Re: [PATCH] docs: Document pahole v1.26 requirement for KF_IMPLICIT_ARGS kfuncs

[Ihor Solodrai]

On 3/24/26 11:46 AM, zhidao su wrote:
> On Tue, 24 Mar 2026 08:12:12 -0700, Alexei Starovoitov wrote:
>> I don't think that's true.
>> At least when implicit args were designed the goal was to avoid
>> pahole dependencies.
>> Please share exact steps to reproduce.
> 
> Here are the exact reproduction steps and code path analysis.

Hi everyone, sorry I'm late to the party.

First of all, a *Nack* to the doc change in isolation, I agree with
Alexei here. Tejun, I suggest to revert it.

Doc change is insufficient and will make things only more confusing.

Kconfig checks for pahole version, as well as Makefile.btf. If we set
a new minimum version then it makes sense to assume it in the kernel
build code, and *disable CONFIG_DEBUG_INFO_BTF* if the version is not
recent enough. Also remove any flags/conditions that depend on pahole
being less than target minimum version. For reference see a patch
where I bumped the minimum version to 1.22 (903922cfa0e6):
https://lore.kernel.org/bpf/20251219181825.1289460-1-ihor.solodrai@xxxxxxxxx/

More importantly, the reported problem is real, but the actual fix is
a bump to v1.27, not v1.26. As usual AI got it about 80% right in the
analysis below (I assume this is AI output, it certainly looks like
one). Although to be fair AI was misled by a human error.

> 
> Reproduction (Ubuntu 24.04, pahole v1.25):
> 
>   $ git clone https://github.com/sched-ext/sched_ext.git
>   $ cd sched_ext && make -j$(nproc) LOCALVERSION=-test
>   $ make -C tools/testing/selftests/sched_ext
>   $ vng --run arch/x86/boot/bzImage --cpus 4 --memory 4G -- \
>       tools/testing/selftests/sched_ext/build/runner 2>&1 | grep "func_proto"
> 
> Result: 23/30 tests fail with:
>   libbpf: extern (func ksym) 'scx_bpf_create_dsq': func_proto [382]
>           incompatible with vmlinux [53813]

This is indeed an error you may get if BTF for kfuncs with
KF_IMPLICIT_ARGS is wrong.

> 
> Root cause:
> 
> The KF_IMPLICIT_ARGS mechanism requires pahole v1.26 for the DECL_TAG
> generation step that enables resolve_btfids to do its btf2btf work:
> 
> 1. scripts/Makefile.btf gates decl_tag_kfuncs on pahole >= 1.26:
> 
>      pahole-flags-$(call test-ge, $(pahole-ver), 126) = ... decl_tag_kfuncs

This is correct in that resolve_btfids expects that pahole ran with
decl_tag_kfuncs, but the version check is and was wrong.

decl_tag_kfuncs was implemented in pahole commit 72e88f29c [1], which
was released in v1.27. But on the kernel build side, the new feature
was simply appended (ebb79e96f1ea [2]) to the list of features under
1.26 check.

[1] https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?id=72e88f29c6f7e14201756e65bd66157427a61aaf
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v7.0-rc5&id=ebb79e96f1ea454fbcc8fe27dfe44e751bd74b4b

> 
> 2. Without decl_tag_kfuncs, pahole does not emit DECL_TAG BTF entries
>    for __bpf_kfunc-annotated functions.
> 
> 3. resolve_btfids/main.c::collect_kfuncs() (line 1002) early-returns
>    when nr_decl_tags == 0:
> 
>      if (!link->nr_decl_tags)
>          return 0;
> 
> 4. With no bpf_kfunc DECL_TAGs, btf2btf() never calls
>    process_kfunc_with_implicit_args() to create _impl variants and
>    strip 'aux' from the original proto.
> 
> 5. Result: vmlinux retains the 3-param proto (with 'aux') for all
>    KF_IMPLICIT_ARGS kfuncs.
> 
> BTF evidence from our pahole v1.25-compiled vmlinux:
> 
>   $ bpftool btf dump file vmlinux | grep -A5 '[53813]'
>   [53813] FUNC_PROTO '(anon)' ret_type_id=... vlen=3
>       'dsq_id' type_id=...
>       'node'   type_id=...
>       'aux'    type_id=...    <-- implicit arg still present, 3-param
>   (no scx_bpf_create_dsq_impl exists)

So far so good.

> 
> With pahole v1.26, resolve_btfids creates scx_bpf_create_dsq_impl
> (3-param, for verifier's find_kfunc_impl_proto) and rewrites
> scx_bpf_create_dsq to 2-param (for libbpf ksym matching).

And this is bullshit. The kernel build with pahole v1.26 has the same
problem:

  # Notice that 'bpf_wq_set_callback_impl' is absent from BTF
  $ bpftool btf dump file vmlinux | grep 'bpf_wq_set'
  [63890] FUNC 'bpf_wq_set_callback' type_id=9991 linkage=static
          'KF_bpf_wq_set_callback' val=29

vs expected (v1.27, v1.31):

  $ bpftool btf dump vmlinux | grep 'bpf_wq_set'
  [64253] FUNC 'bpf_wq_set_callback' type_id=51033 linkage=static
  [64254] FUNC 'bpf_wq_set_callback_impl' type_id=34861 linkage=static
          'KF_bpf_wq_set_callback' val=29

One could've figured this out if they tried to actually run the build.

> 
> You're right that the design goal was to avoid pahole dependencies -
> the implementation could be fixed in resolve_btfids to handle the
> no-DECL_TAG case. But until such a fix lands, the dependency exists
> in practice. Jonathan Corbet suggested raising the minimum version in
> the requirements table to 1.26, which seems the cleanest fix.
> 
> Signed-off-by: zhidao su <suzhidao@xxxxxxxxxx>


zhidao su, I suggest you prepare a proper "minimum pahole version
bump" patch, modifying kconfig checks, Makefile.btf and the
documentation. Please route it through the BPF tree and add me to cc.

Thanks.