From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752948AbdGLWfx (ORCPT <rfc822;w@1wt.eu>);
        Wed, 12 Jul 2017 18:35:53 -0400
Received: from mx1.redhat.com ([209.132.183.28]:34256 "EHLO mx1.redhat.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1751036AbdGLWfv (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 12 Jul 2017 18:35:51 -0400
DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com EEA51624BB
Authentication-Results: ext-mx10.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com
Authentication-Results: ext-mx10.extmail.prod.ext.phx2.redhat.com; spf=pass smtp.mailfrom=jpoimboe@redhat.com
DKIM-Filter: OpenDKIM Filter v2.11.0 mx1.redhat.com EEA51624BB
Date: Wed, 12 Jul 2017 17:35:47 -0500
From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Matthias Kaehlcke <mka@chromium.org>
Cc: Chris J Arges <chris.j.arges@canonical.com>,
        Borislav Petkov <bp@suse.de>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, "H . Peter Anvin" <hpa@zytor.com>,
        x86@kernel.org, linux-kernel@vger.kernel.org,
        Douglas Anderson <dianders@chromium.org>,
        Michael Davidson <md@google.com>, Greg Hackmann <ghackmann@google.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Stephen Hines <srhines@google.com>, Kees Cook <keescook@chromium.org>,
        Arnd Bergmann <arnd@arndb.de>, Bernhard.Rosenkranzer@linaro.org
Subject: Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in
 get_user() inline asm"
Message-ID: <20170712223547.fyra43dizqooosbs@treble>
References: <20170712212744.23660-1-mka@chromium.org>
 <20170712221242.puv5illztsla4nph@treble>
 <20170712222040.GD95735@google.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20170712222040.GD95735@google.com>
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 [10.5.110.39]); Wed, 12 Jul 2017 22:35:51 +0000 (UTC)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, Jul 12, 2017 at 03:20:40PM -0700, Matthias Kaehlcke wrote:
> > This is admittedly an awkward way of achieving this goal, but it's the
> > only way I know how to do it with GCC.
> > 
> > What extra instruction does clang add?
> 
> I was looking at the get_user() call in drm_mode_setcrtc(). The code
> generated by clang without the patch is:
> 
>                         if (get_user(out_id, &set_connectors_ptr[i])) {
> ffffffff81386955:       4a 8d 04 bd 00 00 00    lea    0x0(,%r15,4),%rax
> ffffffff8138695c:       00 
> ffffffff8138695d:       49 03 06                add    (%r14),%rax
> ffffffff81386960:       e8 2b a5 f0 ff          callq  ffffffff81290e90 <__get_user_4>
> 
> And with the patch:
> 
>                         if (get_user(out_id, &set_connectors_ptr[i])) {
> ffffffff81386a56:       4a 8d 04 bd 00 00 00    lea    0x0(,%r15,4),%rax
> ffffffff81386a5d:       00 
> ffffffff81386a5e:       49 03 06                add    (%r14),%rax
> ffffffff81386a61:       48 8b 64 24 28          mov    0x28(%rsp),%rsp
> ffffffff81386a66:       e8 15 a5 f0 ff          callq
> ffffffff81290f80 <__get_user_4>

Hm, that seems odd.  Can you sure the disassembly for the whole
function?

-- 
Josh