[PATCH bpf-next v2 0/6] resolve_btfids: Implement BTF tags emission for kfuncs

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

BTF data for the kernel is generated through the following pipeline:
  * DWARF is emitted by the compiler
  * pahole reads in DWARF and produces BTF
  * resolve_btfids makes kernel-specific btf2btf transformation and
    patches .BTF_ids section

This is orchestrated by link-vmlinux.sh, gen-btf.sh and Makefile.btf
in ./scripts directory.

Historically kernel-specific BTF features were implemented in pahole,
and controlled by the feature flags. This requires kernel build
process to be aware of pahole version used for the build to set
correct runtime arguments for BTF encoding [1].

This is a burden which can be alleviated by splitting kernel/module
BTF generation in two stages:
  1. Generic BTF generation from the kernel source code.
  2. Kernel-specific BTF modifications to support various BPF features.

So far both stages were fused in pahole's BTF encoding. By moving
stage (2) in-tree, the dependency of kernel build on pahole can become
much more loose.

resolve_btfids is already responsible for a few kernel-specific BTF
modifications:
  * .BTF.base generation for modules [2]
  * BTF sorting [3]
  * KF_IMPLICIT_ARGS support [4]

This series completes the migration by emitting BTF kfunc annotations
in-tree: the "bpf_kfunc" and "bpf_fastcall" decl tags and the arena
"address_space(1)" type attribute, dropping the corresponding pahole
feature flags.

The three annotations depend on two pahole feature flags:
"decl_tag_kfuncs" and "attributes".  Since emission is unconditional,
each flag has to be dropped in the same commit as the emission that
replaces it.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/scripts/Makefile.btf?h=v7.1-rc5
[2] https://docs.kernel.org/bpf/btf.html#btf-base-section
[3] https://lore.kernel.org/bpf/20260109130003.3313716-4-dolinux.peng@gmail.com/
[4] https://lore.kernel.org/bpf/20260120222638.3976562-1-ihor.solodrai@linux.dev/
[5] https://lore.kernel.org/bpf/20260722233518.778854-1-ihor.solodrai@linux.dev/
[6] https://lore.kernel.org/bpf/20260617210619.1562858-1-ihor.solodrai@linux.dev/

---

v1->v2:
  * The bottom part of v1 has already been landed [5][6].
  * New patch #1: run btf__dedup() in finalize_btf().
  * Drop the "ensure" pattern. Emission is unconditional; kbuild owns
    the pahole flags, so assume input BTF is not already tagged.
  * Each pahole flag is now dropped in the same commit as the emission
    that replaces it.
  * Fail hard with an error on invalid kfunc declarations such as an
    arena flag naming a missing argument or a non-pointer type.
  * Various cleanups and nits (Andrii, Emil, Jiri, Sashiko).
v1: https://lore.kernel.org/bpf/20260601221805.821394-1-ihor.solodrai@linux.dev/

---

Ihor Solodrai (6):
  resolve_btfids: Deduplicate BTF after btf2btf transformations
  resolve_btfids: Process KF_ARENA_* flags in resolve_btfids
  selftests/bpf: Verify arena type tags in resolve_btfids test
  resolve_btfids: Emit bpf_kfunc and bpf_fastcall decl tags
  selftests/bpf: Verify decl tags emission in resolve_btfids test
  docs, resolve_btfids: Document kfunc BTF annotation emission

 Documentation/bpf/kfuncs.rst                  |   8 +
 Documentation/process/changes.rst             |   7 +-
 scripts/Makefile.btf                          |   7 +-
 tools/bpf/resolve_btfids/main.c               | 192 +++++++++++++++++-
 .../selftests/bpf/prog_tests/resolve_btfids.c | 106 ++++++++++
 tools/testing/selftests/bpf/progs/btf_data.c  |  10 +
 6 files changed, 318 insertions(+), 12 deletions(-)

-- 
2.55.0