From: Andy Lutomirski <luto@amacapital.net>
To: Stas Sergeev <stsp@list.ru>
Cc: X86 ML <x86@kernel.org>, Linux kernel <linux-kernel@vger.kernel.org>
Subject: Re: [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu
Date: Wed, 12 Aug 2015 15:00:48 -0700 [thread overview]
Message-ID: <CALCETrWqf6DJySf8+Hv_s8FLHVsvfsnci3xGXOtixZ=bxm4fsw@mail.gmail.com> (raw)
In-Reply-To: <55CBBFB9.1080201@list.ru>
On Wed, Aug 12, 2015 at 2:50 PM, Stas Sergeev <stsp@list.ru> wrote:
> 13.08.2015 00:37, Andy Lutomirski пишет:
>
>> On Wed, Aug 12, 2015 at 1:55 PM, Stas Sergeev <stsp@list.ru> wrote:
>>>
>>> 12.08.2015 23:47, Andy Lutomirski пишет:
>>>
>>>> On Wed, Aug 12, 2015 at 1:45 PM, Stas Sergeev <stsp@list.ru> wrote:
>>>>>
>>>>> 12.08.2015 23:28, Andy Lutomirski пишет:
>>>>>
>>>>>> On Wed, Aug 12, 2015 at 1:14 PM, Stas Sergeev <stsp@list.ru> wrote:
>>>>>>>
>>>>>>> 12.08.2015 23:01, Andy Lutomirski пишет:
>>>>>>>>
>>>>>>>> On Wed, Aug 12, 2015 at 12:55 PM, Stas Sergeev <stsp@list.ru> 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
next prev parent reply other threads:[~2015-08-12 22:01 UTC|newest]
Thread overview: 121+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-08-12 0:17 [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu Stas Sergeev
2015-08-12 0:38 ` Andy Lutomirski
2015-08-12 8:02 ` Stas Sergeev
2015-08-12 16:19 ` Andy Lutomirski
2015-08-12 17:00 ` Stas Sergeev
2015-08-12 18:25 ` Andy Lutomirski
2015-08-12 18:55 ` Stas Sergeev
2015-08-12 19:20 ` Andy Lutomirski
2015-08-12 19:55 ` Stas Sergeev
2015-08-12 20:01 ` Andy Lutomirski
2015-08-12 20:14 ` Stas Sergeev
2015-08-12 20:28 ` Andy Lutomirski
2015-08-12 20:45 ` Stas Sergeev
2015-08-12 20:47 ` Andy Lutomirski
2015-08-12 20:55 ` Stas Sergeev
2015-08-12 21:37 ` Andy Lutomirski
2015-08-12 21:50 ` Stas Sergeev
2015-08-12 22:00 ` Andy Lutomirski [this message]
2015-08-13 8:39 ` Ingo Molnar
2015-08-13 10:14 ` Stas Sergeev
2015-08-13 12:44 ` Stas Sergeev
2015-08-13 14:58 ` Andy Lutomirski
2015-08-13 15:22 ` Stas Sergeev
2015-08-13 15:38 ` Andy Lutomirski
2015-08-13 16:03 ` Stas Sergeev
2015-08-13 16:09 ` Andy Lutomirski
2015-08-13 16:20 ` Stas Sergeev
2015-08-13 16:24 ` Andy Lutomirski
2015-08-13 16:38 ` Stas Sergeev
2015-08-13 16:42 ` Andy Lutomirski
2015-08-13 16:48 ` Stas Sergeev
2015-08-13 16:59 ` Andy Lutomirski
2015-08-13 17:13 ` Stas Sergeev
2015-08-13 17:17 ` Andy Lutomirski
2015-08-13 18:00 ` Stas Sergeev
2015-08-13 18:05 ` Andy Lutomirski
2015-08-13 18:19 ` Stas Sergeev
2015-08-13 18:25 ` Andy Lutomirski
2015-08-13 18:35 ` Stas Sergeev
2015-08-22 12:38 ` Ingo Molnar
2015-08-22 14:19 ` Stas Sergeev
2015-08-23 6:25 ` Ingo Molnar
2015-08-13 11:08 ` Stas Sergeev
