selftests/bpf: Fix the missing tramp_1 to tramp_40 ops in cfi_stubs

The tramp_1 to tramp_40 ops is not set in the cfi_stubs in the
bpf_testmod_ops. It fails the struct_ops_multi_pages test after
retiring the unsupported_ops in the earlier patch.

This patch initializes them in a loop during the bpf_testmod_init().

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20240722183049.2254692-3-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c

@@ -1024,6 +1024,11 @@ static void bpf_testmod_test_2(int a, int b)
 {
 }
 
+static int bpf_testmod_tramp(int value)
+{
+	return 0;
+}
+
 static int bpf_testmod_ops__test_maybe_null(int dummy,
 					    struct task_struct *task__nullable)
 {
@@ -1080,6 +1085,7 @@ static int bpf_testmod_init(void)
 			.kfunc_btf_id	= bpf_testmod_dtor_ids[1]
 		},
 	};
+	void **tramp;
 	int ret;
 
 	ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_UNSPEC, &bpf_testmod_common_kfunc_set);
@@ -1103,6 +1109,14 @@ static int bpf_testmod_init(void)
 	ret = register_bpf_testmod_uprobe();
 	if (ret < 0)
 		return ret;
+
+	/* Ensure nothing is between tramp_1..tramp_40 */
+	BUILD_BUG_ON(offsetof(struct bpf_testmod_ops, tramp_1) + 40 * sizeof(long) !=
+		     offsetofend(struct bpf_testmod_ops, tramp_40));
+	tramp = (void **)&__bpf_testmod_ops.tramp_1;
+	while (tramp <= (void **)&__bpf_testmod_ops.tramp_40)
+		*tramp++ = bpf_testmod_tramp;
+
 	return 0;
 }