Date:	Wed, 12 Aug 2015 14:37:40 -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 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.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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