Messages in this thread | | | From | Andy Lutomirski <> | Date | Thu, 17 Jul 2014 18:13:12 -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 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
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