[PATCH bpf-next v2 0/8] bpftool: Per-record CO-RE attributes in the C dump

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

bpftool's C dump wraps its output in a clang attribute pragma:

  #pragma clang attribute push (__attribute__((preserve_access_index)), apply_to = record)

GCC-BPF implements preserve_access_index, but not the pragma. Since
the commit 675b4e24bc50 ("bpf: Temporarily define
BPF_NO_PRESEVE_ACCESS_INDEX for GCC") the BPF selftests build GCC
objects with -DBPF_NO_PRESERVE_ACCESS_INDEX, which drops CO-RE from
vmlinux.h altogether.

This series replaces the pragma with an explicit attribute on each
record definition, and uses the same mechanism to annotate the context
types that the verifier rewrites with preserve_static_offset.

Jose proposed per-record attributes in May 2024 [1]. It stalled on a
requirement from preserve_static_offset (PSO) work [2] to add
attributes on selected types only, which a fixed string cannot
express.

BTF decl tags can carry an arbitrary attribute list since commit
51d1b1d42841 ("libbpf: Introduce kflag for type_tags and decl_tags in
BTF"), and btf_dump renders them on record definitions, since
76ea31239021 ("libbpf: Render decl_tags in btf_dump"). This allows
bpftool to put any attribute on any subset of records, with no new
libbpf API.

[1] https://lore.kernel.org/bpf/20240503111836.25275-1-jose.marchesi@oracle.com/
[2] https://lore.kernel.org/bpf/20231208000531.19179-1-eddyz87@gmail.com/

---

v1->v2:
  * Rename __bpf_core_pai to __bpf_relocatable
  * Rename __bpf_core_pso to __bpf_static_offset
  * Use a split BTF to add decl_tags in bpftoo instead of a detached
    copy (Alexei, Alan)
    * New patch "libbpf: Look up BTF strings without making BTF
      modifiable" lets btf__find_str() build its string index without
      making BTF modifiable
    * Dropped patch "bpftool: Dump C from a detached BTF copy"
  * The Clang crash on __builtin_preserve_field_info over PSO types is
    fixed in LLVM. PSO stays on by default; -D__bpf_static_offset=
    remains the documented workaround for Clang 18 through 23
  * GCC-BPF gets __bpf_relocatable from patch #3, so its narrow loads
    from context types fail verification until patch #4 (bpf-ci bot),
    left as is
  * Commit message and documentation rewording
v1: https://lore.kernel.org/bpf/20261016000012.2232059-1-ihor.solodrai@linux.dev/

---

Ihor Solodrai (8):
  libbpf: Look up BTF strings without making BTF modifiable
  selftests/bpf: Cover unknown BTF header-extension bytes in btf dump
  bpftool: Apply __bpf_relocatable per record on BTF dump
  bpftool: Skip __bpf_relocatable on verifier-rewritten context types
  bpftool: Emit __bpf_static_offset on verifier-rewritten context types
  selftests/bpf: Cover record attributes in bpftool BTF dumps
  selftests/bpf: Cover context and socket accesses through vmlinux.h
  bpftool: Document __bpf_relocatable and __bpf_static_offset

 .../bpf/bpftool/Documentation/bpftool-btf.rst |  24 +++
 tools/bpf/bpftool/btf.c                       | 154 ++++++++++++++++--
 tools/lib/bpf/btf.c                           |  36 ++--
 .../bpf/bpftool_btf_dump_sorted.expected      |  48 +++++-
 .../bpf/bpftool_btf_dump_unsorted.expected    |  48 +++++-
 .../bpf/prog_tests/bpftool_btf_dump.c         | 102 +++++++++++-
 .../selftests/bpf/prog_tests/verifier.c       |   2 +
 .../bpf/progs/verifier_ctx_rewritten.c        |  59 +++++++
 8 files changed, 423 insertions(+), 50 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/verifier_ctx_rewritten.c


base-commit: b116ac6087541d3d98031e786adbe9c733cd6df3
-- 
2.56.0