Re: [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu

[Andy Lutomirski <luto@amacapital.net>]

On Mon, Aug 17, 2015 at 11:19 PM, Stas Sergeev <stsp@list.ru> wrote:
> 14.08.2015 04:37, Andy Lutomirski пишет:
>
>> On Thu, Aug 13, 2015 at 6:32 PM, Stas Sergeev <stsp@list.ru> wrote:
>>>
>>> 14.08.2015 04:21, Andy Lutomirski пишет:
>>>
>>>> On Thu, Aug 13, 2015 at 5:50 PM, Stas Sergeev <stsp@list.ru> wrote:
>>>>>
>>>>> 14.08.2015 03:27, Linus Torvalds пишет:
>>>>>>
>>>>>> On Thu, Aug 13, 2015 at 5:17 PM, Stas Sergeev <stsp@list.ru> wrote:
>>>>>>>
>>>>>>> For example because you can as well do:
>>>>>>> prctl(ARCH_SET_SIGNAL_SS, 0)
>>>>>>> which will mean "restore ss in sighandler to its current value",
>>>>>>
>>>>>> I really think a prctl() is the wrong thing to do.
>>>>>>
>>>>>> If you want a signal handler to save/restore segments, I think it
>>>>>> should be a SA_xyz flag to sigaction() (the way we have SA_RESTART
>>>>>
>>>>> Yes, I was proposing the new sigaction() flag in this thread
>>>>> already too. But at the end, prctl() looks better to me because
>>>>> it allows to pass the TLS value to use when restoring FS.
>>>>> The thing is that I am trying to find the similar treatment for
>>>>> both the SS and FS problems. If you don't think they need a
>>>>> similar treatment, then perhaps the Andy's patch is enough.
>>>>>
>>>>>> etc).  And off by default because of the obvious compatibility issues.
>>>>>
>>>>> Of course.
>>>>>
>>>>> So, what we have right now (in the latest Andy's patch) is:
>>>>> 1. lar heuristics
>>>>> 2. new uc_flags flag
>>>>>
>>>>> What it solves: dosemu's regression.
>>>>>
>>>>> What prctl() can give:
>>>>> - fix to dosemu's regression
>>>>> - fix to the TLS problem in the future
>>>>> - no hack and heuristics
>>>>>
>>>>> With SA_xyz you can only solve the SS problem, so it is
>>>>> probably not any better than the uc_flags things coded
>>>>> up by Andy.
>>>>
>>>> I'm leaning slightly toward LAR heuristic + SA_SAVE_SS.
>>>
>>> Stop right here, doesn't the SA_xyz allow to avoid the
>>> lar heuristic? Why would you still need the lar heuristic then?
>>> Just call it SA_RESTORE_SS instead of SA_SAVE_SS, and
>>> the lar heuristic is gone.
>>
>> The LAR heuristic is about five lines of code, and it makes signal
>> delivery more reliable.
>
> Why more reliable? In what case?
>
>>    Sure, we could gate the "regs->ss =
>> __USER_DS" line on a flag, but why?
>
> A few things I can think of why:
> - nested signals (usual for dosemu)

What's the issue with nested signals?

> - using siglongjmp() to return to dosemu (rather than to DOS code)
> Both cases look very scare when using SS from just freed LDT entry.
> How would you even justify and changelog the patch that adds a lar
> heuristic code that no one uses or wants? Since SA_hyz flag allows
> you to do without, why not to just keep things safe and simple?

The LAR heuristic is just for compatibility.

ISTM what DOSEMU should want (on new kernels, anyway) is the ability
to save and restore SS just like any other register, which is what my
patch did.  The issue is that it broke old DOSEMU.  I want to find a
way to keep old DOSEMU working while making things work better for new
code that's aware of new behavior.  That means we want some way
(opt-in or magically compatible with old DOSEMU) to get SS saved and
restored.

Incidentally, I tried implementing the sigaction flag approach.  I
think it's no good.  When we return from a signal, there's no concept
of sigaction -- it's just sigreturn.  Sigreturn can't look up the
sigaction flags -- what if the signal handler calls sigaction itself.
So we either need a per-task flag, a per-sighand flag, or a sigcontext
flag indicating what we should do.

(Yes, I suspect we really might want some way to get FS, GS, and their
bases saved and restored, but I still think we should do that
separately.)

-- 
Andy Lutomirski
AMA Capital Management, LLC