linux-kernel - Re: [RFC 3/4] x86/signal/64: Re-add support for SS in the 64-bit signal context

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Message-ID: <CALCETrWP=O7wyRZn9YWWoqU9XS+jLdX+L3ZjshTO=Rqrxauo8Q@mail.gmail.com>
Date:	Wed, 14 Oct 2015 14:41:05 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Stas Sergeev <stsp@...t.ru>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Borislav Petkov <bp@...en8.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Brian Gerst <brgerst@...il.com>, X86 ML <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC 3/4] x86/signal/64: Re-add support for SS in the 64-bit
 signal context

On Wed, Oct 14, 2015 at 2:37 PM, Stas Sergeev <stsp@...t.ru> wrote:
> 14.10.2015 21:52, Andy Lutomirski пишет:
>>
>> On Wed, Oct 14, 2015 at 11:34 AM, Stas Sergeev <stsp@...t.ru> wrote:
>>>
>>> 14.10.2015 21:06, Andy Lutomirski пишет:
>>>>>
>>>>> Also it doesn't seem to be saying what happens if CS is 32-bit
>>>>> and SS is invalid (the flag is not set).
>>>>
>>>> A new signal will be delivered.  sigreturn doesn't modify its behavior
>>>> in this case -- it does the default thing, which is to honor the SS in
>>>> the saved context.
>>>
>>> Hmm, no, it didn't do this in the past for sure.
>>> It simply ignored SS, no matter to what mode it returns.
>>>
>> What I mean is: it has the behavior it would have normally on a new
>> kernel, which is to honor the saved SS.  I'll try to improve the
>> comment.
>>
>>>>   So it will actually try to use that saved SS
>>>> value, which will fail, causing SIGSEGV.
>>>
>>> So it seems this logic assumes that when dosemu returns to 32bit,
>>> the previous SS is always still valid, am I right with the understanding?
>>> I.e. the one that kernel have saved on a signal delivery (because
>>> old dosemu does not overwrite it).
>>> If it is so, I'd say this assumption is very risky and will likely
>>> not hold. But maybe I am missing the point.
>>>
>> That's the assumption.  If I understand correctly, though, old DOSEMU
>> never actually returns to 32-bit using sigreturn in the first place,
>> since old kernels gave no control over SS.  Doesn't old DOSEMU always
>> return to the 64-bit IRET trampoline?
>
> Ah, so the old progs simply never return to 32bit, so you
> implement the "Right Thing" (tm) for them, thanks. So the whole
> point of UC_STRICT_RESTORE_SS flag is not for the software to
> control it, but just for the kernel itself, so that it knows from
> whether 32 or 64 bit the signal came. This is probably quite
> undocumented in both the comments and the patch description,
> and I was confused because the approaches we discussed before,
> were targeted on the flag that is written by user-space.

Yes, that's the idea.  I'll improve the comments.

> If this my
> understanding is correct and the flag is just an indication rather
> than a requested action, perhaps the name should be different,
> e.g. UC_SIG_FROM_32BIT or the like?
> Anyway, this is minor. :)
> I'll try to test the patch within a few days, thanks for you time!

No problem.  Thanks for being willing to test!

--Andy
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