elf: fix in-kernel kfunc test

We currently test that resolving kfunc works both against in-kernel
as well as module functions. Commit 65eb006d85a2 ("bpf: Move kernel
test kfuncs to bpf_testmod") in the Linux kernel moved the
bpf_kfunc_call_test_mem_len_pass1 kfunc into btf_testmod.

Rewrite the test to use the conntrack kfuncs which were also
introduced in 5.18. They can also be in a module if nf_conntrack is
a module, but our CI kernels and my Fedora host kernel currently
compile it in.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

elf_reader_test.go

@@ -716,7 +716,7 @@ func TestKconfigConfig(t *testing.T) {
 }
 
 func TestKfunc(t *testing.T) {
-	testutils.SkipOnOldKernel(t, "5.18", "bpf_kfunc_call_test_mem_len_pass1")
+	testutils.SkipOnOldKernel(t, "5.18", "kfunc support")
 	testutils.Files(t, testutils.Glob(t, "testdata/kfunc-e*.elf"), func(t *testing.T, file string) {
 		spec, err := LoadCollectionSpec(file)
 		if err != nil {
@@ -734,7 +734,7 @@ func TestKfunc(t *testing.T) {
 		err = spec.LoadAndAssign(&obj, nil)
 		testutils.SkipIfNotSupported(t, err)
 		if err != nil {
-			t.Fatalf("%v+", err)
+			t.Fatalf("%+v", err)
 		}
 		defer obj.Main.Close()
 
@@ -751,7 +751,7 @@ func TestKfunc(t *testing.T) {
 }
 
 func TestInvalidKfunc(t *testing.T) {
-	testutils.SkipOnOldKernel(t, "5.18", "bpf_kfunc_call_test_mem_len_pass1")
+	testutils.SkipOnOldKernel(t, "5.18", "kfunc support")
 
 	if !haveTestmod(t) {
 		t.Skip("bpf_testmod not loaded")

testdata/common.h

@@ -1,13 +1,15 @@
 #pragma once
 
 typedef _Bool bool;
+typedef unsigned char uint8_t;
 typedef unsigned short uint16_t;
 typedef unsigned int uint32_t;
+typedef signed int int32_t;
 typedef unsigned long uint64_t;
 
 enum libbpf_tristate {
-	TRI_NO = 0,
-	TRI_YES = 1,
+	TRI_NO     = 0,
+	TRI_YES    = 1,
 	TRI_MODULE = 2,
 };
 

testdata/kfunc.c

@@ -2,9 +2,21 @@
 
 char __license[] __section("license") = "Dual MIT/GPL";
 
-extern void bpf_kfunc_call_test_mem_len_pass1(void *mem, int len) __ksym;
+// CO-RE type compat checking doesn't allow matches between forward declarations
+// and structs so we can't use forward declarations. Empty structs work just fine.
+struct __sk_buff {};
+struct nf_conn {};
+struct bpf_sock_tuple {};
+struct bpf_ct_opts {};
 
-__section("tc") int call_kfunc() {
-	bpf_kfunc_call_test_mem_len_pass1((void *)0, 0);
+extern struct nf_conn *bpf_skb_ct_lookup(struct __sk_buff *, struct bpf_sock_tuple *, uint32_t, struct bpf_ct_opts *, uint32_t) __ksym;
+extern void bpf_ct_release(struct nf_conn *) __ksym;
+
+__section("tc") int call_kfunc(void *ctx) {
+	char buf[1];
+	struct nf_conn *conn = bpf_skb_ct_lookup(ctx, (void *)buf, 0, (void *)buf, 0);
+	if (conn) {
+		bpf_ct_release(conn);
+	}
 	return 1;
 }