[PATCH bpf-next v1 0/4] libbpf: Render decl_tags in btf_dump

[Ihor Solodrai <ihor.solodrai@linux.dev>]

BTF supports arbitrary __attribute__ encoding via decl tags and type
tags [1]. However btf_dump facility only outputs the attributes
encoded in BTF as type tags, which was implemented in commit
2019c58318b8 ("libbpf: Check the kflag of type tags in btf_dump").

BTF_KIND_DECL_TAG has been left out of btf_dump entirely, independent
of the kflag value. An attribute a BTF producer encoded is silently
dropped from the C output.

Implement decl_tag support in btf_dump. The btf_dump limits rendered
BTF types to records, record members and typedefs. Other types,
particularly functions, are deliberately skipped. Therefore decl_tag
rendering works only for the same subset of target types.

To be efficient, decl_tag dump support requires a reverse index from
the target type id to the list of tags pointing at it.

With decl_tag dump support, provided a validly constructed BTF, it is
now possible to obtain C-syntax output like the following example:

	struct foo {
		int a __attribute__((bar));
		int b: 3 __attribute__((bar));
	} __attribute__((bar));
	typedef struct { ... } __attribute__((bar)) foo_t __attribute__((baz));

vmlinux.h generated in-tree remains unchanged: every decl tag in
vmlinux BTF is a tag on a FUNC, which is not supported by btf_dump.

[1] https://lore.kernel.org/bpf/20250130201239.1429648-1-ihor.solodrai@linux.dev/

---

The series consists of the following:

  * patch #1 is a non-functional refactoring, preparing
    btf_dump_resize() for decl_tag index build
  * patch #2 implements the full decl_tag dump support
  * patch #3 adds the selftests to cover the new feature
  * patch #4 is an independent selftests change relevant to kflagged
    decl_tag handling

Ihor Solodrai (4):
  libbpf: Walk types in btf_dump_resize(), not in mark_referenced()
  libbpf: Render decl_tags in btf_dump
  selftests/bpf: Test btf_dump rendering of decl_tags
  selftests/bpf: Show the tag kflag in the raw BTF dump helper

 tools/lib/bpf/btf_dump.c                      | 210 ++++++++---
 tools/testing/selftests/bpf/btf_helpers.c     |  10 +-
 .../selftests/bpf/prog_tests/btf_dump.c       | 343 +++++++++++++++++-
 .../selftests/bpf/prog_tests/btf_write.c      |  60 +++
 4 files changed, 560 insertions(+), 63 deletions(-)


base-commit: 10c4f610b215bf961235141161992f010cf7e451
-- 
2.55.0