selftests/bpf: Add a failure test for bpf_kptr_xchg() with local kptr

For a bpf_kptr_xchg() with local kptr, if the map value kptr type and
allocated local obj type does not match, with the previous patch,
the below verifier error message will be logged:
  R2 is of type <allocated local obj type> but <map value kptr type> is expected

Without the previous patch, the test will have unexpected success.
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20230822050058.2887354-1-yonghong.song@linux.devSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -5,6 +5,7 @@
 #include <network_helpers.h>
 
 #include "local_kptr_stash.skel.h"
+#include "local_kptr_stash_fail.skel.h"
 static void test_local_kptr_stash_simple(void)
 {
 	LIBBPF_OPTS(bpf_test_run_opts, opts,
@@ -51,10 +52,17 @@ static void test_local_kptr_stash_unstash(void)
 	local_kptr_stash__destroy(skel);
 }
 
-void test_local_kptr_stash_success(void)
+static void test_local_kptr_stash_fail(void)
+{
+	RUN_TESTS(local_kptr_stash_fail);
+}
+
+void test_local_kptr_stash(void)
 {
 	if (test__start_subtest("local_kptr_stash_simple"))
 		test_local_kptr_stash_simple();
 	if (test__start_subtest("local_kptr_stash_unstash"))
 		test_local_kptr_stash_unstash();
+	if (test__start_subtest("local_kptr_stash_fail"))
+		test_local_kptr_stash_fail();
 }

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2023 Meta Platforms, Inc. and affiliates. */
+
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_core_read.h>
+#include "../bpf_experimental.h"
+#include "bpf_misc.h"
+
+struct node_data {
+	long key;
+	long data;
+	struct bpf_rb_node node;
+};
+
+struct map_value {
+	struct node_data __kptr *node;
+};
+
+struct node_data2 {
+	long key[4];
+};
+
+/* This is necessary so that LLVM generates BTF for node_data struct
+ * If it's not included, a fwd reference for node_data will be generated but
+ * no struct. Example BTF of "node" field in map_value when not included:
+ *
+ * [10] PTR '(anon)' type_id=35
+ * [34] FWD 'node_data' fwd_kind=struct
+ * [35] TYPE_TAG 'kptr_ref' type_id=34
+ */
+struct node_data *just_here_because_btf_bug;
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__type(key, int);
+	__type(value, struct map_value);
+	__uint(max_entries, 2);
+} some_nodes SEC(".maps");
+
+SEC("tc")
+__failure __msg("invalid kptr access, R2 type=ptr_node_data2 expected=ptr_node_data")
+long stash_rb_nodes(void *ctx)
+{
+	struct map_value *mapval;
+	struct node_data2 *res;
+	int idx = 0;
+
+	mapval = bpf_map_lookup_elem(&some_nodes, &idx);
+	if (!mapval)
+		return 1;
+
+	res = bpf_obj_new(typeof(*res));
+	if (!res)
+		return 1;
+	res->key[0] = 40;
+
+	res = bpf_kptr_xchg(&mapval->node, res);
+	if (res)
+		bpf_obj_drop(res);
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";