Fix parent not set when building default packet fields (#4706, #4707)

- `Packet._ensure_parent_of()` added.
- Called in `do_init_cached_fields()`.

Author: alxroyer-thales

scapy/packet.py

@@ -319,6 +319,7 @@ def do_init_cached_fields(self, for_dissect_only=False, init_fields=None):
 
                 # Fix: Use `copy_field_value()` instead of just `value.copy()`, in order to duplicate list items as well in case of a list.
                 self.fields[fname] = self.copy_field_value(fname, self.default_fields[fname])
+                self._ensure_parent_of(self.fields[fname])
 
     def prepare_cached_fields(self, flist):
         # type: (Sequence[AnyField]) -> None
@@ -427,6 +428,15 @@ def remove_parent(self, other):
         point to the list owner packet."""
         self.parent = None
 
+    def _ensure_parent_of(self, val):
+        # type: (Any) -> None
+        """Ensures a parent reference with self for val when applicable."""
+        if isinstance(val, Packet):
+            val.parent = self
+        elif isinstance(val, list):
+            for item in val:  # type: Any
+                self._ensure_parent_of(item)
+
     def copy(self) -> Self:
         """Returns a deep copy of the instance."""
         return self._fast_copy(for_clone_with=False)