Fix buffer corruption in metadata retrieval for function type (#35)

* fix buffer corruption in metadata function arg type
* use snprintf instead of sprintf

Author: garethpotter

src/thingset.c

@@ -739,27 +739,27 @@ static inline char *type_to_type_name(const enum thingset_type type)
 static int get_function_arg_types(struct thingset_context *ts, uint16_t parent_id, char *buf,
                                   size_t size)
 {
-    int len = 0;
+    int total_len = 0;
     for (unsigned int i = 0; i < ts->num_objects; i++) {
         if (ts->data_objects[i].parent_id == parent_id) {
-            if (len > 0) {
+            int len = 0;
+            if (total_len > 0) {
                 if (size < 2) {
                     return -THINGSET_ERR_RESPONSE_TOO_LARGE;
                 }
-                len += sprintf(buf, ",");
-                size -= 1;
-                buf += 1;
+                len += snprintf(buf, size, ",");
             }
             char *elementType = type_to_type_name(ts->data_objects[i].type);
-            if (len > size) {
-                return -THINGSET_ERR_RESPONSE_TOO_LARGE;
-            }
-            len += sprintf(buf, "%s", elementType);
+            len += snprintf(buf + len, size - len, "%s", elementType);
             buf += len;
             size -= len;
+            total_len += len;
+            if (total_len > size) {
+                return -THINGSET_ERR_RESPONSE_TOO_LARGE;
+            }
         }
     }
-    return len;
+    return total_len;
 }
 
 int thingset_get_type_name(struct thingset_context *ts, const struct thingset_data_object *obj,
@@ -771,11 +771,11 @@ int thingset_get_type_name(struct thingset_context *ts, const struct thingset_da
             if (sizeof(elementType) > size) {
                 return -THINGSET_ERR_RESPONSE_TOO_LARGE;
             }
-            return sprintf(buf, "%s[]", elementType);
+            return snprintf(buf, size, "%s[]", elementType);
         }
         case THINGSET_TYPE_FN_VOID:
         case THINGSET_TYPE_FN_I32:
-            sprintf(buf, "(");
+            snprintf(buf, size, "(");
             int len = 1 + get_function_arg_types(ts, obj->id, buf + 1, size - 1);
             if (len < 0) {
                 return -THINGSET_ERR_RESPONSE_TOO_LARGE;
@@ -787,18 +787,18 @@ int thingset_get_type_name(struct thingset_context *ts, const struct thingset_da
             size -= len;
             switch (obj->type) {
                 case THINGSET_TYPE_FN_VOID:
-                    len += sprintf(buf, ")->()");
+                    len += snprintf(buf, size, ")->()");
                     break;
                 case THINGSET_TYPE_FN_I32:
-                    len += sprintf(buf, ")->(i32)");
+                    len += snprintf(buf, size, ")->(i32)");
                     break;
                 default:
                     break;
             }
             return len;
         default: {
             char *type = type_to_type_name(obj->type);
-            return sprintf(buf, "%s", type);
+            return snprintf(buf, size, "%s", type);
         }
     }
 }