Add null terminator for char* based json string unescape, and precond…

…ition on input length (#2303)

* Add null terminator for char* based json string unescape, and precondition on input length

* Add remark comment about precondition and null terimination

* Update doc comment typo

* Remove the test cases about insufficient space.

* Fix clang formatting.

* Add null terminator tests.

* Test that the null terminator is there at the end.

* Fix build errors.

sdk/inc/azure/core/az_json.h

@@ -807,6 +807,10 @@ AZ_NODISCARD az_result az_json_reader_skip_children(az_json_reader* ref_json_rea
  * @retval #AZ_OK The string is returned.
  * @retval #AZ_ERROR_NOT_ENOUGH_SPACE \p destination does not have enough size.
  *
+ * @remarks The buffer referred to by \p destination must have a size that is at least 1 byte bigger
+ * than the \p json_string #az_span for the \p destination string to be zero-terminated.
+ * Content is copied from the source buffer, while unescaping and then `\0` is added at the end.
+ *
  * @remarks This API can also be used to perform in place unescaping.
  */
 AZ_NODISCARD az_result az_json_string_unescape(

sdk/src/azure/core/az_json_token.c

@@ -333,7 +333,8 @@ AZ_NODISCARD az_result az_json_string_unescape(
 {
   _az_PRECONDITION_VALID_SPAN(json_string, 1, false);
   _az_PRECONDITION_NOT_NULL(destination);
-  _az_PRECONDITION(destination_max_size > 0);
+  // The destination needs to be larger than the input, for null terminator.
+  _az_PRECONDITION(destination_max_size > az_span_size(json_string));
 
   int32_t position = 0;
   int32_t span_size = az_span_size(json_string);
@@ -370,6 +371,8 @@ AZ_NODISCARD az_result az_json_string_unescape(
     position++;
   }
 
+  destination[position] = 0;
+
   if (out_string_length != NULL)
   {
     *out_string_length = position;

sdk/tests/core/test_az_json.c

@@ -3029,6 +3029,31 @@ static void test_az_json_string_unescape(void** state)
     assert_int_equal(AZ_ERROR_UNEXPECTED_END, result);
   }
 
+  // null terminator
+  {
+    uint8_t buffer[3];
+    az_span test_span = AZ_SPAN_FROM_BUFFER(buffer);
+    test_span._internal.ptr[0] = 'a';
+    test_span._internal.ptr[1] = 'b';
+    test_span._internal.ptr[2] = 'c';
+
+    az_span expected = AZ_SPAN_FROM_STR("ab");
+
+    char destination[59] = { 0 };
+    destination[2] = 'd'; // verify that 'd' is overwritten with 0
+    az_span destination_span = AZ_SPAN_FROM_BUFFER(destination);
+
+    int final_size;
+    az_result result
+        = az_json_string_unescape(az_span_slice(test_span, 0, 2), destination, 59, &final_size);
+
+    destination_span = az_span_slice(destination_span, 0, final_size);
+
+    assert_int_equal(0, (int)destination[2]);
+    assert_int_equal(AZ_OK, result);
+    assert_true(az_span_is_content_equal(expected, destination_span));
+  }
+
   // only escapes
   {
     az_span original = AZ_SPAN_FROM_STR("\\b\\f\\n\\r\\t\\\\");
@@ -3105,17 +3130,6 @@ static void test_az_json_string_unescape(void** state)
     assert_true(az_span_is_content_equal(expected, destination_span));
   }
 
-  // insufficient space
-  {
-    az_span json = AZ_SPAN_FROM_STR(" { \"name\": \"some value string\" , \"code\" : 123456 } ");
-    char destination[5] = { 0 };
-    int final_size;
-
-    az_result result = az_json_string_unescape(json, destination, 4, &final_size);
-
-    assert_int_equal(AZ_ERROR_NOT_ENOUGH_SPACE, result);
-  }
-
   // magic test
   {
     az_span original = AZ_SPAN_FROM_STR("A\\\"\"Z");
@@ -3246,6 +3260,25 @@ static void test_az_json_string_unescape_same_buffer(void** state)
     _az_span_free(&json);
   }
 
+  // null terminator
+  {
+    uint8_t buffer[3];
+    az_span test_span = AZ_SPAN_FROM_BUFFER(buffer);
+    test_span._internal.ptr[0] = 'a';
+    test_span._internal.ptr[1] = 'b';
+    test_span._internal.ptr[2] = 'c'; // verify that 'c' is overwritten with 0
+
+    az_span expected = AZ_SPAN_FROM_STR("ab");
+
+    int final_size;
+    az_result result = az_json_string_unescape(
+        az_span_slice(test_span, 0, 2), (char*)az_span_ptr(test_span), 59, &final_size);
+
+    assert_int_equal(0, test_span._internal.ptr[2]);
+    assert_int_equal(AZ_OK, result);
+    assert_true(az_span_is_content_equal(expected, az_span_slice(test_span, 0, final_size)));
+  }
+
   // only escapes
   {
     az_span original = az_span_create_from_str(strdup("\\b\\f\\n\\r\\t\\\\"));
@@ -3318,18 +3351,6 @@ static void test_az_json_string_unescape_same_buffer(void** state)
     assert_int_equal(AZ_ERROR_UNEXPECTED_END, result);
     _az_span_free(&original);
   }
-
-  // insufficient space
-  {
-    az_span json = az_span_create_from_str(
-        strdup(" { \"name\": \"some value string\" , \"code\" : 123456 } "));
-    int final_size;
-
-    az_result result = az_json_string_unescape(json, (char*)az_span_ptr(json), 5, &final_size);
-
-    assert_int_equal(AZ_ERROR_NOT_ENOUGH_SPACE, result);
-    _az_span_free(&json);
-  }
 }
 
 int test_az_json()