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 S1754570AbbHNAuX (ORCPT <rfc822;w@1wt.eu>);
	Thu, 13 Aug 2015 20:50:23 -0400
Received: from smtp35.i.mail.ru ([94.100.177.95]:46485 "EHLO smtp35.i.mail.ru"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1752135AbbHNAuW (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Thu, 13 Aug 2015 20:50:22 -0400
Subject: Re: [regression] x86/signal/64: Fix SS handling for signals delivered
 to 64-bit programs breaks dosemu
To: Linus Torvalds <torvalds@linux-foundation.org>
References: <55CA90B4.2010205@list.ru> <55CCD921.4040301@list.ru>
 <CA+55aFycGSrN27gP4kh6eLZdkSxFvVqS8cZn1YJairD3+z3Zpg@mail.gmail.com>
 <CALCETrUDYw3oobs27QNraCQo8z3eR-3xoBmQE-fQkGvh=Ax0sA@mail.gmail.com>
 <CA+55aFz_KF5p0LcxroCVJ5zPrUmthR6X0Q1YWKMepDYKn=J-sA@mail.gmail.com>
 <20150813200823.GS2059@uranus>
 <CA+55aFyYNgPoh0VxqcGUxB-_z7oK+zB18YhJQQrozdb_gLAp5g@mail.gmail.com>
 <55CD0F29.4070604@gmail.com>
 <CA+55aFyqb29r+1YP2iYStFOij9X0QDWqjgmee=JcTkB-q8iEdA@mail.gmail.com>
 <55CD13F3.1070904@list.ru>
 <CALCETrVX-Op1U-RED9bkb3R9v7jcu5O1ur1Yt08ppZ0VFyZx7A@mail.gmail.com>
 <55CD1968.7070002@list.ru>
 <CALCETrUJS9kgNgggbnYJrLr93ARtocZBAGDVuudWahM7JhQuwQ@mail.gmail.com>
 <55CD1F79.2010508@list.ru>
 <CALCETrWf=rRZLJ=RWS_s2NDN5GNZT87DKTgt7eS81ncrXxi5hA@mail.gmail.com>
 <55CD2F9A.9040300@list.ru>
 <CALCETrV4Xbcw9JtFgoR6BOz=TTnj7KEuzOJeLcELur6yJT7E9Q@mail.gmail.com>
 <55CD3382.5070506@list.ru>
 <CA+55aFxRbtQd372SKQYRKfkpZ+BSqj6EwdB96n+SqUaxJ-LXuA@mail.gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
        Raymond Jennings <shentino@gmail.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Pavel Emelyanov <xemul@parallels.com>,
        Linux kernel <linux-kernel@vger.kernel.org>
From: Stas Sergeev <stsp@list.ru>
Message-ID: <55CD3B3F.9040203@list.ru>
Date: Fri, 14 Aug 2015 03:50:07 +0300
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101
 Thunderbird/38.1.0
MIME-Version: 1.0
In-Reply-To: <CA+55aFxRbtQd372SKQYRKfkpZ+BSqj6EwdB96n+SqUaxJ-LXuA@mail.gmail.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 8bit
X-Mras: Ok
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

14.08.2015 03:27, Linus Torvalds пишет:
> On Thu, Aug 13, 2015 at 5:17 PM, Stas Sergeev <stsp@list.ru> wrote:
>> For example because you can as well do:
>> prctl(ARCH_SET_SIGNAL_SS, 0)
>> which will mean "restore ss in sighandler to its current value",
> I really think a prctl() is the wrong thing to do.
>
> If you want a signal handler to save/restore segments, I think it
> should be a SA_xyz flag to sigaction() (the way we have SA_RESTART
Yes, I was proposing the new sigaction() flag in this thread
already too. But at the end, prctl() looks better to me because
it allows to pass the TLS value to use when restoring FS.
The thing is that I am trying to find the similar treatment for
both the SS and FS problems. If you don't think they need a
similar treatment, then perhaps the Andy's patch is enough.

> etc).  And off by default because of the obvious compatibility issues.
Of course.

So, what we have right now (in the latest Andy's patch) is:
1. lar heuristics
2. new uc_flags flag

What it solves: dosemu's regression.

What prctl() can give:
- fix to dosemu's regression
- fix to the TLS problem in the future
- no hack and heuristics

With SA_xyz you can only solve the SS problem, so it is
probably not any better than the uc_flags things coded
up by Andy.