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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.

A separate GN target build_id_or_noop is available which provides an empty build ID on platforms where GNU build ID is not available while still providing a real GNU build ID where supported.

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.

/* 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:

/* 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. It is possible to call pw::build_info::LogBuildId() function to print it (as hexadecimal).

Python API reference

.. py:function:: read_build_id_from_section(elf_file: BinaryIO) -> \
                     bytes | None

  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) -> \
                     bytes | None

  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) -> bytes | None

  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) -> \
                     Path | None

  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.

$ python -m pw_build_info.build_id my_device_image.elf
d43cce74f18522052f77a1fa3fb7a25fe33f40dd