Re: [PATCH bpf-next v2 6/6] docs, resolve_btfids: Document kfunc BTF annotation emission

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2026-08-05 at 16:06 -0700, Ihor Solodrai wrote:

...

> diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
> index cbde86d082cc..c60fc574e8b0 100644
> --- a/Documentation/bpf/kfuncs.rst
> +++ b/Documentation/bpf/kfuncs.rst
> @@ -472,6 +472,14 @@ type. An example is shown below::
>          }
>          late_initcall(init_subsystem);
>  
> +At kernel build time the ``resolve_btfids`` tool discovers all kfuncs from the
> +registered ``BTF_SET8_KFUNCS`` sets and emits their BTF annotations into the

Note that this is a single occurrence of the word BTF_SET8_KFUNCS in
this .rst file. Also The wording "registered" is confusing as the
above code snippet shows the usage of register_btf_kfunc_id_set()
function, which is completely unrelated.

> +kernel's BTF; these annotations were historically produced by pahole. For each

I'd skip a note about pahole, it does not convey usable information.

> +discovered kfunc ``resolve_btfids`` emits a ``bpf_kfunc`` BTF decl tag, a
> +``bpf_fastcall`` decl tag when the kfunc is flagged ``KF_FASTCALL``, and the
> +``address_space(1)`` type attribute on the return value and/or arguments flagged
> +``KF_ARENA_RET``, ``KF_ARENA_ARG1`` or ``KF_ARENA_ARG2`` (see section 2.8).
> +
>  2.7  Specifying no-cast aliases with ___init
>  --------------------------------------------
>  
> diff --git a/Documentation/process/changes.rst b/Documentation/process/changes.rst
> index 1ca8c5f73ad0..6d1dbe4abf0f 100644
> --- a/Documentation/process/changes.rst
> +++ b/Documentation/process/changes.rst
> @@ -147,10 +147,9 @@ Since Linux 5.2, if CONFIG_DEBUG_INFO_BTF is selected, the build system
>  generates BTF (BPF Type Format) from DWARF in vmlinux, a bit later from kernel
>  modules as well.  This requires pahole v1.22 or later.
>  
> -Since Linux 7.0, kfuncs annotated with KF_IMPLICIT_ARGS require pahole v1.26
> -or later.  Without it, such kfuncs will have incorrect BTF prototypes in
> -vmlinux, causing BPF programs to fail to load with a "func_proto incompatible
> -with vmlinux" error.  Many sched_ext kfuncs are affected.
> +Kfunc BTF annotations (the bpf_kfunc and bpf_fastcall decl tags and the arena
> +address_space(1) type attribute) are emitted in-tree by resolve_btfids from the
> +BTF_KFUNCS sets, so they no longer depend on a specific pahole version.

Just drop the whole paragraph?

>  
>  It is found in the 'dwarves' or 'pahole' distro packages or from
>  https://fedorapeople.org/~acme/dwarves/.
> diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
> index a1812985a61a..717e76ce96a7 100644
> --- a/scripts/Makefile.btf
> +++ b/scripts/Makefile.btf
> @@ -14,6 +14,9 @@ pahole-flags-$(call test-ge, $(pahole-ver), 125)	+= --skip_encoding_btf_inconsis
>  else
>  
>  # Switch to using --btf_features for v1.26 and later.
> +#
> +# kfunc BTF annotations (bpf_kfunc/bpf_fastcall decl tags and the arena
> +# address_space(1) type attribute) are emitted by resolve_btfids, not pahole.

What's the point of this comment?

>  pahole-flags-$(call test-ge, $(pahole-ver), 126)  = -j$(JOBS) --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func
>  
>  pahole-flags-$(call test-ge, $(pahole-ver), 131) += --btf_features=layout

...