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 - 12:43:17 EST

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.

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