Message-ID: <20190724140533.yxwbq4mlqzrviaf5@treble> References: <20190724023946.yxsz5im22fz4zxrn@treble> <20190724074732.GJ3402@hirez.programming.kicks-ass.net> <20190724125525.kgybu3nnpvwlcz2c@treble> <20190724133516.GB31381@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190724133516.GB31381@hirez.programming.kicks-ass.net> User-Agent: NeoMutt/20180716 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.43]); Wed, 24 Jul 2019 14:05:41 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 24, 2019 at 03:35:16PM +0200, Peter Zijlstra wrote: > On Wed, Jul 24, 2019 at 07:55:25AM -0500, Josh Poimboeuf wrote: > > On Wed, Jul 24, 2019 at 09:47:32AM +0200, Peter Zijlstra wrote: > > > On Tue, Jul 23, 2019 at 09:43:24PM -0500, Josh Poimboeuf wrote: > > > > On Thu, Jul 18, 2019 at 10:40:09PM +0200, Thomas Gleixner wrote: > > > > > > > > > drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool: .altinstr_replacement+0x86: redundant UACCESS disable > > > > > > > > Looking at this one, I think I agree with objtool. > > > > > > > > PeterZ, Linus, I know y'all discussed this code a few months ago. > > > > > > > > __copy_from_user() already does a CLAC in its error path. So isn't the > > > > user_access_end() redundant for the __copy_from_user() error path? > > > > > > Hmm, is this a result of your c705cecc8431 ("objtool: Track original function across branches") ? > > > > > > I'm thinking it might've 'overlooked' the CLAC in the error path before > > > (because it didn't have a related function) and now it sees it and > > > worries about it. > > > > > > Then again, I'm not seeing this warning on my GCC builds; so what's > > > happening? > > > > According to the github issue[1] my patch doesn't fix the warning with > > Clang. So questions remain: > > I was thinking your patch resulted in the warning due to the exception > code gaining a ->func. I had the same thought. > But then that doesn't make sense either, because all that lives in > copy_user_64.S which is a completely different translation unit. Hm? __copy_from_user() uses raw_copy_from_user() to do the STAC/CLAC in a header file for the __builtin_constant_p() case. > > a) what is objtool actually warning about? > > CLAC with AC already clear. Either we do double CLAC at the end, or we > do CLAC without having done STAC first. > > The issue isn't BAD(tm), as AC clear is the safe state, but it typically > indicates confused code flow. But as I said my patch didn't fix the Clang warning. Or is there another redundant UACCESS disable you know about? > > b) why doesn't objtool detect the case I found? > > With GCC you mean? Yes, that is really really weird. With both compilers... > Let me go stare at objdump output for this file (which doesn't build > with: > > make O=defconfig-build/ drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o > ) -- Josh