Date:	Thu, 13 Aug 2015 14:08:48 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Andy Lutomirski <luto@...capital.net>
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

13.08.2015 01:00, Andy Lutomirski пишет:
> 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.
Unfortunately, this doesn't help.
I made a simple patch that checks the kernel version:
https://github.com/stsp/dosemu2/commit/098413ef8de98972ca795e078351ae9f3cc07ffe
but iret is still used when switching to DOS code from dosemu
code, rather than from signal handler. So espfix64 doesn't help
much.
I guess the only real solution to this would be to "rewrite" dosemu
so that the DOS code is run in a clone(CLONE_VM) separate process...
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