json: Fix calculation of object size

The calculation of the object size may be incorrect when the size of
a field is smaller than the struct alignment. When such a struct is
used in an array field, the decoded object contains wrong values.

The alignment influences the object size. For example the following
struct has a calculated object size of 8 bytes, however due to
alignment the real size of the struct is 12 bytes:

struct test_bool {
  bool b1; /* offset 0, size 1 */
           /* 3-byte padding */
  int i1;  /* offset 4, size 4 */
  bool b2; /* offset 8, size 1 */
           /* 3-byte padding */
};

This commit changes the object size calculation and computes the size
with the offset and size of the last field in the struct (rounded up
by the struct alignment).

Fixes: #85121
Signed-off-by: Christoph Winklhofer <[email protected]>

Author: cjwinklhofer

lib/utils/json.c

@@ -635,13 +635,17 @@ static ptrdiff_t get_elem_size(const struct json_obj_descr *descr)
 		size_t i;
 
 		for (i = 0; i < descr->object.sub_descr_len; i++) {
-			total += get_elem_size(&descr->object.sub_descr[i]);
-
 			if (descr->object.sub_descr[i].align_shift > align_shift) {
 				align_shift = descr->object.sub_descr[i].align_shift;
 			}
 		}
 
+		i = descr->object.sub_descr_len;
+		if (i > 0) {
+			total = descr->object.sub_descr[i - 1].offset +
+				get_elem_size(&descr->object.sub_descr[i - 1]);
+		}
+
 		return ROUND_UP(total, 1 << align_shift);
 	}
 	default:

tests/lib/json/src/main.c

@@ -205,6 +205,27 @@ static const struct json_obj_descr outer_descr[] = {
 				num_elements, element_descr, ARRAY_SIZE(element_descr))
 };
 
+struct test_alignment_nested {
+	bool bool1;
+	int int1;
+	bool bool2;
+};
+
+struct test_alignment_bool {
+	struct test_alignment_nested array[3];
+	size_t num_elements;
+};
+
+static const struct json_obj_descr alignment_nested_descr[] = {
+	JSON_OBJ_DESCR_PRIM(struct test_alignment_nested, bool1, JSON_TOK_TRUE),
+	JSON_OBJ_DESCR_PRIM(struct test_alignment_nested, int1, JSON_TOK_NUMBER),
+	JSON_OBJ_DESCR_PRIM(struct test_alignment_nested, bool2, JSON_TOK_TRUE),
+};
+
+static const struct json_obj_descr alignment_bool_descr[] = {
+	JSON_OBJ_DESCR_OBJ_ARRAY(struct test_alignment_bool, array, 3, num_elements,
+				 alignment_nested_descr, ARRAY_SIZE(alignment_nested_descr)) };
+
 ZTEST(lib_json_test, test_json_encoding)
 {
 	struct test_struct ts = {
@@ -1362,4 +1383,27 @@ ZTEST(lib_json_test, test_json_array_alignment)
 	zassert_equal(o.array[1].int3, 6, "Element 1 int3 not decoded correctly");
 }
 
+ZTEST(lib_json_test, test_json_array_alignment_bool)
+{
+	char encoded[] = "{\"array\":["
+			 "{\"bool1\":true,\"int1\":1,\"bool2\":false},"
+			 "{\"bool1\":true,\"int1\":2,\"bool2\":false}"
+			 "]}";
+
+	struct test_alignment_bool o = { 0 };
+	int64_t ret = json_obj_parse(encoded, sizeof(encoded) - 1, alignment_bool_descr,
+				     ARRAY_SIZE(alignment_bool_descr), &o);
+
+	zassert_false(ret < 0, "json_obj_parse returned error %d", ret);
+	zassert_equal(o.num_elements, 2, "Number of elements not decoded correctly");
+
+	zassert_equal(o.array[0].bool1, true, "Element 0 bool1 not decoded correctly");
+	zassert_equal(o.array[0].int1, 1, "Element 0 int1 not decoded correctly");
+	zassert_equal(o.array[0].bool2, false, "Element 0 bool2 not decoded correctly");
+
+	zassert_equal(o.array[1].bool1, true, "Element 1 bool1 not decoded correctly");
+	zassert_equal(o.array[1].int1, 2, "Element 1 int1 not decoded correctly");
+	zassert_equal(o.array[1].bool2, false, "Element 1 bool2 not decoded correctly");
+}
+
 ZTEST_SUITE(lib_json_test, NULL, NULL, NULL, NULL, NULL);