ARM Linux EABI: unwinding through a segfault handler

[Andrew Haley]

On 08/25/2011 05:57 PM, David Daney wrote:
> On 08/25/2011 05:26 AM, 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.
> 
> For what it's worth, I did the equivalent on MIPS.
> 
> Once you do this, it is a de facto ABI.  Probably the ARM linux 
> maintainers should be consulted to see if they are willing to consider 
> the possibility of never changing it.

I think Joseph Meyers is the author of both ends of this, and I think he
reads this list.

Andrew.