Date:	Wed, 12 Aug 2015 15:00:48 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Stas Sergeev <stsp@...t.ru>
Cc:	X86 ML <x86@...nel.org>,
	Linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [regression] x86/signal/64: Fix SS handling for signals delivered
 to 64-bit programs breaks dosemu

On Wed, Aug 12, 2015 at 2:50 PM, Stas Sergeev <stsp@...t.ru> wrote:
> 13.08.2015 00:37, Andy Lutomirski пишет:
>
>> On Wed, Aug 12, 2015 at 1:55 PM, Stas Sergeev <stsp@...t.ru> wrote:
>>>
>>> 12.08.2015 23:47, Andy Lutomirski пишет:
>>>
>>>> On Wed, Aug 12, 2015 at 1:45 PM, Stas Sergeev <stsp@...t.ru> wrote:
>>>>>
>>>>> 12.08.2015 23:28, Andy Lutomirski пишет:
>>>>>
>>>>>> On Wed, Aug 12, 2015 at 1:14 PM, Stas Sergeev <stsp@...t.ru> wrote:
>>>>>>>
>>>>>>> 12.08.2015 23:01, Andy Lutomirski пишет:
>>>>>>>>
>>>>>>>> On Wed, Aug 12, 2015 at 12:55 PM, Stas Sergeev <stsp@...t.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
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