selftests/bpf: test maximum recursion depth for bpf_core_types_are_compat()

bpf_core_types_are_compat() was limited to 2 recursion levels, which are
enough to parse a function prototype.
Add a test which checks the existence of a function prototype, so to
test the bpf_core_types_are_compat() code path.

The test for the recursion limit being hit is done in a separate object,
because the kernel failure makes the whole load to fail.

Sample run log with extra prints:

	[ 5689.913751] bpf_core_apply_relo_insn:1200 cands->len: 2
	[ 5689.913902] bpf_core_types_are_compat:6896: ret: 1
	[ 5689.913994] bpf_core_types_are_compat:6896: ret: 0
	[ 5689.914025] bpf_core_apply_relo_insn:1200 cands->len: 2
	[ 5689.914141] bpf_core_types_are_compat:6896: ret: 0
	[ 5689.914246] bpf_core_types_are_compat:6896: ret: 0
	test_core_kern_lskel:PASS:open_and_load 0 nsec
	test_core_kern_lskel:PASS:attach(core_relo_proto) 0 nsec
	test_core_kern_lskel:PASS:bpf_core_type_exists 0 nsec
	test_core_kern_lskel:PASS:!bpf_core_type_exists 0 nsec
	#41 core_kern_lskel:OK
	[ 5689.915267] bpf_core_apply_relo_insn:1200 cands->len: 2
	[ 5689.915399] bpf_core_types_are_compat:6896: ret: 0
	[ 5689.915504] bpf_core_types_are_compat:6896: ret: -22
	test_core_kern_overflow_lskel:PASS:open_and_load 0 nsec
	#42 core_kern_overflow_lskel:OK
	Summary: 2/0 PASSED, 0 SKIPPED, 0 FAILED
	Successfully unloaded bpf_testmod.ko.

Signed-off-by: Matteo Croce <[email protected]>

Author: teknoraver

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 +13,11 @@
 #define CREATE_TRACE_POINTS
 #include "bpf_testmod-events.h"
 
+typedef int (*func_proto_typedef___match)(long);
+typedef int (*func_proto_typedef___overflow)(func_proto_typedef___match);
+func_proto_typedef___match funcp = NULL;
+func_proto_typedef___overflow funcp_of = NULL;
+
 DEFINE_PER_CPU(int, bpf_testmod_ksym_percpu) = 123;
 
 noinline void

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

@@ -7,8 +7,21 @@
 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");
+
+cleanup:
 	core_kern_lskel__destroy(skel);
 }

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

@@ -0,0 +1,13 @@
+// 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 +101,18 @@ int balancer_ingress(struct __sk_buff *ctx)
 	return 0;
 }
 
+typedef int (*func_proto_typedef___match)(long);
+typedef void (*func_proto_typedef___doesnt_match)(char*);
+
+int proto_out[2];
+
+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);
+
+	return 0;
+}
+
 char LICENSE[] SEC("license") = "GPL";

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

@@ -0,0 +1,21 @@
+// 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___match)(long);
+typedef int (*func_proto_typedef___overflow)(func_proto_typedef___match);
+
+int proto_out;
+
+SEC("raw_tracepoint/sys_enter")
+int core_relo_proto(void *ctx)
+{
+	proto_out = bpf_core_type_exists(func_proto_typedef___overflow);
+
+	return 0;
+}
+
+char LICENSE[] SEC("license") = "GPL";