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

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Message-ID: <55CCD054.5020600@list.ru>
Date:	Thu, 13 Aug 2015 20:13:56 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Ingo Molnar <mingo@...nel.org>, X86 ML <x86@...nel.org>,
	Linux kernel <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Brian Gerst <brgerst@...il.com>,
	Borislav Petkov <bp@...en8.de>,
	Stas Sergeev <stsp@...rs.sourceforge.net>
Subject: Re: [regression] x86/signal/64: Fix SS handling for signals delivered
 to 64-bit programs breaks dosemu

13.08.2015 19:59, Andy Lutomirski пишет:
> On Thu, Aug 13, 2015 at 9:48 AM, Stas Sergeev <stsp@...t.ru> wrote:
>> 13.08.2015 19:42, Andy Lutomirski пишет:
>>
>>> On Thu, Aug 13, 2015 at 9:38 AM, Stas Sergeev <stsp@...t.ru> wrote:
>>>> 13.08.2015 19:24, Andy Lutomirski пишет:
>>>>
>>>>> On Thu, Aug 13, 2015 at 9:20 AM, Stas Sergeev <stsp@...t.ru> wrote:
>>>>>> 13.08.2015 19:09, Andy Lutomirski пишет:
>>>>>>
>>>>>>> On Thu, Aug 13, 2015 at 9:03 AM, Stas Sergeev <stsp@...t.ru> wrote:
>>>>>>>> 13.08.2015 18:38, Andy Lutomirski пишет:
>>>>>>>>>
>>>>>>>>> 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
>>>> <http://lxr.free-electrons.com/ident?v=2.4.37;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.
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