From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753444AbbHMQtN (ORCPT ); Thu, 13 Aug 2015 12:49:13 -0400 Received: from smtp10.mail.ru ([94.100.181.92]:39065 "EHLO smtp10.mail.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751996AbbHMQtJ (ORCPT ); Thu, 13 Aug 2015 12:49:09 -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> <55CCC812.5010101@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: <55CCCA78.8030806@list.ru> Date: Thu, 13 Aug 2015 19:48:56 +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 19:42, Andy Lutomirski пишет: > 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. But that's too late to affect the signal _delivery_ anyhow, no? Any idea about the flag that can control both delivery and return?