pw_build_info/docs.rst

.. _module-pw_build_info:

=============
pw_build_info
=============

.. warning::
  This module is incomplete, but the build ID integration is ready for use.

pw_build_info provides tooling, build integration, and libraries for generating,
embedding, and parsing build-related information that is embedded into
binaries. Simple numeric version numbering doesn't typically express things
like where the binary originated, what devices it's compatible with, whether
local changes were present when the binary was built, and more. pw_build_info
simplifies the process of integrating rich version metadata to answer more
complex questions about compiled binaries.

-------------
GNU build IDs
-------------
This module provides C++ and python libraries for reading GNU build IDs
generated by the link step of a C++ executable. These build IDs are essentially
hashes of the final linked binary, meaning two identical binaries will have
identical build IDs. This can be used to accurately identify matching
binaries.

Linux executables that depend on the ``build_id`` GN target will automatically
generate GNU build IDs. Windows and macOS binaries cannot use this target as
the implementation of GNU build IDs depends on the ELF file format.

Getting started
===============
To generate GNU build IDs as part of your firmware image, you'll need to update
your embedded target's linker script.

Updating your linker script
---------------------------
If your project has a custom linker scipt, you'll need to update it to include
a section to contain the generated build ID. This section should be placed
alongside the ``.text`` and ``.rodata`` sections, and named
``.note.gnu.build-id``.

.. code-block:: none

  /* Main executable code. */
  .code : ALIGN(4)
  {
    . = ALIGN(4);
    /* Application code. */
    *(.text)
    *(.text*)
    KEEP(*(.init))
    KEEP(*(.fini))
    ...
  } >FLASH

  /* GNU build ID section. */
  .note.gnu.build-id :
  {
    . = ALIGN(4);
    gnu_build_id_begin = .;
    *(.note.gnu.build-id);
  } >FLASH

  /* Explicitly initialized global and static data. (.data) */
  .static_init_ram : ALIGN(4)
  {
    *(.data)
    *(.data*)
    ...
  } >RAM AT> FLASH


Alternatively, you can copy the following linker snippet into a pre-existing
section. This makes reading the build ID slower, so whenever possibe prefer
creating a dedicated section for the build ID.

.. literalinclude:: build_id_linker_snippet.ld

An example of directly inserting a build ID into an existing section is
provided below:

.. code-block:: none

  /* Main executable code. */
  .code : ALIGN(4)
  {
    . = ALIGN(4);
    /* Application code. */
    *(.text)
    *(.text*)
    KEEP(*(.init))
    KEEP(*(.fini))

    . = ALIGN(4);
    gnu_build_id_begin = .;
    *(.note.gnu.build-id);

    ...
  } >FLASH

If your linker script is auto-generated, you may be able to use the
``INSERT AFTER`` linker script directive to append the build ID as seen in the
Linux host support for pw_build_info's build ID integration:

.. literalinclude:: add_build_id_to_default_linker_script.ld

Generating the build ID
-----------------------
When you depend on ``"$dir_pw_build_info:build_id``, a GNU build ID will be
generated at the final link step of any binaries that depend on that library
(whether directly or transitively). Those binaries will be able to read the
build ID by calling ``pw::build_info::BuildId()``. Note that the build ID
is not a string, but raw binary data, so to print it you'll need to convert
it to hex or base64.

Python API reference
====================

.. py:function:: read_build_id_from_section(elf_file: BinaryIO) -> \
                     Optional[bytes]

  Reads a GNU build ID from an ELF binary by searching for a
  ``.note.gnu.build-id`` section.

.. py:function:: read_build_id_from_symbol(elf_file: BinaryIO) -> \
                     Optional[bytes]

  Reads a GNU build ID from an ELF binary by searching for a
  ``gnu_build_id_begin`` symbol. This can be a rather slow operation.

.. py:function:: read_build_id(elf_file: BinaryIO) -> Optional[bytes]

  Reads a GNU build ID from an ELF binary, first checking for a GNU build ID
  section and then falling back to search for a ``gnu_build_id_begin`` symbol.

.. py:function:: find_matching_elf(uuid: bytes, search_dir: Path) -> \
                     Optional[Path]

  Recursively searches a directory for an ELF file with a matching UUID.

  Warning: This can take on the order of several seconds.

Python utility
==============
GNU build IDs can be parsed out of ELF files using the ``build_id`` python tool.
Simply point the tool to a binary with a GNU build ID and the build ID will be
printed out if it is found.

.. code-block:: sh

  $ python -m pw_build_info.build_id my_device_image.elf
  d43cce74f18522052f77a1fa3fb7a25fe33f40dd