PATCH: PR other/48007: Unwind library doesn't work with UNITS_PER_WORD > sizeof (void *)

[H.J. Lu]

On Tue, Mar 22, 2011 at 3:17 PM, Ian Lance Taylor <iant@google.com> wrote:
> "Ulrich Weigand" <uweigand@de.ibm.com> writes:
>
>> Ian Lance Taylor wrote:
>>> Andrew Pinski <pinskia@gmail.com> writes:
>>> > On Tue, Mar 22, 2011 at 9:42 AM, Ian Lance Taylor <iant@google.com> wrote:
>>> >>
>>> >> Any target on which _Unwind_Word is larger than void * is broken today,
>>> >> so I don't think we need to care about that case.
>>> >
>>> > So a MIPS N32 is broken?  Lots of people use that target already and
>>> > nothing like this has showed up yet.
>>>
>>> That is a fair question.  It does seem to me that it must be broken in
>>> some cases.  _Unwind_GetGRPtr will return &context->reg[index], which is
>>> a void** cast to void*.  We will then pass that to _Unwind_SetGRPtr.  If
>>> we later call _Unwind_SetGR on that register, it will write a value of
>>> size _Unwind_Word through that pointer.  Similarly if we call
>>> _Unwind_GetGR, it will read a value of size _Unwind_Word.  In both
>>> cases, we will be accessing a 4-byte field as an 8-byte value.
>>>
>>> If MIPS N32 works today, then something must be ensuring that that
>>> sequence can never occur, or that for some reason it never matters.
>>
>> Well, the whole problem only occurs when using the "by value" mechanism,
>> which is only triggered by DW_CFA_val_... CFI statements, which are new
>> in Dwarf-3 and seem to be very rarely used: they are apparently never
>> generated by GCC, and the only files I could find in current glibc that
>> create such CFI by hand are Intel-specific.
>
> I see, thanks.
>
> H.J., what is the failure mode that you are seeing?

Yes, those come from hand written CFI statements for signal handler.

> This suggests that, at least, when sizeof(_Unwind_Word) > sizeof(void*),
> we need to extend _Unwind_Context with a new array at the end, and use
> that new array when by_value[REG] is true.

Will there be a case where 2 copies of unwind-dw2.c are within the same
process?

-- 
H.J.