selftests/bpf: Ensure the unsupported struct_ops prog cannot be loaded

There is an existing "bpf_tcp_ca/unsupp_cong_op" test to ensure
the unsupported tcp-cc "get_info" struct_ops prog cannot be loaded.

This patch adds a new test in the bpf_testmod such that the
unsupported ops test does not depend on other kernel subsystem
where its supporting ops may be changed in the future.
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20240722183049.2254692-4-martin.lau@linux.devSigned-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.h

@@ -35,6 +35,7 @@ struct bpf_testmod_ops {
 	void (*test_2)(int a, int b);
 	/* Used to test nullable arguments. */
 	int (*test_maybe_null)(int dummy, struct task_struct *task);
+	int (*unsupported_ops)(void);
 
 	/* The following fields are used to test shadow copies. */
 	char onebyte;

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

@@ -9,6 +9,7 @@
 #include "struct_ops_nulled_out_cb.skel.h"
 #include "struct_ops_forgotten_cb.skel.h"
 #include "struct_ops_detach.skel.h"
+#include "unsupported_ops.skel.h"
 
 static void check_map_info(struct bpf_map_info *info)
 {
@@ -311,5 +312,6 @@ void serial_test_struct_ops_module(void)
 		test_struct_ops_forgotten_cb();
 	if (test__start_subtest("test_detach_link"))
 		test_detach_link();
+	RUN_TESTS(unsupported_ops);
 }
 

tools/testing/selftests/bpf/progs/unsupported_ops.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_misc.h"
+#include "../bpf_testmod/bpf_testmod.h"
+
+char _license[] SEC("license") = "GPL";
+
+SEC("struct_ops/unsupported_ops")
+__failure
+__msg("attach to unsupported member unsupported_ops of struct bpf_testmod_ops")
+int BPF_PROG(unsupported_ops)
+{
+	return 0;
+}
+
+SEC(".struct_ops.link")
+struct bpf_testmod_ops testmod = {
+	.unsupported_ops = (void *)unsupported_ops,
+};