Re: [Libunwind-devel] local vars

[Lassi Tuura]

Hi,

> It's actually a little messier even than that, because the whole thing
> about backing up one call instruction only makes sense for regular
> stack frames. A signal handler should not back up any instructions.
> (We ran into a related but different problem recently -- see
> https://bugzilla.mozilla.org/show_bug.cgi?id=695985 ).

We actually know the state inside libunwind, it's dwarf_cursor's use_prev_instr.
If set we subtract one from the IP before use (= call), if zero we use IP as-is
(= signal, etc.). Indeed you have to be very careful not to look after the call
instruction - doing so does result in bad unwind decisions in real life. It's
not unusual for compiler to place 'call' as a last instruction in a function
(and thus FDE), or for the very next instruction to push/pop stack.

There's unw_is_signal_frame() to return the info, sort of. There isn't an API to
retrieve use_prev_instr as such that I know of.

The fast trace code (= unw_backtrace()) accounts for use_prev_instr to adjust 
the
IP addresses it reports. For other unwind methods you can use 
unw_is_signal_frame()
instead and subtract one from returned IP if it returns zero.

Regards,
Lassi