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: <55CCC812.5010101@list.ru>
Date:	Thu, 13 Aug 2015 19:38:42 +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 19:24, 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.
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