[PATCH bpf-next 0/4] Subskeleton support for BPF libraries

[Delyan Kratunov <delyank@fb.com>]

In the quest for ever more modularity, a new need has arisen - the ability to
access data associated with a BPF library from a corresponding userspace library.
The catch is that we don't want the userspace library to know about the structure of the
final BPF object that the BPF library is linked into.

In pursuit of this modularity, this patch series introduces *subskeletons.*
Subskeletons are similar in use and design to skeletons with a couple of differences:

1. The generated storage types do not rely on contiguous storage for the library's
variables because they may be interspersed randomly throughout the final BPF object's sections.

2. Subskeletons do not own objects and instead require a loaded bpf_object* to
be passed at runtime in order to be initialized. By extension, symbols are resolved at
runtime by parsing the final object's BTF. This has the interesting effect that the same
userspace code can interoperate with the library BPF code *linked into different final objects.*

3. Currently, only globals are supported though the codegen can be extended to support
non-owning pointers to maps, progs, links, etc.

Areas that are RFC/TODO:
* AFAICT, the ELF section names are the only way to find the correct maps in the final
linked object. As a result, I've added bpf_map__section_name so bpftool can use the section
names in the codegen. Do let me know if there's a better design I'm missing.

* The bpf_object__{open,destroy}_subskeleton approach mirrors the corresponding skeleton
support functionality. Do let me know if there's anything that needs to exist in it to ensure
forward compatibility. (Unfortunately, I don't see any way for subskeletons to work with older
libbpf versions, so I'd rather introduce all the new APIs they may need in a single version.)


Delyan Kratunov (4):
  libbpf: expose map elf section name
  bpftool: add support for subskeletons
  libbpf: add subskeleton scaffolding
  selftests/bpf: test subskeleton functionality

 tools/bpf/bpftool/gen.c                       | 322 +++++++++++++++++-
 tools/lib/bpf/libbpf.c                        |  84 +++++
 tools/lib/bpf/libbpf.h                        |  23 ++
 tools/lib/bpf/libbpf.map                      |   7 +
 tools/lib/bpf/libbpf_version.h                |   2 +-
 tools/testing/selftests/bpf/Makefile          |  18 +-
 .../selftests/bpf/prog_tests/subskeleton.c    |  38 +++
 .../bpf/prog_tests/subskeleton_lib.c          |  29 ++
 .../selftests/bpf/progs/test_subskeleton.c    |  20 ++
 .../bpf/progs/test_subskeleton_lib.c          |  22 ++
 10 files changed, 553 insertions(+), 12 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/subskeleton.c
 create mode 100644 tools/testing/selftests/bpf/prog_tests/subskeleton_lib.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_subskeleton.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_subskeleton_lib.c

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2.34.1