linux-kernel - Re: sigaltstack breaks swapcontext()

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Date:	Mon, 7 Mar 2016 23:02:25 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: sigaltstack breaks swapcontext()

09.01.2016 04:48, Andy Lutomirski пишет:
> On Fri, Jan 8, 2016 at 5:43 PM, Stas Sergeev <stsp@...t.ru> wrote:
>> 09.01.2016 02:24, Andy Lutomirski пишет:
>>> 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?