selftests/bpf: Test bpf_core_types_are_compat() functionality.

Add several tests to check bpf_core_types_are_compat() functionality:
- candidate type name exists and types match
- candidate type name exists but types don't match
- nested func protos at kernel recursion limit
- nested func protos above kernel recursion limit. Such bpf prog
  is rejected during the load.
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220204005519.60361-3-mcroce@linux.microsoft.com

tools/testing/selftests/bpf/Makefile

@@ -330,7 +330,7 @@ LINKED_SKELS := test_static_linked.skel.h linked_funcs.skel.h		\
 
 LSKELS := kfunc_call_test.c fentry_test.c fexit_test.c fexit_sleep.c \
 	test_ringbuf.c atomics.c trace_printk.c trace_vprintk.c \
-	map_ptr_kern.c core_kern.c
+	map_ptr_kern.c core_kern.c core_kern_overflow.c
 # Generate both light skeleton and libbpf skeleton for these
 LSKELS_EXTRA := test_ksyms_module.c test_ksyms_weak.c kfunc_call_test_subprog.c
 SKEL_BLACKLIST += $$(LSKELS)

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

@@ -13,6 +13,10 @@
 #define CREATE_TRACE_POINTS
 #include "bpf_testmod-events.h"
 
+typedef int (*func_proto_typedef)(long);
+typedef int (*func_proto_typedef_nested1)(func_proto_typedef);
+typedef int (*func_proto_typedef_nested2)(func_proto_typedef_nested1);
+
 DEFINE_PER_CPU(int, bpf_testmod_ksym_percpu) = 123;
 
 noinline void
@@ -31,6 +35,9 @@ struct bpf_testmod_btf_type_tag_2 {
 
 noinline int
 bpf_testmod_test_btf_type_tag_user_1(struct bpf_testmod_btf_type_tag_1 __user *arg) {
+	BTF_TYPE_EMIT(func_proto_typedef);
+	BTF_TYPE_EMIT(func_proto_typedef_nested1);
+	BTF_TYPE_EMIT(func_proto_typedef_nested2);
 	return arg->a;
 }
 

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

@@ -7,8 +7,22 @@
 void test_core_kern_lskel(void)
 {
 	struct core_kern_lskel *skel;
+	int link_fd;
 
 	skel = core_kern_lskel__open_and_load();
-	ASSERT_OK_PTR(skel, "open_and_load");
+	if (!ASSERT_OK_PTR(skel, "open_and_load"))
+		return;
+
+	link_fd = core_kern_lskel__core_relo_proto__attach(skel);
+	if (!ASSERT_GT(link_fd, 0, "attach(core_relo_proto)"))
+		goto cleanup;
+
+	/* trigger tracepoints */
+	usleep(1);
+	ASSERT_TRUE(skel->bss->proto_out[0], "bpf_core_type_exists");
+	ASSERT_FALSE(skel->bss->proto_out[1], "!bpf_core_type_exists");
+	ASSERT_TRUE(skel->bss->proto_out[2], "bpf_core_type_exists. nested");
+
+cleanup:
 	core_kern_lskel__destroy(skel);
 }

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

+// SPDX-License-Identifier: GPL-2.0
+
+#include "test_progs.h"
+#include "core_kern_overflow.lskel.h"
+
+void test_core_kern_overflow_lskel(void)
+{
+	struct core_kern_overflow_lskel *skel;
+
+	skel = core_kern_overflow_lskel__open_and_load();
+	if (!ASSERT_NULL(skel, "open_and_load"))
+		core_kern_overflow_lskel__destroy(skel);
+}

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

@@ -101,4 +101,20 @@ int balancer_ingress(struct __sk_buff *ctx)
 	return 0;
 }
 
+typedef int (*func_proto_typedef___match)(long);
+typedef int (*func_proto_typedef___doesnt_match)(char *);
+typedef int (*func_proto_typedef_nested1)(func_proto_typedef___match);
+
+int proto_out[3];
+
+SEC("raw_tracepoint/sys_enter")
+int core_relo_proto(void *ctx)
+{
+	proto_out[0] = bpf_core_type_exists(func_proto_typedef___match);
+	proto_out[1] = bpf_core_type_exists(func_proto_typedef___doesnt_match);
+	proto_out[2] = bpf_core_type_exists(func_proto_typedef_nested1);
+
+	return 0;
+}
+
 char LICENSE[] SEC("license") = "GPL";

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

+// SPDX-License-Identifier: GPL-2.0
+#include "vmlinux.h"
+
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_core_read.h>
+
+typedef int (*func_proto_typedef)(long);
+typedef int (*func_proto_typedef_nested1)(func_proto_typedef);
+typedef int (*func_proto_typedef_nested2)(func_proto_typedef_nested1);
+
+int proto_out;
+
+SEC("raw_tracepoint/sys_enter")
+int core_relo_proto(void *ctx)
+{
+	proto_out = bpf_core_type_exists(func_proto_typedef_nested2);
+
+	return 0;
+}
+
+char LICENSE[] SEC("license") = "GPL";