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

[Vineet Gupta <vineet.gupta@linux.dev>]

+CC David's new email

On 10/23/26 3:26 PM, Ihor Solodrai wrote:
> 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