On 10/27/2011 12:53 PM, Paul Brook wrote: >> On 10/27/2011 02:15 AM, Paul Brook wrote: >>>>>> So, suggestions welcome. Is there a nice way to detect a signal >>>>>> frame? >>> >>> That just makes me ask why are you're trying to detect a signal frame in >>> the first place? >> >> Because I need backtrace() to work when called from a signal handler. > > You've missed several logic step there :-) On ARM the frame unwinding is > performed by the personality routine. The unwinding library has no business > poking its nose in.
OK, so I need a way to fix the personality routine. >>> Short story is that the standard EABI unwinding opcodes can't describe a >>> signal frame accurately. Especially if we don't know (when building >>> glibc) whether the application will be using VFP. >>> >>> The good news is that The EABI unwinding tables delegate all interesting >>> behavior to the personality routine (c.f. DWARF unwinding where the frame >>> description is parsed by libunwind). In order to accurately unwind >>> signal frames you need to have the sa_restorer functions reference a >>> custom personality routine that does the unwinding. >>> >>> If something is specifying a non-default sa_restorer, then you loose. >>> Don't Do That :-) >>> >>> While Richard is correct that the ARM EABI only requires unwinding >>> information at call sites, in practice as long as you use and accept the >>> limitations imposed by -fnon-call-exceptions, and ignore stack >>> overflows, it should be sufficient. >> >> Sure, but without the fix I provided it doesn't work. The problem is >> that the return address points to the faulting instruction, not the >> following instruction. In the generic unwinder code, the boolean >> ip_before_insn is set to cope with this. The ARM unwinder does not >> set this, so the backtrace does not work. > > IMO you're fixing the wrong problem. Instead you should advance the IP when > unwinding the frame. Given the unwinding tables for sa_restorer are already > borken you may as well fix both at once :-) I'm trying to understand what you mean, but your comments are rather obscure. Which file do you think I should change? Just tell me where to look and I'll do it. Andrew.
