Messages in this thread | | | From | Andy Lutomirski <> | Date | Sat, 12 Jul 2014 14:17:08 -0700 | Subject | Re: [PATCH 2/2] x86_64,signal: Remove 'fs' and 'gs' from sigcontext |
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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.
--Andy
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