selftests/bpf: add another struct_ops callback use case test

Add a test which tests the case that was just fixed. Kernel has full
type information about callback, but user explicitly nulls out the
reference to declaratively set BPF program reference.

Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Martin KaFai Lau <[e-mail]>

tools/testing/selftests/bpf/prog_tests/test_struct_ops_module.c

···
 #include <time.h>
 
 #include "struct_ops_module.skel.h"
+#include "struct_ops_nulled_out_cb.skel.h"
 
 static void check_map_info(struct bpf_map_info *info)
 {
···
 	struct_ops_module__destroy(skel);
 }
 
+/* validate that it's ok to "turn off" callback that kernel supports */
+static void test_struct_ops_nulled_out_cb(void)
+{
+	struct struct_ops_nulled_out_cb *skel;
+	int err;
+
+	skel = struct_ops_nulled_out_cb__open();
+	if (!ASSERT_OK_PTR(skel, "skel_open"))
+		return;
+
+	/* kernel knows about test_1, but we still null it out */
+	skel->struct_ops.ops->test_1 = NULL;
+
+	err = struct_ops_nulled_out_cb__load(skel);
+	if (!ASSERT_OK(err, "skel_load"))
+		goto cleanup;
+
+	ASSERT_FALSE(bpf_program__autoload(skel->progs.test_1_turn_off), "prog_autoload");
+	ASSERT_LT(bpf_program__fd(skel->progs.test_1_turn_off), 0, "prog_fd");
+
+cleanup:
+	struct_ops_nulled_out_cb__destroy(skel);
+}
+
 void serial_test_struct_ops_module(void)
 {
 	if (test__start_subtest("test_struct_ops_load"))
···
 		test_struct_ops_not_zeroed();
 	if (test__start_subtest("test_struct_ops_incompatible"))
 		test_struct_ops_incompatible();
+	if (test__start_subtest("test_struct_ops_null_out_cb"))
+		test_struct_ops_nulled_out_cb();
 }
 

tools/testing/selftests/bpf/progs/struct_ops_nulled_out_cb.c

···
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2024 Meta Platforms, Inc. and affiliates. */
+#include <vmlinux.h>
+#include <bpf/bpf_tracing.h>
+#include "../bpf_testmod/bpf_testmod.h"
+
+char _license[] SEC("license") = "GPL";
+
+int rand;
+int arr[1];
+
+SEC("struct_ops/test_1")
+int BPF_PROG(test_1_turn_off)
+{
+	return arr[rand]; /* potentially way out of range access */
+}
+
+SEC(".struct_ops.link")
+struct bpf_testmod_ops ops = {
+	.test_1 = (void *)test_1_turn_off,
+};
+