From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753482AbbHMRAF (ORCPT ); Thu, 13 Aug 2015 13:00:05 -0400 Received: from mail-ob0-f173.google.com ([209.85.214.173]:34117 "EHLO mail-ob0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753373AbbHMRAD convert rfc822-to-8bit (ORCPT ); Thu, 13 Aug 2015 13:00:03 -0400 MIME-Version: 1.0 In-Reply-To: <55CCCA78.8030806@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> <55CCCA78.8030806@list.ru> From: Andy Lutomirski Date: Thu, 13 Aug 2015 09:59:42 -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 9:48 AM, Stas Sergeev wrote: > 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? I think my LAR patch should cover the signal delivery part. --Andy