Re: [PATCH 2/2] x86_64,signal: Remove 'fs' and 'gs' from sigcontext

[Andy Lutomirski <luto@amacapital.net>]

On Sat, Jul 12, 2014 at 2:17 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Sat, Jul 12, 2014 at 11:52 AM, Andi Kleen <andi@firstfloor.org> wrote:
>> On Sat, Jul 12, 2014 at 11:40:03AM -0700, H. Peter Anvin wrote:
>>> Because you are doing something weird (like Pin, for example) and take an asynchronous fault?
>>
>> But even for pin that would need executing 16 bit code, or really weird
>> 32bit code. AFAIK for 32bit the only good use case was NX emulation
>> (and old virtualization) which are both completely obsolete.
>
> Nothing particularly weird is needed.  Set a non-default stack segment
> (e.g. any 16-bit ss) and take *any* fault.  This could be something
> asynchronous or even a normal synchronous fault.  Return from the
> signal handler: boom.
>
> We know that people use 16-bit stack segments: it's what prompted the
> whole espfix64 thing.
>
>>
>> I don't think it's worth messing with the signal handlers for 16bit
>> code. If there's any problem with saving/restoring state that emulator
>> can always handle it by itself.
>>
>
> How?
>
> I can think of at least two vaguely feasible ways.  The program could
> ptrace itself, trap sigreturn, and fix ss.  Or it could restore
> registers itself and return using far ret or iret.  Both suck.

You can't even hack around this with ptrace -- ptrace silently fails
to write ss for non-TIF_IA32 tasks.

--Andy