[v2,bpf-next,0/9] libbpf: BTF writer APIs

Message

Andrii Nakryiko Sept. 26, 2020, 1:13 a.m. UTC

This patch set introduces a new set of BTF APIs to libbpf that allow to
conveniently produce BTF types and strings. Internals of struct btf were
changed such that it can transparently and automatically switch to writable
mode, which allows appending BTF types and strings. This will allow for libbpf
itself to do more intrusive modifications of program's BTF (by rewriting it,
at least as of right now), which is necessary for the upcoming libbpf static
linking. But they are complete and generic, so can be adopted by anyone who
has a need to produce BTF type information.

One such example outside of libbpf is pahole, which was actually converted to
these APIs (locally, pending landing of these changes in libbpf) completely
and shows reduction in amount of custom pahole code necessary and brings nice
savings in memory usage (about 370MB reduction at peak for my kernel
configuration) and even BTF deduplication times (one second reduction,
23.7s -> 22.7s). Memory savings are due to avoiding pahole's own copy of
"uncompressed" raw BTF data. Time reduction comes from faster string
search and deduplication by relying on hashmap instead of BST used by pahole's
own code. Consequently, these APIs are already tested on real-world
complicated kernel BTF, but there is also pretty extensive selftest doing
extra validations.

Selftests in patch #9 add a set of generic ASSERT_{EQ,STREQ,ERR,OK} macros
that are useful for writing shorter and less repretitive selftests. I decided
to keep them local to that selftest for now, but if they prove to be useful in
more contexts we should move them to test_progs.h. And few more (e.g.,
inequality tests) macros are probably necessary to have a more complete set.

Cc: Arnaldo Carvalho de Melo <[e-mail]>

v1->v2:
  - fixed comments (John);
  - renamed btf__append_xxx() into btf__add_xxx() (Alexei);
  - added btf__find_str() in addition to btf__add_str();
  - btf__new_empty() now sets kernel FD to -1 initially.

Andrii Nakryiko (9):
  libbpf: refactor internals of BTF type index
  libbpf: remove assumption of single contiguous memory for BTF data
  libbpf: generalize common logic for managing dynamically-sized arrays
  libbpf: extract generic string hashing function for reuse
  libbpf: allow modification of BTF and add btf__add_str API
  libbpf: add btf__new_empty() to create an empty BTF object
  libbpf: add BTF writing APIs
  libbpf: add btf__str_by_offset() as a more generic variant of
    name_by_offset
  selftests/bpf: test BTF writing APIs

 tools/lib/bpf/bpf.c                           |    2 +-
 tools/lib/bpf/bpf.h                           |    2 +-
 tools/lib/bpf/btf.c                           | 1343 +++++++++++++++--
 tools/lib/bpf/btf.h                           |   44 +
 tools/lib/bpf/btf_dump.c                      |    9 +-
 tools/lib/bpf/hashmap.h                       |   12 +
 tools/lib/bpf/libbpf.map                      |   23 +
 tools/lib/bpf/libbpf_internal.h               |    3 +
 .../selftests/bpf/prog_tests/btf_write.c      |  278 ++++
 9 files changed, 1596 insertions(+), 120 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/btf_write.c