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

[zhidao su]

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@xxxxxxxxx>
Closes: https://lore.kernel.org/bpf/e0ca748d-3204-4160-b37d-0f76cbac8c6c@xxxxxxxxx/
Signed-off-by: zhidao su <suzhidao@xxxxxxxxxx>
---
 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
 
-- 
2.43.0