Re: ARM Linux EABI: unwinding through a segfault handler

[Daniel Jacobowitz <drow@false.org>]

On Mon, Aug 29, 2011 at 11:18 AM, Ken Werner <ken.werner@linaro.org> wrote:
> On 08/25/2011 02:26 PM, Andrew Haley wrote:
>>
>> Throwing an exception through a segfault handler doesn't always work
>> on ARM: the attached example fails on current gcc trunk.
>>
>> panda-9:~ $ g++ segv.cc -fnon-call-exceptions -g
>> panda-9:~ $ ./a.out
>> terminate called after throwing an instance of 'FoobarException*'
>> Aborted
>>
>> The bug is that _Unwind_GetIPInfo doesn't correctly set ip_before_insn.
>> Instead, it always sets it to zero; it should be set to 1 if this
>> is a frame created by a signal handler:
>>
>>
>> #define _Unwind_GetIPInfo(context, ip_before_insn) \
>>   (*ip_before_insn = 0, _Unwind_GetGR (context, 15)&  ~(_Unwind_Word)1)
>>
>>
>> Fixing this on ARM is hard because signal frames aren't specially
>> marked as they are on systems that use DWARF unwinder data.  I have
>> a patch that works on systems where the signal restorer is exactly
>>
>>         mov     r7, $SYS_rt_sigreturn
>>         swi     0x0
>>
>> It works as a proof of concept, but it's fugly.
>>
>> So, suggestions welcome.  Is there a nice way to detect a signal frame?
>
> Libunwind also reads the IP to detect signal frames on ARM Linux:
> http://git.savannah.gnu.org/gitweb/?p=libunwind.git;a=blob;f=src/arm/Gis_signal_frame.c;hb=HEAD
>
> I'd also be interested if there are better approaches to detect them. :)

There aren't better ways - this is pretty much the standard for
on-stack signal frames :-)

I thought we used a handler in GLIBC that was properly annotated,
nowadays, but I might be mistaken.

-- 
Thanks,
Daniel