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

On 8/5/26 5:01 PM, [email protected] wrote:
>> diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
>> index cbde86d082cce..c60fc574e8b06 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
>> +kernel's BTF; these annotations were historically produced by pahole. For each
>> +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).
> 
> Emil Tsalapatis suggested in v1 replacing "address_space(1)" with "arena"
> throughout the documentation for better readability. This nit was not
> addressed in v2.
> 
> The term "address_space(1)" appears in four locations across this commit
> (kfuncs.rst, changes.rst, Makefile.btf, and main.c). Would using "arena"
> be clearer, especially since the code already defines TYPE_ATTR_ARENA?

Nonsense feedback from the bot. Ignoring.

> 
>> +
>>  2.7  Specifying no-cast aliases with ___init
>>  --------------------------------------------
>>
>> diff --git a/Documentation/process/changes.rst b/Documentation/process/changes.rst
>> index 1ca8c5f73ad00..6d1dbe4abf0f0 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.
>>
>>  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 a1812985a61aa..717e76ce96a71 100644
>> --- a/scripts/Makefile.btf
>> +++ b/scripts/Makefile.btf
>> @@ -14,6 +14,9 @@ pahole-flags-$(call test-ge, $(pahole-ver), 125)	+= --skip_encoding_btf_inconsis
>>  else
>>
>>  # Switch to using --btf_features for v1.26 and later.
>> +#
>> +# kfunc BTF annotations (bpf_kfunc/bpf_fastcall decl tags and the arena
>> +# address_space(1) type attribute) are emitted by resolve_btfids, not pahole.
>>  pahole-flags-$(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
>> diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
>> index ea25ca34aa006..66f4014e6aa17 100644
>> --- a/tools/bpf/resolve_btfids/main.c
>> +++ b/tools/bpf/resolve_btfids/main.c
>> @@ -58,6 +58,17 @@
>>   *             __BTF_ID__func__vfs_fallocate__5:
>>   *             .zero 4
>>   *	       .word (1 << 3) | (1 << 1) | (1 << 2)
>> + *
>> + * In addition to resolving BTF IDs, resolve_btfids performs kernel-specific
>> + * BTF-to-BTF transformations for kfuncs found in BTF_SET8_KFUNCS sets. For
>> + * each such kfunc it:
>> + *
>> + *   - emits a "bpf_kfunc" decl tag, and "bpf_fastcall" when KF_FASTCALL is set;
>> + *   - wraps the return value and/or arguments flagged KF_ARENA_RET,
>> + *     KF_ARENA_ARG1 or KF_ARENA_ARG2 with the "address_space(1)" type attribute;
>> + *   - rewrites the prototype of KF_IMPLICIT_ARGS kfuncs.
>> + *
>> + * These kfunc annotations were historically produced by pahole.
>>   */
>>
>>  #define  _GNU_SOURCE
> 
> 
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
> 
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/31056256483