From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753498AbbHMQYg (ORCPT ); Thu, 13 Aug 2015 12:24:36 -0400 Received: from mail-ob0-f169.google.com ([209.85.214.169]:33359 "EHLO mail-ob0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752706AbbHMQYf convert rfc822-to-8bit (ORCPT ); Thu, 13 Aug 2015 12:24:35 -0400 MIME-Version: 1.0 In-Reply-To: <55CCC3E1.9060603@list.ru> References: <55CBA4CE.1040108@list.ru> <55CBA909.3020306@list.ru> <55CBB053.7050803@list.ru> <55CBB2CC.9090600@list.ru> <55CBBFB9.1080201@list.ru> <20150813083949.GA17091@gmail.com> <55CC911D.3080607@list.ru> <55CCB625.3000900@list.ru> <55CCBFDC.5000207@list.ru> <55CCC3E1.9060603@list.ru> From: Andy Lutomirski Date: Thu, 13 Aug 2015 09:24:14 -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: Ingo Molnar , X86 ML , Linux kernel , Linus Torvalds , "H. Peter Anvin" , Thomas Gleixner , Brian Gerst , Borislav Petkov , Stas Sergeev 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 9:20 AM, Stas Sergeev wrote: > 13.08.2015 19:09, Andy Lutomirski пишет: > >> On Thu, Aug 13, 2015 at 9:03 AM, Stas Sergeev wrote: >>> >>> 13.08.2015 18:38, Andy Lutomirski пишет: >>>> >>>> >>>> So... what do we do about it? We could revert the whole mess. We >>>> could tell everyone to fix their DOSEMU, which violates policy and is >>>> especially annoying given how much effort we've put into keeping >>>> 16-bit mode fully functional lately. We could add yet more heuristics >>>> and teach sigreturn to ignore the saved SS value in sigcontext if the >>>> saved CS is 64-bit and the saved SS is unusable. >>> >>> Andy, why do you constantly ignore the proposal to make >>> new behaviour explicitly controlable? You don't have to agree >>> with it, but you could at least comment on that possibility >>> and/or mention it with the ones you listed above. >> >> I'm not sure what the proposal is exactly. >> >> We could add a new uc_flags flag. If set, it means that >> sigcontext->ss is valid and should be used by sigreturn. If clear, >> then we ignore sigcontext->ss and just restore __USER_DS. >> >> The problem is that, by itself, this won't fix old DOSEMU. We somehow >> need to either detect that something funny is going on or just leave >> the flag clear by default. >> >> We could do this: always save SS to sigcontext->ss, but only restore >> sigcontext->ss if userspace explicitly sets the flag before sigreturn. >> If we do that, we'd need to also add my patch to preserve the actual >> HW SS selector if possible so that old DOSEMU knows what SS to program >> into its trampoline. >> >> This at least lets *new* DOSEMU set the flag and get the improved >> behavior. I still don't know what effect it'll have on Wine and CRIU. >> >> Stas, is that what you were thinking, or were you thinking of something >> else? > > Not quite. > I mean the flag that will control not only sigreturn, but > the signal delivery as well. This may probably be a sigaction() > flag or some other. If not set - ss is ignored by both signal > delivery and sigreturn(). If set - ss is saved/restored (and in > the future - also fs/gs). > Is such a flag possible? Maybe. I think I'm more nervous about adding new flags in sigaction than I am in uc_flags. --Andy