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

From: Andy Lutomirski <luto@amacapital.net>
To: Stas Sergeev <stsp@list.ru>
Cc: Ingo Molnar <mingo@kernel.org>, X86 ML <x86@kernel.org>,
	Linux kernel <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Brian Gerst <brgerst@gmail.com>, Borislav Petkov <bp@alien8.de>,
	Stas Sergeev <stsp@users.sourceforge.net>
Subject: Re: [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu
Date: Thu, 13 Aug 2015 09:24:14 -0700	[thread overview]
Message-ID: <CALCETrWo6WSykpef0f9Nfrt7wUhPcePESff6h4hak_ffAMkbzA@mail.gmail.com> (raw)
In-Reply-To: <55CCC3E1.9060603@list.ru>

On Thu, Aug 13, 2015 at 9:20 AM, Stas Sergeev <stsp@list.ru> wrote:
> 13.08.2015 19:09, Andy Lutomirski пишет:
>
>> On Thu, Aug 13, 2015 at 9:03 AM, Stas Sergeev <stsp@list.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.

--Andy