Re: [PATCH 3/3] dwarf-reader: Ignore zero length location expressions from DW_AT_location

[Matthias Maennich <maennich@google.com>]

Hi Mark!

On Thu, Oct 29, 2020 at 03:00:20PM +0100, Mark Wielaard wrote:
>Hi,
>
>On Thu, 2020-10-29 at 12:30 +0000, Matthias Maennich wrote:
>> On Thu, Oct 29, 2020 at 12:21:00PM +0000, Matthias Maennich wrote:
>> > Location expressions might occasionally be of length 0. E.g. a reason
>> > for that are thread local variables that do not exactly have a location
>> > to refer to. Compilers/Linkers may choose an odd representation. E.g.
>> > see the dwarfdump output for the added testcase based on libandroid.so
>> > (from AOSP).
>
>I don't think it is "odd", it is simply an empty location expression,
>which is defined as no location known.

You are right. I will use the 'empty location expression' terminology
instead.

>
>> > The DW_AT_location is properly read by elfutils' dwarf_location(), but
>> > is not useful for us to proceed with. Hence early exit on this.
>> >
>> > 	* src/abg-dwarf-reader.cc (die_location_expr): Ignore zero
>> > 	  length location expressions.
>> > 	* tests/data/Makefile.am: Add new test files.
>> > 	* tests/data/test-read-dwarf/test-libandroid.so: New test file.
>> > 	* tests/data/test-read-dwarf/test-libandroid.so.abi: Likewise.
>> > 	* tests/test-read-dwarf.cc: Add new test case.
>> >
>> > Reported-by: Dan Albert <danalbert@google.com>
>> > Reviewed-by: Giuliano Procida <gprocida@google.com>
>> > Cc: Mark Wielaard <mark@klomp.org>
>> > Signed-off-by: Matthias Maennich <maennich@google.com>
>
>> [...]
>
>>   // Ignore invalid location expressions where reading them succeeded but their
>>   // length is 0.
>>   result &= len > 0;
>
>The actual code change looks correct, but I would like the comment to
>not say they are invalid (if they were dwarf_getlocation would have
>returned an error), but that an empty location expression represents an
>unknown location (and so it is indeed not very useful).
>
>See also DWARFv5 2.6.1.1.1 Empty Location Descriptions
>
>   An empty location description consists of a DWARF expression
>   containing no operations. It represents a piece or all of an object
>   that is present in the source but not in the object code (perhaps
>   due to optimization).
>

I dropped the "invalid" wording everywhere after your last comment, but
forgot to change this very line :-/

I updated code change and commit message and pushed a new version to
Github:
https://github.com/metti/libabigail/commit/eb8c694cbc1e8a8c3d08872930af1a30df9f6d05

Cheers,
Matthias

>Cheers,
>
>Mark