From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753537AbbHMQnN (ORCPT ); Thu, 13 Aug 2015 12:43:13 -0400 Received: from mail-ob0-f177.google.com ([209.85.214.177]:34347 "EHLO mail-ob0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753517AbbHMQnJ convert rfc822-to-8bit (ORCPT ); Thu, 13 Aug 2015 12:43:09 -0400 MIME-Version: 1.0 In-Reply-To: <55CCC812.5010101@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> <55CCC812.5010101@list.ru> From: Andy Lutomirski Date: Thu, 13 Aug 2015 09:42:49 -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: 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 9:38 AM, Stas Sergeev wrote: > 13.08.2015 19:24, Andy Lutomirski пишет: > >> 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. > > Isn't uc_flags read-only for the user? > I look into setup_rt_frame > () and see > --- > /* Create the ucontext. */ > err |= __put_user(0, &frame->uc.uc_flags); > --- > so it doesn't look like the flag that user can use to _request_ > something from the kernel. And I am talking about exactly > the flag to request the new behaviour, as only that can remove > the regression completely without patching dosemu. User code could rewrite it in the signal handler to request something. --Andy