libretro: report where the core was executing when it dies

[WizzardSK]

A crash in the Android core currently leaves nothing behind. The report in #159 is a RetroArch log that stops mid-sentence in the middle of microVU1 compiling VU1 programs — RetroArch cannot log a signal that kills the process, and the core installs no reporter of its own. The only sigaction in the tree is the fastmem page fault filter.

So the question a crash in a recompiled core turns on goes unanswered: was the program counter in generated code, or in the core’s own text? A backtrace cannot say — JIT pages carry no unwind information, so the unwinder stops at the signal frame. /proc/self/maps can, because generated code lives in an anonymous mapping and compiled code lives in the .so.

This installs a handler for SIGSEGV, SIGBUS, SIGILL, SIGFPE, SIGABRT and SIGTRAP that prints the signal and its code, the faulting thread’s name, pc/lr/sp/fp and the fault address, and the /proc/self/maps line each of those falls in. Output goes to logcat as well as stderr, since logcat is where an Android crash is actually readable.

Ordering

It installs from retro_init(), before vtlb reaches HostSys::InstallPageFaultHandler(). That is the whole trick: fastmem takes SIGSEGV (and SIGBUS on aarch64) and chains to whatever it displaced when a fault is not one of its own, so going in first puts this on the end of that chain instead of out of it. Fastmem’s own handling is untouched and this only ever sees faults fastmem has already declined. Having reported, it chains onward to whatever was there before it, so the process still dies the way it would have.

Testing

Faulting on purpose, both ways round:

• a store to 0x1234 reports SIGSEGV, with pc and lr resolved to the executable and the fault address unmapped;
• jumping into an anonymous PROT_EXEC page holding an undefined encoding reports SIGILL, with pc in ... rwxp 00000000 00:00 0 and lr pointing back at the caller — the shape a codegen bug would take.

Both then die with the default action, 139 and 132. Compiles clean with -Wall on this base; non-POSIX targets get an empty CrashHandler_Install().