[PATCH] docs: Raise minimum pahole version to 1.26 for KF_IMPLICIT_ARGS kfuncs

[zhidao su]

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 (e.g. scx_bpf_create_dsq, scx_bpf_kick_cpu).

The root cause: scripts/Makefile.btf passes --btf_features=decl_tag_kfuncs
to pahole only when pahole >= 1.26.  Without that flag, pahole emits no
DECL_TAG BTF entries for __bpf_kfunc-annotated functions.  As a result,
resolve_btfids/main.c::collect_kfuncs() finds no bpf_kfunc DECL_TAGs,
short-circuits at line 1002, and btf2btf() never creates the _impl
variants or strips the implicit 'aux' argument from the visible proto.
The vmlinux BTF retains the 3-param prototype while BPF programs declare
the 2-param version, triggering the mismatch.

Raise the minimum version in the requirements table from 1.22 to 1.26
and add a note explaining the failure mode, so users understand why
their BPF programs fail on distributions shipping pahole v1.25 (e.g.
Ubuntu 24.04 LTS).

Suggested-by: Jonathan Corbet <corbet@xxxxxxx>
Signed-off-by: zhidao su <suzhidao@xxxxxxxxxx>
---
 Documentation/process/changes.rst | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/Documentation/process/changes.rst b/Documentation/process/changes.rst
index 6b373e193548..02068d72a101 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                 1.22             pahole --version
+pahole                 1.26             pahole --version
 util-linux             2.10o            mount --version
 kmod                   13               depmod -V
 e2fsprogs              1.41.4           e2fsck -V
@@ -145,6 +145,11 @@ 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.
+
 It is found in the 'dwarves' or 'pahole' distro packages or from
 https://fedorapeople.org/~acme/dwarves/.
 
-- 
2.43.0