[Mesa-dev] [PATCH 1/2] util: Add utility build-id code.

[Nicholas Miell]

On 02/14/2017 12:30 PM, Chad Versace wrote:
> On Tue 14 Feb 2017, Matt Turner wrote:
>> Provides the ability to read the .note.gnu.build-id section of ELF
>> binaries, which is inserted by the --build-id=... flag to ld.
>> ---
>>  configure.ac              |   2 +
>>  src/util/Makefile.sources |   2 +
>>  src/util/build_id.c       | 109 ++++++++++++++++++++++++++++++++++++++++++++++
>>  src/util/build_id.h       |  56 ++++++++++++++++++++++++
>>  4 files changed, 169 insertions(+)
>>  create mode 100644 src/util/build_id.c
>>  create mode 100644 src/util/build_id.h
>
>
>> +AC_CHECK_FUNC([dl_iterate_phdr], [DEFINES="$DEFINES -DHAVE_DL_ITERATE_PHDR"])
>
> Nice. I wasn't aware of dl_iterate_phdr(). My code for querying the
> build-id was less slick. It used open(2) on the library, then manually
> parsed the ElfW(Ehdr) and ElfW(Shdr) to find the build-id node.

I also reinvented the build ID lookup wheel and just to record this 
knowledge publicly for posterity:

The struct link_map l_addr field seems to corresponds to the struct 
dl_phdr_info dlpi_addr field.

You can retrieve the struct link_map for a symbol by passing the 
RTLD_DL_LINKMAP flag to dladdr1() or for a library handle returned by 
dlopen() by passing RTLD_DI_LINKMAP to dlinfo().

This means you can find the note section for a loaded library directly 
without having to resort to string comparisons against library names, 
which is probably more future-proof.