binaryxml: Fix stringpool insertion padding (#135)

frida_tools/apk.py

@@ -264,7 +264,7 @@ def get_string(self, index: int) -> str:
 
         string = None
         if self.utf8:
-            # Ignore UTF-16 length
+            # Ignore number of characters
             n = struct.unpack("<B", self.stream.read(1))[0]
             if n & 0x80:
                 n = ((n & 0x7F) << 8) | struct.unpack("<B", self.stream.read(1))[0]
@@ -299,10 +299,10 @@ def append_str(self, add: str) -> int:
         offset = len(chunk_data) - 8 - self.strings_offset + 4
 
         if self.utf8:
-            # UTF-16 length (ignored)
-            chunk_data.extend(struct.pack("<B", len(add)))
-            # UTF-8 length
-            chunk_data.extend(struct.pack("<B", len(add)))
+            assert len(add.encode("utf-8")) < 128  # multi-byte len strings not supported yet
+            length_in_characters = len(add)
+            length_in_bytes = len(add.encode("utf-8"))
+            chunk_data.extend(struct.pack("<BB", length_in_characters, length_in_bytes))
 
             chunk_data.extend(add.encode("utf-8"))
             # Insert a UTF-8 NUL
@@ -313,6 +313,11 @@ def append_str(self, add: str) -> int:
             # Insert a UTF-16 NUL
             chunk_data.extend([0, 0])
 
+        # pad to a multiple of 4 bytes
+        if len(chunk_data) % 4 != 0:
+            alignment_padding = [0] * (4 - len(chunk_data) % 4)
+            chunk_data.extend(alignment_padding)
+
         # Insert a new offset at the end of the existing offsets
         chunk_data[end : end + 4] = struct.pack("<I", offset)