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

From: Andy Lutomirski
Date: Thu Aug 13 2015 - 13:18:11 EST

On Thu, Aug 13, 2015 at 10:13 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
> 13.08.2015 19:59, Andy Lutomirski ÐÐÑÐÑ:
>> On Thu, Aug 13, 2015 at 9:48 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
>>> 13.08.2015 19:42, Andy Lutomirski ÐÐÑÐÑ:
>>>> On Thu, Aug 13, 2015 at 9:38 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
>>>>> 13.08.2015 19:24, Andy Lutomirski ÐÐÑÐÑ:
>>>>>> On Thu, Aug 13, 2015 at 9:20 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
>>>>>>> 13.08.2015 19:09, Andy Lutomirski ÐÐÑÐÑ:
>>>>>>>> On Thu, Aug 13, 2015 at 9:03 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
>>>>>>>>> 13.08.2015 18:38, Andy Lutomirski ÐÐÑÐÑ:
>>>>>>>>>> So... what do we do about it? We could revert the whole mess.
>>>>>>>>>> We
>>>>>>>>>> could tell everyone to fix their DOSEMU, which violates policy and
>>>>>>>>>> is
>>>>>>>>>> especially annoying given how much effort we've put into keeping
>>>>>>>>>> 16-bit mode fully functional lately. We could add yet more
>>>>>>>>>> heuristics
>>>>>>>>>> and teach sigreturn to ignore the saved SS value in sigcontext if
>>>>>>>>>> the
>>>>>>>>>> saved CS is 64-bit and the saved SS is unusable.
>>>>>>>>> Andy, why do you constantly ignore the proposal to make
>>>>>>>>> new behaviour explicitly controlable? You don't have to agree
>>>>>>>>> with it, but you could at least comment on that possibility
>>>>>>>>> and/or mention it with the ones you listed above.
>>>>>>>> I'm not sure what the proposal is exactly.
>>>>>>>> We could add a new uc_flags flag. If set, it means that
>>>>>>>> sigcontext->ss is valid and should be used by sigreturn. If clear,
>>>>>>>> then we ignore sigcontext->ss and just restore __USER_DS.
>>>>>>>> The problem is that, by itself, this won't fix old DOSEMU. We
>>>>>>>> somehow
>>>>>>>> need to either detect that something funny is going on or just leave
>>>>>>>> the flag clear by default.
>>>>>>>> We could do this: always save SS to sigcontext->ss, but only restore
>>>>>>>> sigcontext->ss if userspace explicitly sets the flag before
>>>>>>>> sigreturn.
>>>>>>>> If we do that, we'd need to also add my patch to preserve the actual
>>>>>>>> HW SS selector if possible so that old DOSEMU knows what SS to
>>>>>>>> program
>>>>>>>> into its trampoline.
>>>>>>>> This at least lets *new* DOSEMU set the flag and get the improved
>>>>>>>> behavior. I still don't know what effect it'll have on Wine and
>>>>>>>> CRIU.
>>>>>>>> Stas, is that what you were thinking, or were you thinking of
>>>>>>>> something
>>>>>>>> else?
>>>>>>> Not quite.
>>>>>>> I mean the flag that will control not only sigreturn, but
>>>>>>> the signal delivery as well. This may probably be a sigaction()
>>>>>>> flag or some other. If not set - ss is ignored by both signal
>>>>>>> delivery and sigreturn(). If set - ss is saved/restored (and in
>>>>>>> the future - also fs/gs).
>>>>>>> Is such a flag possible?
>>>>>> Maybe. I think I'm more nervous about adding new flags in sigaction
>>>>>> than I am in uc_flags.
>>>>> Isn't uc_flags read-only for the user?
>>>>> I look into setup_rt_frame
>>>>> <;i=setup_rt_frame>() and
>>>>> see
>>>>> ---
>>>>> /* Create the ucontext. */
>>>>> err |= __put_user(0, &frame->uc.uc_flags);
>>>>> ---
>>>>> so it doesn't look like the flag that user can use to _request_
>>>>> something from the kernel. And I am talking about exactly
>>>>> the flag to request the new behaviour, as only that can remove
>>>>> the regression completely without patching dosemu.
>>>> User code could rewrite it in the signal handler to request something.
>>> But that's too late to affect the signal _delivery_ anyhow, no?
>>> Any idea about the flag that can control both delivery and return?
>> I think my LAR patch should cover the signal delivery part.
> Ah, I see your point now.
> But that's not what I mean, as it doesn't cover fs/gs, which
> is what Linus is looking to revert now too (I am building the
> testing kernels now).
> So you obviously don't want the flag that will control all 3
> things together without any lar heuristics, but I don't understand why...
> Yes, your heuristic+uc_flag may work, but IMHO far from
> perfection and TLS problem is not covered. I can test such
> a patch but I don't understand why you don't want the flag
> that will just control all things together.

The fs/gs patch doesn't change anything, so there's nothing to
control. It just renamed fields that did nothing. (It turns out they
did something back before arch_prctl existed, but there's only a
narrow range of kernels like that, and I'm not at all convinced that
those kernels are ABI-compatible with modern kernels at all. This is
all pre-git.)

Sure, it might make sense to change TLS behavior in signals at some
point, but I don't think we're there yet. We need to deal with
fsgsbase first, and that's a *huge* can of worms.


Andy Lutomirski
AMA Capital Management, LLC
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