On Sat, Jul 12, 2014 at 2:17 PM, Andy Lutomirski <l...@amacapital.net> wrote: > On Sat, Jul 12, 2014 at 11:52 AM, Andi Kleen <a...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
