Date:	Wed, 2 Sep 2015 15:25:15 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Stas Sergeev <stsp@...t.ru>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raymond Jennings <shentino@...il.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	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, Sep 2, 2015 at 3:25 PM, Stas Sergeev <stsp@...t.ru> wrote:
> 03.09.2015 00:39, Andy Lutomirski пишет:
>
>> On Wed, Sep 2, 2015 at 2:01 PM, Stas Sergeev <stsp@...t.ru> wrote:
>>>
>>> 02.09.2015 22:06, Andy Lutomirski пишет:
>>>
>>>> On Wed, Sep 2, 2015 at 11:23 AM, Stas Sergeev <stsp@...t.ru> wrote:
>>>>>
>>>>> 02.09.2015 21:17, Andy Lutomirski пишет:
>>>>>>
>>>>>> On Wed, Sep 2, 2015 at 10:46 AM, Stas Sergeev <stsp@...t.ru> wrote:
>>>>>>>
>>>>>>> 02.09.2015 17:21, Andy Lutomirski пишет:
>>>>>>>>>>
>>>>>>>>>> This should work for old DOSEMU.  It's a bit gross, but it has the
>>>>>>>>>> nice benefit that everyone (even things that aren't DOSEMU) gain
>>>>>>>>>> the
>>>>>>>>>> ability to catch signals thrown from bogus SS contexts, which
>>>>>>>>>> probably
>>>>>>>>>> improves debugability.  It's also nice to not have the SA flag.
>>>>>>>>>
>>>>>>>>> Pros:
>>>>>>>>> - No new SA flag
>>>>>>>>> - May improve debugability in some unknown scenario where people
>>>>>>>>> do not want to just use the new flag to get their things improved
>>>>>>>>>
>>>>>>>>> Cons:
>>>>>>>>> - Does not allow to cleanly use siglongjmp(), as then there is a
>>>>>>>>> risk
>>>>>>>>> to jump to 64bit code with bad SS
>>>>>>>>
>>>>>>>> What's the issue here?  I don't understand.
>>>>>>>>
>>>>>>>> On musl, (sig)longjmp just restores rsp, rbx, rbp, and r12-r15, so
>>>>>>>> it
>>>>>>>> won't be affected.  AFAIK all implementations of siglongjmp are
>>>>>>>> likely
>>>>>>>> to call sigprocmask or similar, and that will clobber SS.  I'm not
>>>>>>>> aware of an implementation of siglongjmp that uses sigreturn.
>>>>>>>
>>>>>>> I am not saying siglongjmp() will be affected.
>>>>>>> Quite the opposite: it won't, which is bad. :)
>>>>>>> If you have always correct SS, you can use siglongjmp(). If you have
>>>>>>> broken SS at times, siglongjmp() will be an asking for troubles, as
>>>>>>> it exactly does not restore SS.
>>>>>>> dosemu could do a good use of siglongjmp() to get back to 64bit code
>>>>>>> from its sighandler.
>>>>>>
>>>>>> This seems like it would be relying unpleasantly heavily on libc
>>>>>> internals.
>>>>>
>>>>> Could you please clarify?
>>>>> If kernel always passes the right SS to the sighandler, then what's
>>>>> the problem?
>>>>
>>>> What's the exact siglongjmp usage you have in mind?  Signal context
>>>> isn't normally involved AFAIK.
>>>
>>> dosemu needs 2 return pathes:
>>> 1. to DOS code
>>> 2. to 64bit code (dosemu is not all in a sighandler, right?)
>>>
>>> How it is currently achieved:
>>> dosemu1:
>>> 1. sigreturn() + iret (to DOS)
>>> 2. modify sigcontext -> sigreturn() (to 64bit asm helper)
>>>
>>> dosemu2:
>>> 1. sigreturn() + iret (to DOS)
>>> 2. modify sigcontext -> sigreturn() -> longjmp() (to 64bit C-coded)
>>
>> So you're modifying sigcontext such that it returns to a C function
>> that calls longjmp?
>
> Yes.
>
>>> How dosemu2 is supposed to do this:
>>> 1. sigreturn() (to DOS)
>>> 2. siglongjmp() (to 64bit C-coded)
>>
>> This should work fine on any kernel, right?
>
> 1 - not.
> 2 - maybe.
> If, as you say, siglongjmp() restores SS, I need to try it out.
> (there is also a problem that most siglongjmp() implementations
> are incompatible with sigaltstack(), but this is not what you can fix).
>

1 - definitely needs kernel changes.  I was referring to #2.

2 - siglongjmp probably varies in its behavior across different libc
implementations.  My point is that siglongjmp isn't a kernel-provided
thing.

>> For backwards compat, we either need the default behavior to be
>> unchanged, or we need the default behavior to be something that works
>> with existing dosemu.  For existing dosemu, the only interesting cases
>> (I think) are signal delivery from *valid* 16-bit context, in which
>> case we need to preserve SS so that the signal handler can read it out
>> with mov ..., %ss, and sigreturn to 64-bit mode for the IRET
>> trampoline.  For sigreturn, IIUC old dosemu will replace the saved CS
>> with a 64-bit code segment selector and won't touch the saved SS
>> because it doesn't know about the saved SS.  Those dosemu versions
>> don't care what SS actually contains after sigreturn, because they're
>> immediately going to change it again using IRET.  So we just need to
>> make sure we return without faulting.
>>
>> New dosemu2 would like to sigreturn directly back to 16-bit mode, so
>> it needs the kernel to honor the saved ss value and restore it,
>> possibly changed by dosemu.
>>
>> We obviously can't require old dosemu to set an SA flag to keep
>> working.  But, if we can get away with it, I think it's somewhat
>> preferable not to require new DOSEMU to set an SA flag either.
>>
>> This has one major benefit at least: if new dosemu loads some random
>> library that installs some async signal handler (SIGALRM for example),
>> everything will work with regard to CS and SS.
>
> This case is covered if we do both things together: use
> your heuristic when SA_hyz is not set, and don't use it
> when its set. In this case dosemu2 will be able to request
> the proper SS delivery for its sighandlers, but the 3rd-party
> sighandlers will work too.
> I think we have never discussed the possibility of doing
> both things together, even though I have proposed it many
> times.
> After discussing this full-blown solution, we can think about
> reducing it, either by removing the heuristic or by removing
> SA_hyz, but discussing the full one would be nice too.
> Your opinion is likely that no one will use this SA_hyz in
> presence of the heuristic that "seems to work anyway".
> But in the light of extending it for TLS (with a new flag),
> I wouldn't be so sure. You can also document it as a
> needed flag when user code touches SS, and then it will
> be used. dosemu1 code that doesn't use it, will eventually
> be forgotten. So IMHO whether it will be used, is fully up
> to how will you market it. :)

I'll think about it.  I'll think about FS and GS, too, although that's
still a longer-term thing.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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