Message-ID: <55CBB2CC.9090600@list.ru>
Date:	Wed, 12 Aug 2015 23:55:40 +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

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?
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