docs, resolve_btfids: Document kfunc BTF annotation emission

resolve_btfids now emits the bpf_kfunc and bpf_fastcall BTF decl tags and
the arena address_space(1) type attribute for kfuncs, which were
previously produced by pahole.

Reflect this in the in-tree comments and documentation.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://patch.msgid.link/20260807032029.78092-7-ihor.solodrai@linux.dev
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

Documentation/bpf/kfuncs.rst

@@ -472,6 +472,13 @@ type. An example is shown below::
         }
         late_initcall(init_subsystem);
 
+At kernel build time the ``resolve_btfids`` tool finds all kfuncs declared with
+``BTF_KFUNCS_START()`` and emits their BTF annotations into the kernel's BTF.
+For each kfunc it 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).
+
 2.7  Specifying no-cast aliases with ___init
 --------------------------------------------
 

Documentation/process/changes.rst

@@ -147,11 +147,6 @@ 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/.
 

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