2015-08-13 15:37 ` Linus Torvalds
2015-08-13 15:43 ` Andy Lutomirski
2015-08-13 16:19 ` Linus Torvalds
2015-08-13 16:23 ` Andy Lutomirski
2015-08-13 16:34 ` Linus Torvalds
2015-08-13 16:43 ` Linus Torvalds
2015-08-13 16:44 ` Andy Lutomirski
2015-08-13 17:00 ` Brian Gerst
2015-08-18 6:29 ` Stas Sergeev
2015-08-18 22:42 ` Andy Lutomirski
2015-08-18 22:47 ` Andy Lutomirski
2015-08-19 9:35 ` Stas Sergeev
2015-08-19 15:46 ` Andy Lutomirski
2015-08-19 16:30 ` Stas Sergeev
2015-09-02 5:12 ` Andy Lutomirski
2015-09-02 9:17 ` Stas Sergeev
2015-09-02 14:21 ` Andy Lutomirski
2015-09-02 15:02 ` Andy Lutomirski
2015-09-02 17:46 ` Stas Sergeev
2015-09-02 18:17 ` Andy Lutomirski
2015-09-02 18:23 ` Stas Sergeev
2015-09-02 19:06 ` Andy Lutomirski
2015-09-02 21:01 ` Stas Sergeev
2015-09-02 21:39 ` Andy Lutomirski
2015-09-02 22:25 ` Stas Sergeev
2015-09-02 22:25 ` Andy Lutomirski
2015-09-02 23:01 ` Stas Sergeev
2015-08-19 10:10 ` Stas Sergeev
2015-08-19 15:35 ` Andy Lutomirski
2015-08-14 8:10 ` Cyrill Gorcunov
2015-08-13 17:51 ` Stas Sergeev
2015-08-13 18:35 ` Linus Torvalds
2015-08-13 18:41 ` Andy Lutomirski
2015-08-13 19:05 ` Stas Sergeev
2015-08-13 19:49 ` Andy Lutomirski
2015-08-13 20:09 ` Stas Sergeev
2015-08-13 19:53 ` Linus Torvalds
2015-08-13 20:08 ` Cyrill Gorcunov
2015-08-13 20:09 ` Linus Torvalds
2015-08-13 21:42 ` Raymond Jennings
2015-08-13 21:46 ` Linus Torvalds
2015-08-13 22:01 ` Raymond Jennings
2015-08-13 22:05 ` Stas Sergeev
2015-08-13 23:05 ` Linus Torvalds
2015-08-13 23:18 ` Linus Torvalds
2015-08-13 23:35 ` Raymond Jennings
2015-08-13 23:43 ` Stas Sergeev
2015-08-14 0:02 ` Linus Torvalds
2015-08-13 22:02 ` Stas Sergeev
2015-08-13 22:11 ` Andy Lutomirski
2015-08-13 22:25 ` Stas Sergeev
2015-08-13 22:29 ` Andy Lutomirski
2015-08-13 22:51 ` Stas Sergeev
2015-08-13 23:00 ` Andy Lutomirski
2015-08-13 23:17 ` Stas Sergeev
2015-08-14 0:00 ` Stas Sergeev
2015-08-14 0:05 ` Andy Lutomirski
2015-08-14 0:17 ` Stas Sergeev
2015-08-14 0:27 ` Linus Torvalds
2015-08-14 0:50 ` Stas Sergeev
2015-08-14 1:21 ` Andy Lutomirski
2015-08-14 1:32 ` Stas Sergeev
2015-08-14 1:37 ` Andy Lutomirski
2015-08-14 2:03 ` Stas Sergeev
2015-08-18 6:19 ` Stas Sergeev
2015-08-14 0:08 ` Linus Torvalds
2015-08-14 0:24 ` Andy Lutomirski
2015-08-14 0:40 ` Linus Torvalds
2015-08-14 7:22 ` Cyrill Gorcunov
2015-08-14 10:02 ` Pavel Emelyanov
2015-08-14 10:53 ` Cyrill Gorcunov
2015-08-13 18:57 ` Stas Sergeev
2015-08-13 19:01 ` Andy Lutomirski
2015-08-13 19:13 ` Stas Sergeev
2015-08-13 19:37 ` Linus Torvalds
2015-08-13 19:59 ` Stas Sergeev
2015-08-13 20:07 ` Linus Torvalds
2015-08-18 6:40 ` Stas Sergeev
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