From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754575AbbHMXA7 (ORCPT ); Thu, 13 Aug 2015 19:00:59 -0400 Received: from mail-ob0-f180.google.com ([209.85.214.180]:35153 "EHLO mail-ob0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754021AbbHMXA6 convert rfc822-to-8bit (ORCPT ); Thu, 13 Aug 2015 19:00:58 -0400 MIME-Version: 1.0 In-Reply-To: <55CD1F79.2010508@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> From: Andy Lutomirski Date: Thu, 13 Aug 2015 16:00:38 -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 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. --Andy