Re: sigaltstack breaks swapcontext()

From: Andy Lutomirski
Date: Mon Mar 07 2016 - 16:10:35 EST

On Mon, Mar 7, 2016 at 12:02 PM, Stas Sergeev <stsp@xxxxxxx> wrote:
> 09.01.2016 04:48, Andy Lutomirski ÐÐÑÐÑ:
>> On Fri, Jan 8, 2016 at 5:43 PM, Stas Sergeev <stsp@xxxxxxx> wrote:
>>> 09.01.2016 02:24, Andy Lutomirski ÐÐÑÐÑ:
>>>> It's not sigaltstack that I'm thinking about. It's signal delivery.
>>>> If you end up in DOS mode with SP coincidentally pointing to the
>>>> sigaltstack (but with different SS so it's not really the
>>>> sigaltstack), then the signal delivery will malfunction.
>>> Will you take care of this one?
>>> Looks quite dangerous for dosemu! And absolutely
>>> undebuggable: you never know when you hit it.
>> I'll try to remember to tack it on to the sigcontext series.
> How is this one going?
> There seem to be one more bug in sigcontext handling.
> dosemu have this code:
> ---
> /*
> * FIRST thing to do in signal handlers - to avoid being trapped into
> int0x11
> * forever, we must restore the eflags.
> */
> loadflags(eflags_fs_gs.eflags);
> ---
> I quickly checked the kernel code, and it seems the
> flags are indeed forgotten, even on ia32! I think the
> most dangerous flags are AC and NT. But most of
> others are important too. IMHO the safe defaults
> should be forced when entering the sighandler.
> Would you mind taking a look at this problem too?

Clearing NT seems sane.

Clearing AC seems like an ABI break, so I'd be a bit nervous about
clearing AC unconditionally. We could add yet another SS flag (sigh),
or we could make the change. As a more conservative option, we could
make it so that AC is cleared on entry to an alignment check signal.


Andy Lutomirski
AMA Capital Management, LLC