Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752094AbbHLWBK (ORCPT ); Wed, 12 Aug 2015 18:01:10 -0400 Received: from mail-ob0-f172.google.com ([209.85.214.172]:34928 "EHLO mail-ob0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751092AbbHLWBI convert rfc822-to-8bit (ORCPT ); Wed, 12 Aug 2015 18:01:08 -0400 MIME-Version: 1.0 In-Reply-To: <55CBBFB9.1080201@list.ru> References: <55CA90B4.2010205@list.ru> <55CAFD9F.2070001@list.ru> <55CB7BAE.9090503@list.ru> <55CB9697.1050602@list.ru> <55CBA4CE.1040108@list.ru> <55CBA909.3020306@list.ru> <55CBB053.7050803@list.ru> <55CBB2CC.9090600@list.ru> <55CBBFB9.1080201@list.ru> From: Andy Lutomirski Date: Wed, 12 Aug 2015 15:00:48 -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: X86 ML , Linux kernel 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 Content-Length: 4922 Lines: 119 On Wed, Aug 12, 2015 at 2:50 PM, Stas Sergeev wrote: > 13.08.2015 00:37, Andy Lutomirski пишет: > >> On Wed, Aug 12, 2015 at 1:55 PM, Stas Sergeev wrote: >>> >>> 12.08.2015 23:47, Andy Lutomirski пишет: >>> >>>> On Wed, Aug 12, 2015 at 1:45 PM, Stas Sergeev wrote: >>>>> >>>>> 12.08.2015 23:28, Andy Lutomirski пишет: >>>>> >>>>>> On Wed, Aug 12, 2015 at 1:14 PM, Stas Sergeev wrote: >>>>>>> >>>>>>> 12.08.2015 23:01, Andy Lutomirski пишет: >>>>>>>> >>>>>>>> On Wed, Aug 12, 2015 at 12:55 PM, Stas Sergeev wrote: >>>>>>>>> >>>>>>>>> 12.08.2015 22:20, Andy Lutomirski пишет: >>>>>>>>>> >>>>>>>>>> current kernels, it stays switched. If we change this, it won't >>>>>>>>>> stay >>>>>>>>>> switched. Even ignoring old ABI, it's not really clear to me what >>>>>>>>>> the >>>>>>>>>> right thing to do is. >>>>>>>>> >>>>>>>>> There can be the following cases: >>>>>>>>> - switch_userspace_thread() switches fs to non-zero selector >>>>>>>>> - switch_userspace_thread() switches the fs base via syscall >>>>>>>>> - switch_userspace_thread() switches fs in sigcontext >>>>>>>>> - switch_userspace_thread() switches fs_base in sigcontext (???) >>>>>>>>> What exactly case do you have in mind? >>>>>>>>> I'd say, the way x86_32 is doing things - is good, but the >>>>>>>>> bases... perhaps, in ideal world, they should be a part of >>>>>>>>> the sigcontext as well? >>>>>>>> >>>>>>>> Any of the above. What do you want the kernel to do, and how does >>>>>>>> the >>>>>>>> kernel know you want to do that? The kernel has to pick *some* >>>>>>>> semantics here. >>>>>>> >>>>>>> Assuming the bases are made the part of a sigcontext, >>>>>>> I'd say there would be no ambiguities remained at all: >>>>>>> whatever you change in a sigcontext, will be "applied" by >>>>>>> the sigreturn(). Whatever you put in the registers >>>>>>> (either segregs or MSRs), is valid until sigreturn(), then >>>>>>> forgotten forever. >>>>>>> The mess only comes in when some things are part of >>>>>>> sigcontext and some are not. But if you have _all_ things >>>>>>> accessable in sigcontext, then the user has a way of expressing >>>>>>> his needs very clearly: he'll either touch sigcontext or direct >>>>>>> values, depending on what he need. >>>>>>> >>>>>>> Is this right? >>>>>> >>>>>> Maybe, except that doing this might break existing code (Wine and Java >>>>>> come to mind). I'm not really sure. >>>>> >>>>> Yes, but that's why I was talking about some new >>>>> flag. Maybe a new sigaction() flag? Or something else that >>>>> will allow the user to request explicitly the new handling >>>>> where the things are all switched by the kernel. Then >>>>> the old programs that don't use that flag, will remain >>>>> unaffected. I realize this may be a lot of work... But please >>>>> note that there will be no more a chance like this one, >>>>> when things are already badly broken. :) >>>> >>>> I think that, with my patch, we get the best of both worlds. We keep >>>> the old behavior in cases where it would work, and we switch to the >>>> new behavior in cases where the old behavior would result in killing >>>> the task. >>> >>> But I mean also fs/TLS. >>> There is a chance now to fix things for good, all at once. :) >>> With such an ss patch applied to stable, there will be no more >>> such a chance ever. What's your opinion on the possibility of >>> fixing the TLS problem? >>> Also I am not sure about the sigreturn()'s detection: is it >>> a subject of the subsequent patch, or you dropped an idea? >> >> I think these things shouldn't be conflated. If we can fix it >> transparently (i.e. if my patch works), then I think we should do >> something like my patch. > > OK. > I'll try to test the patch tomorrow, but I think the sigreturn()'s > capability detection is still needed to easily replace the iret trampoline > in userspace (without generating a signal and testing by hands). > Can of course be done with a run-time kernel version check... That feature is so specialized that I think you should just probe it. void foo(...) { sigcontext->ss = 7; } modify_ldt(initialize descriptor 0); sigaction(SIGUSR1, foo, SA_SIGINFO); if (ss == 7) yay; Fortunately, all kernels that restore ss also have espfix64, so you don't need to worry about esp[31:16] corruption on those kernels either. I suppose we could add a new uc_flag to indicate that ss is saved and restored, though. Ingo, hpa: any thoughts on that? There will always be some kernel versions that save and restore ss but don't set the flag, though. --Andy -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/