Emit __ksym extern for local @KFunC declarations

Signed-off-by: Cong Wang <cwang@multikernel.io>

Author: congwang-mk

src/ebpf_c_codegen.ml

@@ -3048,11 +3048,14 @@ let generate_declarations_in_source_order_unified ctx ir_multi_prog ~_btf_path _
         emit_blank_line ctx
 
     | Ir.IRDeclKfuncDecl kfunc_decl ->
-        (* Emit a function-signature declaration so eBPF programs can call the kfunc.
-           Kernel-provided kfuncs need `extern ... __ksym;`; locally-defined @kfunc
-           functions just need a forward prototype. Standard BPF helpers are skipped
-           entirely - libbpf's bpf_helpers.h already declares them, and a __ksym
-           extern would clash with that declaration. *)
+        (* Emit `extern ... __ksym;` so libbpf resolves the symbol against the
+           kernel's BTF at load time. Both kinds of kfunc are external to the
+           eBPF object: kernel-provided ones live in vmlinux, and locally-defined
+           @kfunc bodies live in the sibling kernel module compiled alongside the
+           eBPF program. Without __ksym, bpftool's skeleton generator looks for
+           BTF inside the .o and fails. Standard BPF helpers are skipped entirely
+           - libbpf's bpf_helpers.h already declares them, and a __ksym extern
+           would clash with that declaration. *)
         let name = kfunc_decl.Ir.ikfunc_name in
         if kfunc_decl.Ir.ikfunc_is_extern && Bpf_helpers.is_bpf_helper name then
           ()
@@ -3064,13 +3067,8 @@ let generate_declarations_in_source_order_unified ctx ir_multi_prog ~_btf_path _
               ) params)
           in
           let return_type_str = kfunc_signature_type_to_c kfunc_decl.Ir.ikfunc_return_type in
-          if kfunc_decl.Ir.ikfunc_is_extern then
-            emit_line ctx (sprintf "extern %s %s(%s) __ksym;"
-              return_type_str name params_str)
-          else (
-            emit_line ctx "/* kfunc declaration */";
-            emit_line ctx (sprintf "%s %s(%s);" return_type_str name params_str)
-          );
+          emit_line ctx (sprintf "extern %s %s(%s) __ksym;"
+            return_type_str name params_str);
           emit_blank_line ctx
         )
   ) ir_multi_prog.Ir.source_declarations;

tests/test_kfunc_attribute.ml

@@ -151,12 +151,18 @@ let test_ebpf_kfunc_declarations () =
 
   (* @kfunc declarations are lowered into the IR; codegen needs no side-channel *)
   let (generated_code, _) = Ebpf_c_codegen.compile_multi_to_c_with_analysis ir in
-  
-  (* Check that kfunc declarations are generated *)
-  check bool "Contains kfunc declaration" true
-    (try ignore (Str.search_forward (Str.regexp "bool security_check") generated_code 0); true with Not_found -> false);
-  check bool "Contains kfunc call" true
-    (try ignore (Str.search_forward (Str.regexp "security_check(") generated_code 0); true with Not_found -> false)
+
+  let contains substr =
+    try ignore (Str.search_forward (Str.regexp_string substr) generated_code 0); true
+    with Not_found -> false
+  in
+  (* Local @kfuncs are external to the eBPF object - they live in the sibling
+     kernel module - so they must be declared with __ksym, like vmlinux kfuncs.
+     Without __ksym, bpftool gen skeleton fails with "failed to find BTF for
+     extern" because libbpf looks for the symbol's BTF inside the .o file. *)
+  check bool "Local @kfunc declared as __ksym extern" true
+    (contains "extern bool security_check(__u64 addr) __ksym;");
+  check bool "Contains kfunc call" true (contains "security_check(")
 
 (** Test kernel module print function translation *)
 let test_kernel_print_translation () =