From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754613AbbHNAGM (ORCPT ); Thu, 13 Aug 2015 20:06:12 -0400 Received: from mail-ob0-f171.google.com ([209.85.214.171]:34612 "EHLO mail-ob0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753344AbbHNAGK convert rfc822-to-8bit (ORCPT ); Thu, 13 Aug 2015 20:06:10 -0400 MIME-Version: 1.0 In-Reply-To: <55CD2F9A.9040300@list.ru> References: <55CA90B4.2010205@list.ru> <55CCD921.4040301@list.ru> <20150813200823.GS2059@uranus> <55CD0F29.4070604@gmail.com> <55CD13F3.1070904@list.ru> <55CD1968.7070002@list.ru> <55CD1F79.2010508@list.ru> <55CD2F9A.9040300@list.ru> From: Andy Lutomirski Date: Thu, 13 Aug 2015 17:05:49 -0700 Message-ID: Subject: Re: [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu To: Stas Sergeev Cc: Linus Torvalds , Raymond Jennings , Cyrill Gorcunov , Pavel Emelyanov , Linux kernel Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 13, 2015 at 5:00 PM, Stas Sergeev wrote: > 14.08.2015 02:00, Andy Lutomirski пишет: > >> On Thu, Aug 13, 2015 at 3:51 PM, Stas Sergeev wrote: >>> >>> 14.08.2015 01:29, Andy Lutomirski пишет: >>>> >>>> On Thu, Aug 13, 2015 at 3:25 PM, Stas Sergeev wrote: >>>>> >>>>> 14.08.2015 01:11, Andy Lutomirski пишет: >>>>> >>>>>> Now suppose you set some magic flag and jump (via sigreturn, >>>>>> trampoline, whatever) into DOS code. The DOS code loads 0x7 into FS >>>>>> and then gets #GP. You land in a signal handler. As far as the >>>>>> kernel's concerned, the FS base register is whatever the base of LDT >>>>>> entry 0 is. What else is the kernel supposed to shove in there? >>>>> >>>>> The same as what happens when you do in userspace: >>>>> --- >>>>> asm ("mov $0,%%fs\n"); >>>>> prctl(ARCH_SET_FS, my_tls_base); >>>>> --- >>>>> >>>>> This was the trick I did before gcc started to use FS in prolog, >>>>> now I have to do this in asm. >>>>> But how simpler for the kernel is to do the same? >>>>> >>>>>> I think that making this work fully in the kernel would require a >>>>>> full-blown FS equivalent of sigaltstack, and that seems like overkill. >>>>> >>>>> Setting selector and base is what you call an "equivalent of >>>>> sigaltstack"? >>>> >>>> Yes. sigaltstack says "hey, kernel! here's my SP for signal >>>> handling." I think we'd need something similar to tell the kernel >>>> what my_tls_base is. Using the most recent thing passed to >>>> ARCH_SET_FS is no good because WRFSBASE systems might not use >>>> ARCH_SET_FS, and we can't break DOSEMU on Ivy Bridge and newer as soon >>>> as we enable WRFSBASE. >>> >>> If someone uses WRFSBASE and wants things to be preserved >>> in a sighandler, he'll just not set the aforementioned flag. No >>> regression. >>> Whoever wants to use that flag properly, will not use WRFSBASE, >>> and will use ARCH_SET_FS or set_thread_area(). >>> What exactly breakage do you have in mind? >> >> DOSEMU, when you set that flag, WRFSBASE gets enabled, and glibc's >> threading library starts using WRFSBASE instead of arch_prctl. > > Hmm, how about the following: > > prctl(ARCH_SET_SIGNAL_FS, my_tls) > If my_tls==NULL - use current fsbase (including one of WRFSBASE). > If my_tls==(void)-1 - don't restore. > > Can this work? Certainly, but why? ISTM user code should do this itself with a little bit of asm unless there's a good reason it wouldn't work. A good reason it wouldn't work is that high-performance applications need this and an extra syscall is too slow, but IMO that would need evidence. --Andy -- Andy Lutomirski AMA Capital Management, LLC