selftests/bpf: Add a test for out of bound rdonly buf access

If the bpf program contains out of bound access w.r.t. a
particular map key/value size, the verification will be
still okay, e.g., it will be accepted by verifier. But
it will be rejected during link_create time. A test
is added here to ensure link_create failure did happen
if out of bound access happened.
  $ ./test_progs -n 4
  ...
  #4/23 rdonly-buf-out-of-bound:OK
  ...

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200723184124.591700-1-yhs@fb.com

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

@@ -20,6 +20,7 @@
 #include "bpf_iter_bpf_array_map.skel.h"
 #include "bpf_iter_bpf_percpu_array_map.skel.h"
 #include "bpf_iter_bpf_sk_storage_map.skel.h"
+#include "bpf_iter_test_kern5.skel.h"
 
 static int duration;
 
@@ -865,6 +866,25 @@ static void test_bpf_sk_storage_map(void)
 	bpf_iter_bpf_sk_storage_map__destroy(skel);
 }
 
+static void test_rdonly_buf_out_of_bound(void)
+{
+	DECLARE_LIBBPF_OPTS(bpf_iter_attach_opts, opts);
+	struct bpf_iter_test_kern5 *skel;
+	struct bpf_link *link;
+
+	skel = bpf_iter_test_kern5__open_and_load();
+	if (CHECK(!skel, "bpf_iter_test_kern5__open_and_load",
+		  "skeleton open_and_load failed\n"))
+		return;
+
+	opts.map_fd = bpf_map__fd(skel->maps.hashmap1);
+	link = bpf_program__attach_iter(skel->progs.dump_bpf_hash_map, &opts);
+	if (CHECK(!IS_ERR(link), "attach_iter", "unexpected success\n"))
+		bpf_link__destroy(link);
+
+	bpf_iter_test_kern5__destroy(skel);
+}
+
 void test_bpf_iter(void)
 {
 	if (test__start_subtest("btf_id_or_null"))
@@ -911,4 +931,6 @@ void test_bpf_iter(void)
 		test_bpf_percpu_array_map();
 	if (test__start_subtest("bpf_sk_storage_map"))
 		test_bpf_sk_storage_map();
+	if (test__start_subtest("rdonly-buf-out-of-bound"))
+		test_rdonly_buf_out_of_bound();
 }

tools/testing/selftests/bpf/progs/bpf_iter_test_kern5.c

@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+#include "bpf_iter.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+struct key_t {
+	int a;
+	int b;
+	int c;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, 3);
+	__type(key, struct key_t);
+	__type(value, __u64);
+} hashmap1 SEC(".maps");
+
+__u32 key_sum = 0;
+
+SEC("iter/bpf_map_elem")
+int dump_bpf_hash_map(struct bpf_iter__bpf_map_elem *ctx)
+{
+	void *key = ctx->key;
+
+	if (key == (void *)0)
+		return 0;
+
+	/* out of bound access w.r.t. hashmap1 */
+	key_sum += *(__u32 *)(key + sizeof(struct key_t));
+	return 0;
+}