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

Andy Lutomirski
2015-10-14T18:53:00Z

On Wed, Oct 14, 2015 at 11:34 AM, Stas Sergeev <stsp@list.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