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 S1751318AbbCTMEZ (ORCPT <rfc822;w@1wt.eu>);
	Fri, 20 Mar 2015 08:04:25 -0400
Received: from mail-lb0-f176.google.com ([209.85.217.176]:35584 "EHLO
	mail-lb0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751004AbbCTMEV (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 20 Mar 2015 08:04:21 -0400
Date: Fri, 20 Mar 2015 15:04:19 +0300
From: Cyrill Gorcunov <gorcunov@gmail.com>
To: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
        Pavel Emelyanov <xemul@parallels.com>, Oleg Nesterov <oleg@redhat.com>,
        Andrey Wagin <avagin@gmail.com>, Andy Lutomirski <luto@kernel.org>,
        Ingo Molnar <mingo@kernel.org>, Andi Kleen <andi@firstfloor.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Al Viro <viro@zeniv.linux.org.uk>,
        X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Borislav Petkov <bp@alien8.de>, Pavel Emelyanov <xemul@openvz.org>
Subject: Re: [PATCH v3 1/2] x86_64,signal: Fix SS handling for signals
 delivered to 64-bit programs
Message-ID: <20150320120419.GS27066@moon>
References: <20150318183133.GB1832@redhat.com>
 <20150318185027.GB17491@moon>
 <CANaxB-ySx5R6-9yBGSEpr9iO+FDZDuiP_PNMYJihHAUDiVAoAg@mail.gmail.com>
 <20150318200247.GA6355@redhat.com>
 <CALCETrXzGb+5sBNhoD+mC7mYGBiAmtzj=jMo7R=gdsThuPHPUg@mail.gmail.com>
 <5509EF82.60900@parallels.com>
 <CALCETrWWU-izMJ688HP8_V6OLfN5V8+W0E-RGhRR=yBOUaYecQ@mail.gmail.com>
 <20150319073512.GA27066@moon>
 <CAK1hOcPXcjuqsLkb1x2qJcj5MEoc9beC2tzR_GpuGwL1wkgAAg@mail.gmail.com>
 <20150320115622.GR27066@moon>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20150320115622.GR27066@moon>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, Mar 20, 2015 at 02:56:22PM +0300, Cyrill Gorcunov wrote:
> > >
> > > fwiw currently we're passing zero in this __pad0 (replying to your
> > > previous email, so we can workaround in the kernel assuming zero
> > > as a special case, not that good but better than nothing).
> > 
> > Special-casing zero sounds not that bad to me.
> > It can be removed after a few years - just don't forget
> > to document it in a good comment: why we have special
> > case? What software required it?
> > In which version of that software the need to have this hack
> > was eliminated?
> 
> To be fair, such special case would be ideal for us, so that
> if noone object against such hack, i would cook a patch.

Denys, note though that Andy pointed a downside for such approach as well:

 | The only real down side I can see to special casing zero is that it
 | really is possible to end up with zero in there.  For example, the
 | SIGSEGV you get do to the failed sigreturn probably has sigcontext->ss
 | == 0 :)

which I don't know how to resolve.