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 S1753753Ab1JJLTO (ORCPT <rfc822;w@1wt.eu>);
	Mon, 10 Oct 2011 07:19:14 -0400
Received: from mail-vw0-f46.google.com ([209.85.212.46]:50001 "EHLO
	mail-vw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1753623Ab1JJLTN convert rfc822-to-8bit (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Mon, 10 Oct 2011 07:19:13 -0400
MIME-Version: 1.0
In-Reply-To: <CAObL_7HUhxsOVHYFovgxYu7r5YNju+n4Q45GZ9viCCW-GrxWhg@mail.gmail.com>
References: <20111003090846.GA25136@localhost.pp.htv.fi>
	<CAObL_7GSQFr_PRtKY=H7r=yD7Qk-xvgbg5bV1=nU-qayG-mJdw@mail.gmail.com>
	<20111003173359.GA3072@localhost.pp.htv.fi>
	<CAObL_7Fi3mw5Azyhxx-vsdNuhJm8RewmYS97QvWa0F=cbYvegg@mail.gmail.com>
	<CAFLxGvzpvJ1mgdcbrq+3h5EA+VOhZDR9BY27uWGgsmV4TWYb4A@mail.gmail.com>
	<20111005223055.GG14406@localhost.pp.htv.fi>
	<CAObL_7GJ_WztLEgBhPhJGhDOJmPqiPXcHPqS=1_Fa=S9_PmpNw@mail.gmail.com>
	<CAObL_7Hhdfneba2C_4Bg+M=4LCmebFL0BvZKfw0P6xO=JQukcg@mail.gmail.com>
	<CAObL_7HL1M00D7XDCtMgO1kDSh64N7LvwB0DVd7c8Afi-Ki-Gw@mail.gmail.com>
	<CAFLxGvx0Cpu0ePytdCSSzfO2or3ZL3j5QrghOE5eWHZCJt6C6g@mail.gmail.com>
	<CAObL_7FJy=BFmtuTd+CE_uF2+ra6LnQWGUp=g+1py_V9ukYvJg@mail.gmail.com>
	<CA+55aFyskjs9qPkoFom16veQXy5a-beWF95A7mCFexqanRiiKQ@mail.gmail.com>
	<CAObL_7HUhxsOVHYFovgxYu7r5YNju+n4Q45GZ9viCCW-GrxWhg@mail.gmail.com>
Date: Mon, 10 Oct 2011 13:19:12 +0200
Message-ID: <CAFLxGvx0QMrvG_ZsFE6C_xHn9JpWJ-aW-V=K9pm3OoXZc=NKxQ@mail.gmail.com>
Subject: Re: [3.1 patch] x86: default to vsyscall=native
From: richard -rw- weinberger <richard.weinberger@gmail.com>
To: Andrew Lutomirski <luto@mit.edu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Adrian Bunk <bunk@stusta.de>, "H. Peter Anvin" <hpa@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        x86@kernel.org, linux-kernel@vger.kernel.org
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 8BIT
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, Oct 7, 2011 at 2:48 AM, Andrew Lutomirski <luto@mit.edu> wrote:
> On Thu, Oct 6, 2011 at 11:34 AM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>> On Thu, Oct 6, 2011 at 11:16 AM, Andrew Lutomirski <luto@mit.edu> wrote:
>>>
>>> Fixing it will be annoying because the attached fancier version needs
>>> to work, too.  I could implement the whole mess in software, but it
>>> might be nicer to arrange for uaccess errors to stash some information
>>> somewhere (like in the thread_struct cr2 variable).
>>
>> That should be easy enough to do. Just add it to the
>> "fixup_exception()" case in no_context().
>
> This code is rather messy.  We stash the cr2, err, and trap fields of
> sigcontext in thread_struct and we *never* reset them until the next
> segfault.  So userspace sees stale garbage on every signal that isn't
> a (genuine) segfault.  I can imagine this breaking UML is remarkably
> bizarre ways even without vsyscall emulation because UML actually
> seems to rely on that stuff to determine the source of a signal.
>

>>From UML's point of view the current situation is odd.
UML will no longer run on top of a default 3.1 kernel.

Why is this odd?
One of the major reasons why people are still using UML is because you
can run it as non-privileged user on any x86 Linux host.
An user which has root privileges can setup and use KVM which is much
nicer than UML...

-- 
Thanks,
//richard