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

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Date:	Wed, 14 Oct 2015 11:52:39 -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 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?

--Andy
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