selftests/bpf: Add write to hashmap to array_map iter test

Modify iter prog in existing bpf_iter_bpf_array_map.c, which currently
dumps arraymap key/val, to also do a write of (val, key) into a
newly-added hashmap. Confirm that the write succeeds as expected by
modifying the userspace runner program.

Before a change added in an earlier commit - considering PTR_TO_BUF reg
a valid input to helpers which expect MAP_{KEY,VAL} - the verifier
would've rejected this prog change due to type mismatch. Since using
current iter's key/val to access a separate map is a reasonable usecase,
let's add support for it.

Note that the test prog cannot directly write (val, key) into hashmap
via bpf_map_update_elem when both come from iter context because key is
marked MEM_RDONLY. This is due to bpf_map_update_elem - and other basic
map helpers - taking ARG_PTR_TO_MAP_{KEY,VALUE} w/o MEM_RDONLY type
flag. bpf_map_{lookup,update,delete}_elem don't modify their
input key/val so it should be possible to tag their args READONLY, but
due to the ubiquitous use of these helpers and verifier checks for
type == MAP_VALUE, such a change is nontrivial and seems better to
address in a followup series.

Also fixup some 'goto's in test runner's map checking loop.
Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20221020160721.4030492-4-davemarchevsky@fb.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -941,10 +941,10 @@ static void test_bpf_array_map(void)
 {
 	__u64 val, expected_val = 0, res_first_val, first_val = 0;
 	DECLARE_LIBBPF_OPTS(bpf_iter_attach_opts, opts);
-	__u32 expected_key = 0, res_first_key;
+	__u32 key, expected_key = 0, res_first_key;
+	int err, i, map_fd, hash_fd, iter_fd;
 	struct bpf_iter_bpf_array_map *skel;
 	union bpf_iter_link_info linfo;
-	int err, i, map_fd, iter_fd;
 	struct bpf_link *link;
 	char buf[64] = {};
 	int len, start;
@@ -1001,12 +1001,20 @@ static void test_bpf_array_map(void)
 	if (!ASSERT_EQ(skel->bss->val_sum, expected_val, "val_sum"))
 		goto close_iter;
 
+	hash_fd = bpf_map__fd(skel->maps.hashmap1);
 	for (i = 0; i < bpf_map__max_entries(skel->maps.arraymap1); i++) {
 		err = bpf_map_lookup_elem(map_fd, &i, &val);
-		if (!ASSERT_OK(err, "map_lookup"))
-			goto out;
-		if (!ASSERT_EQ(i, val, "invalid_val"))
-			goto out;
+		if (!ASSERT_OK(err, "map_lookup arraymap1"))
+			goto close_iter;
+		if (!ASSERT_EQ(i, val, "invalid_val arraymap1"))
+			goto close_iter;
+
+		val = i + 4;
+		err = bpf_map_lookup_elem(hash_fd, &val, &key);
+		if (!ASSERT_OK(err, "map_lookup hashmap1"))
+			goto close_iter;
+		if (!ASSERT_EQ(key, val - 4, "invalid_val hashmap1"))
+			goto close_iter;
 	}
 
 close_iter:

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

@@ -19,13 +19,20 @@ struct {
 	__type(value, __u64);
 } arraymap1 SEC(".maps");
 
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, 10);
+	__type(key, __u64);
+	__type(value, __u32);
+} hashmap1 SEC(".maps");
+
 __u32 key_sum = 0;
 __u64 val_sum = 0;
 
 SEC("iter/bpf_map_elem")
 int dump_bpf_array_map(struct bpf_iter__bpf_map_elem *ctx)
 {
-	__u32 *key = ctx->key;
+	__u32 *hmap_val, *key = ctx->key;
 	__u64 *val = ctx->value;
 
 	if (key == (void *)0 || val == (void *)0)
@@ -35,6 +42,18 @@ int dump_bpf_array_map(struct bpf_iter__bpf_map_elem *ctx)
 	bpf_seq_write(ctx->meta->seq, val, sizeof(__u64));
 	key_sum += *key;
 	val_sum += *val;
+
+	/* workaround - It's necessary to do this convoluted (val, key)
+	 * write into hashmap1, instead of simply doing
+	 *   bpf_map_update_elem(&hashmap1, val, key, BPF_ANY);
+	 * because key has MEM_RDONLY flag and bpf_map_update elem expects
+	 * types without this flag
+	 */
+	bpf_map_update_elem(&hashmap1, val, val, BPF_ANY);
+	hmap_val = bpf_map_lookup_elem(&hashmap1, val);
+	if (hmap_val)
+		*hmap_val = *key;
+
 	*val = *key;
 	return 0;
 }