Re: [PATCH] libdw: fix offset for sig8 lookup in dwarf_formref_die

[Josh Stone <jistone at redhat dot com>]

On 01/14/2015 08:47 AM, Mark Wielaard wrote:
> On Wed, 2015-01-14 at 09:26 -0500, Jason P. Leasure wrote:
>> The type_offset of a type unit header is relative to the beginning
>> of the type unit header.
>>
>> Signed-off-by: Jason P. Leasure <jpleasu@super.org>
>> ---
>>  libdw/dwarf_formref_die.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/libdw/dwarf_formref_die.c b/libdw/dwarf_formref_die.c
>> index 63f6697..8b92e22 100644
>> --- a/libdw/dwarf_formref_die.c
>> +++ b/libdw/dwarf_formref_die.c
>> @@ -95,7 +95,7 @@ dwarf_formref_die (attr, result)
>>  
>>        datap = cu->dbg->sectiondata[IDX_debug_types]->d_buf;
>>        size = cu->dbg->sectiondata[IDX_debug_types]->d_size;
>> -      offset = cu->type_offset;
>> +      offset = cu->start + cu->type_offset;
> 
> Thanks, I believe this is correct. I am surprised we didn't encounter
> this earlier. Do you happen to have a testcase for it?

It's a regression from commit 9202665816763, before which cu->start was
used with the offset everywhere.

I can see this in my dwarvish tool with Jason's example source.  You
just need a ref_sig8 that's not in the first type_unit, cu->start > 0.
So here, struct A has a ref_sig8 to struct B in the second type_unit.

In the bad case I see "signature  ref_sig8  [30] 0", where those last
two bits are supposed to be the offset and tag.

I see "signature  ref_sig8  [72] structure_type" with 0.160, or with
master and this patch, and it expands the tree of attributes from there.

Of course you won't want a GUI for tests, but it should be easy to craft
this one directly.