Re: [PATCH] kbuild: fix decl_tag_kfuncs gating to require pahole v1.27

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

On 3/26/26 5:24 PM, zhidao su wrote:
> decl_tag_kfuncs was implemented in pahole v1.27 (dwarves commit
> 72e88f29c6f7 [1]).  However, ebb79e96f1ea ("kbuild: bpf: Tell pahole
> to DECL_TAG kfuncs") appended it to the list of --btf_features flags
> already gated on pahole >= 1.26, making it silently inactive on any
> pahole v1.26 build.
> 
> Without decl_tag_kfuncs, pahole does not emit DECL_TAG BTF entries for
> __bpf_kfunc-annotated functions.  resolve_btfids then skips
> process_kfunc_with_implicit_args(), so KF_IMPLICIT_ARGS kfuncs keep
> their full prototype (including the implicit aux parameter) in vmlinux
> BTF.  BPF programs loading such kfuncs as weak ksyms fail with:
> 
>   libbpf: extern (func ksym) 'scx_bpf_create_dsq': func_proto [N]
>           incompatible with vmlinux [M]
> 
> Fix by splitting decl_tag_kfuncs into its own >= 127 line in
> Makefile.btf.  Add a PAHOLE_HAS_DECL_TAG_KFUNCS Kconfig symbol so
> build rules can gate on this feature explicitly.  Update
> Documentation/process/changes.rst to reflect the corrected minimum
> pahole version for KF_IMPLICIT_ARGS support.
> 
> [1] https://github.com/acmel/dwarves/commit/72e88f29c6f7e14201756e65bd66157427a61aaf
> 
> Fixes: ebb79e96f1ea ("kbuild: bpf: Tell pahole to DECL_TAG kfuncs")
> Reported-by: Ihor Solodrai <ihor.solodrai@linux.dev>
> Closes: https://lore.kernel.org/bpf/e0ca748d-3204-4160-b37d-0f76cbac8c6c@linux.dev/

No. Please read the feedback behind this link carefully and check the
patch bumping to v1.22 before submitting a new version.

Hint: you can feed it to your "AI agent" too, it should be able to
figure it out.

We don't need a new kconfig flag to enforce higher minimum pahole version.

pw-bot: cr

> Signed-off-by: zhidao su <suzhidao@xiaomi.com>
> ---
>  Documentation/process/changes.rst |  4 ++--
>  lib/Kconfig.debug                 | 10 ++++++++++
>  scripts/Makefile.btf              |  6 +++++-
>  3 files changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/process/changes.rst b/Documentation/process/changes.rst
> index 02068d72a101..bfe98e38c9c0 100644
> --- a/Documentation/process/changes.rst
> +++ b/Documentation/process/changes.rst
> @@ -38,7 +38,7 @@ bash                   4.2              bash --version
>  binutils               2.30             ld -v
>  flex                   2.5.35           flex --version
>  bison                  2.0              bison --version
> -pahole                 1.26             pahole --version
> +pahole                 1.27             pahole --version
>  util-linux             2.10o            mount --version
>  kmod                   13               depmod -V
>  e2fsprogs              1.41.4           e2fsck -V
> @@ -145,7 +145,7 @@ 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
> +Since Linux 7.0, kfuncs annotated with KF_IMPLICIT_ARGS require pahole v1.27
>  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.
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 93f356d2b3d9..38ef8b6ce98d 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -425,6 +425,16 @@ config PAHOLE_HAS_LANG_EXCLUDE
>  	  otherwise it would emit malformed kernel and module binaries when
>  	  using DEBUG_INFO_BTF_MODULES.
>  
> +config PAHOLE_HAS_DECL_TAG_KFUNCS
> +	def_bool PAHOLE_VERSION >= 127
> +	help
> +	  Support for the decl_tag_kfuncs BTF feature, which is required for
> +	  kfuncs annotated with KF_IMPLICIT_ARGS to receive correct BTF
> +	  prototypes.  Without it, such kfuncs retain their full prototype
> +	  (including the implicit aux parameter) in vmlinux BTF, causing BPF
> +	  programs to fail to load with a "func_proto incompatible" error.
> +	  Implemented in pahole v1.27 (commit 72e88f29c).
> +
>  config DEBUG_INFO_BTF_MODULES
>  	bool "Generate BTF type information for kernel modules"
>  	default y
> diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
> index 562a04b40e06..c804643c5040 100644
> --- a/scripts/Makefile.btf
> +++ b/scripts/Makefile.btf
> @@ -14,7 +14,11 @@ pahole-flags-$(call test-ge, $(pahole-ver), 125)	+= --skip_encoding_btf_inconsis
>  else
>  
>  # Switch to using --btf_features for v1.26 and later.
> -pahole-flags-$(call test-ge, $(pahole-ver), 126)  = -j$(JOBS) --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs
> +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
> +
> +# decl_tag_kfuncs was implemented in pahole v1.27 (commit 72e88f29c).
> +# It was incorrectly gated on >= 126 by ebb79e96f1ea.
> +pahole-flags-$(call test-ge, $(pahole-ver), 127) += --btf_features=decl_tag_kfuncs
>  
>  pahole-flags-$(call test-ge, $(pahole-ver), 130) += --btf_features=attributes
>