json: Fix multidimensional array support

This patch fixes support for encoding and decoding multidimensional arrays
as described by the JSON_OBJ_DESCR_ARRAY_ARRAY() macro.

Currently, the JSON array encoding and decoding functions, arr_encode()
and arr_parse(), expect array elements to be of object or primitive type.
However, arrays may be nested and so an array's elements may also be
arrays.

In order to support nested arrays, two special cases must be considered:

1. The array of objects/arrays sub-descriptor is described by two
`json_obj_descr` structs and so two instead of one `json_obj_descr`
structs must be skipped when iterating over the JSON descriptor to get to
an array's elements.
2. The implicit array item count field has to be considered for the
parent itself and all its child array items when calculating an element's
size.

Fixes #50801

Signed-off-by: Markus Fuchs <[email protected]>

Author: mrfuchs

lib/os/json.c

@@ -519,8 +519,17 @@ static ptrdiff_t get_elem_size(const struct json_obj_descr *descr)
 	case JSON_TOK_TRUE:
 	case JSON_TOK_FALSE:
 		return sizeof(bool);
-	case JSON_TOK_ARRAY_START:
-		return descr->array.n_elements * get_elem_size(descr->array.element_descr);
+	case JSON_TOK_ARRAY_START: {
+		ptrdiff_t size;
+
+		size = descr->array.n_elements * get_elem_size(descr->array.element_descr);
+		/* Consider additional item count field for array objects */
+		if (descr->field_name_len > 0) {
+			size = size + sizeof(size_t);
+		}
+
+		return size;
+	}
 	case JSON_TOK_OBJECT_START: {
 		ptrdiff_t total = 0;
 		size_t i;
@@ -542,37 +551,44 @@ static int arr_parse(struct json_obj *obj,
 		     const struct json_obj_descr *elem_descr,
 		     size_t max_elements, void *field, void *val)
 {
-	ptrdiff_t elem_size = get_elem_size(elem_descr);
-	void *last_elem = (char *)field + elem_size * max_elements;
-	size_t *elements = NULL;
-	struct json_token value;
+	void *value = val;
+	size_t *elements = (size_t *)((char *)value + elem_descr->offset);
+	ptrdiff_t elem_size;
+	void *last_elem;
+	struct json_token tok;
 
-	if (val) {
-		elements = (size_t *)((char *)val + elem_descr->offset);
+	/* For nested arrays, skip parent descriptor to get elements */
+	if (elem_descr->type == JSON_TOK_ARRAY_START) {
+		elem_descr = elem_descr->array.element_descr;
 	}
 
-	__ASSERT_NO_MSG(elem_size > 0);
+	*elements = 0;
+	elem_size = get_elem_size(elem_descr);
+	last_elem = (char *)field + elem_size * max_elements;
 
-	if (elements) {
-		*elements = 0;
-	}
+	__ASSERT_NO_MSG(elem_size > 0);
 
-	while (!arr_next(obj, &value)) {
-		if (value.type == JSON_TOK_ARRAY_END) {
+	while (!arr_next(obj, &tok)) {
+		if (tok.type == JSON_TOK_ARRAY_END) {
 			return 0;
 		}
 
 		if (field == last_elem) {
 			return -ENOSPC;
 		}
 
-		if (decode_value(obj, elem_descr, &value, field, NULL) < 0) {
-			return -EINVAL;
+		/* For nested arrays, update value to current field,
+		 * so it matches descriptor's offset to length field
+		 */
+		if (elem_descr->type == JSON_TOK_ARRAY_START) {
+			value = field;
 		}
 
-		if (elements) {
-			(*elements)++;
+		if (decode_value(obj, elem_descr, &tok, field, value) < 0) {
+			return -EINVAL;
 		}
+
+		(*elements)++;
 		field = (char *)field + elem_size;
 	}
 
@@ -835,7 +851,7 @@ static int arr_encode(const struct json_obj_descr *elem_descr,
 		      const void *field, const void *val,
 		      json_append_bytes_t append_bytes, void *data)
 {
-	ptrdiff_t elem_size = get_elem_size(elem_descr);
+	ptrdiff_t elem_size;
 	/*
 	 * NOTE: Since an element descriptor's offset isn't meaningful
 	 * (array elements occur at multiple offsets in `val'), we use
@@ -851,6 +867,13 @@ static int arr_encode(const struct json_obj_descr *elem_descr,
 		return ret;
 	}
 
+	/* For nested arrays, skip parent descriptor to get elements */
+	if (elem_descr->type == JSON_TOK_ARRAY_START) {
+		elem_descr = elem_descr->array.element_descr;
+	}
+
+	elem_size = get_elem_size(elem_descr);
+
 	for (i = 0; i < n_elem; i++) {
 		/*
 		 * Though "field" points at the next element in the