[LLVMdev] Verifying unwind info/debugging a crash in _Unwind_Backtrace() on OSX

[Alexander Potapenko]

Turns out to be a real bug in the X86 asm backend:
http://llvm.org/bugs/show_bug.cgi?id=20800
The compact unwind info for this function contains the wrong stack frame size.

On Thu, Aug 28, 2014 at 6:12 PM, Alexander Potapenko <glider at google.com> wrote:
> Turns out OSX uses a different format of unwind info, the so-called
> compact unwind encoding.
> Some of its contents can be obtained from a thin (non-universal)
> binary by `llvm-objdump -unwind-info`
> Right now I'm trying to make Clang use the _Unwind_Backtrace
> implementation from libc++abi instead of the system one so that it's
> possible to debug it.
>
> On Wed, Aug 27, 2014 at 6:48 PM, Yury Gribov <y.gribov at samsung.com> wrote:
>>> Is there a way to check whether the unwind info is correct? Could this
>>> be a bug in the system implementation of _Unwind_Backtrace, which
>>> probably should not crash in the middle of a valid stack trace?
>>
>> I'm not experienced with OSX but AFAIK unwinders generally trust unwind
>> bytecode so if the latter is broken and causes SP miscalculation, unwinder
>> will happily read invalid memory and segfault.
>>
>> You may want to examine unwind tables for Asan runtime (via readelf -u) and
>> compare those to e.g. assembler code of __asan_report_error.
>>
>> -Y
>
>
>
> --
> Alexander Potapenko
> Software Engineer
> Google Moscow



-- 
Alexander Potapenko
Software Engineer
Google Moscow