From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753174AbbHMQiv (ORCPT ); Thu, 13 Aug 2015 12:38:51 -0400 Received: from smtp13.mail.ru ([94.100.181.94]:44120 "EHLO smtp13.mail.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752484AbbHMQit (ORCPT ); Thu, 13 Aug 2015 12:38:49 -0400 Subject: Re: [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu To: Andy Lutomirski 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> Cc: Ingo Molnar , X86 ML , Linux kernel , Linus Torvalds , "H. Peter Anvin" , Thomas Gleixner , Brian Gerst , Borislav Petkov , Stas Sergeev From: Stas Sergeev Message-ID: <55CCC812.5010101@list.ru> Date: Thu, 13 Aug 2015 19:38:42 +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: 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: X-Mailing-List: linux-kernel@vger.kernel.org 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.