feat: Improve wasm string null-termintor checks

runtime/include/runtime.h

@@ -24,10 +24,10 @@ void         stub_init(i32 offset);
 void *       worker_thread__main(void *return_code);
 
 /**
- * TODO: ???
- * @param offset TODO: ????
- * @param bounds_check TODO: ???
- * @return TODO: ???
+ * Translates WASM offsets into runtime VM pointers
+ * @param offset an offset into the WebAssembly linear memory
+ * @param bounds_check the size of the thing we are pointing to
+ * @return void pointer to something in WebAssembly linear memory
  **/
 static inline void *
 get_memory_ptr_void(u32 offset, u32 bounds_check)
@@ -36,24 +36,30 @@ get_memory_ptr_void(u32 offset, u32 bounds_check)
 }
 
 /**
- * TODO: ???
- * @param offset TODO: ????
- * @return TODO: ???
+ * Get a single-byte extended ASCII character from WebAssembly linear memory
+ * @param offset an offset into the WebAssembly linear memory
+ * @return char at the offset
  **/
-static inline char *
-get_memory_string(u32 offset)
+static inline char
+get_memory_character(u32 offset)
 {
-	char *naive_ptr = get_memory_ptr_for_runtime(offset, 1);
-	int   i         = 0;
-
-	while (true) {
-		// Keep bounds checking the waters over and over until we know it's safe (we find a terminating
-		// character)
-		char ith_element = get_memory_ptr_for_runtime(offset, i + 1)[i];
+	char result = get_memory_ptr_for_runtime(offset, 1)[0];
+	return result;
+}
 
-		if (ith_element == '\0') return naive_ptr;
-		i++;
+/**
+ * Get a null-terminated String from WebAssembly linear memory
+ * @param offset an offset into the WebAssembly linear memory
+ * @param max_length the maximum expected length in characters
+ * @return pointer to the string or NULL if max_length is reached without finding null-terminator
+ **/
+static inline char *
+get_memory_string(u32 offset, u32 max_length)
+{
+	for (int i = 0; i < max_length; i++) {
+		if (get_memory_character(offset + i) == '\0') return get_memory_ptr_void(offset, 1);
 	}
+	return NULL;
 }
 
 /**

runtime/src/libc/uvio.c

@@ -183,7 +183,7 @@ i32
 wasm_open(i32 path_off, i32 flags, i32 mode)
 {
 	uv_fs_t req  = UV_FS_REQ_INIT();
-	char *  path = get_memory_string(path_off);
+	char *  path = get_memory_string(path_off, 4096);
 
 	int iofd = current_sandbox__initialize_io_handle();
 	if (iofd < 0) return -1;
@@ -296,7 +296,7 @@ struct wasm_stat {
 i32
 wasm_stat(u32 path_str_offset, i32 stat_offset)
 {
-	char *            path     = get_memory_string(path_str_offset);
+	char *            path     = get_memory_string(path_str_offset, 4096);
 	struct wasm_stat *stat_ptr = get_memory_ptr_void(stat_offset, sizeof(struct wasm_stat));
 
 	struct stat stat;
@@ -388,7 +388,7 @@ wasm_fstat(i32 filedes, i32 stat_offset)
 i32
 wasm_lstat(i32 path_str_offset, i32 stat_offset)
 {
-	char *            path     = get_memory_string(path_str_offset);
+	char *            path     = get_memory_string(path_str_offset, 4096);
 	struct wasm_stat *stat_ptr = get_memory_ptr_void(stat_offset, sizeof(struct wasm_stat));
 
 	struct stat stat;
@@ -664,7 +664,7 @@ wasm_getcwd(u32 buf_offset, u32 buf_size)
 u32
 wasm_unlink(u32 path_str_offset)
 {
-	char *  str = get_memory_string(path_str_offset);
+	char *  str = get_memory_string(path_str_offset, 4096);
 	uv_fs_t req = UV_FS_REQ_INIT();
 	debuglog("[%p] start[%s]\n", uv_fs_get_data(&req), str);
 	uv_fs_unlink(get_thread_libuv_handle(), &req, str, wasm_fs_callback);