--- a/python/mozbuild/mozbuild/backend/tup.py
+++ b/python/mozbuild/mozbuild/backend/tup.py
@@ -44,19 +44,18 @@ from ..frontend.context import (
     ObjDirPath,
 )
 
 
 class BackendTupfile(object):
     """Represents a generated Tupfile.
     """
 
-    def __init__(self, srcdir, objdir, environment, topsrcdir, topobjdir):
+    def __init__(self, objdir, environment, topsrcdir, topobjdir):
         self.topsrcdir = topsrcdir
-        self.srcdir = srcdir
         self.objdir = objdir
         self.relobjdir = mozpath.relpath(objdir, topobjdir)
         self.environment = environment
         self.name = mozpath.join(objdir, 'Tupfile')
         self.rules_included = False
         self.shell_exported = False
         self.defines = []
         self.host_defines = []
@@ -193,20 +192,19 @@ class TupOnly(CommonBackend, PartialBack
 
         # These are 'group' dependencies - All rules that list these as an output
         # will be built before any rules that list this as an input.
         self._installed_idls = '$(MOZ_OBJ_ROOT)/<installed-idls>'
         self._installed_files = '$(MOZ_OBJ_ROOT)/<installed-files>'
 
     def _get_backend_file(self, relativedir):
         objdir = mozpath.join(self.environment.topobjdir, relativedir)
-        srcdir = mozpath.join(self.environment.topsrcdir, relativedir)
         if objdir not in self._backend_files:
             self._backend_files[objdir] = \
-                    BackendTupfile(srcdir, objdir, self.environment,
+                    BackendTupfile(objdir, self.environment,
                                    self.environment.topsrcdir, self.environment.topobjdir)
         return self._backend_files[objdir]
 
     def _get_backend_file_for(self, obj):
         return self._get_backend_file(obj.relativedir)
 
     def _py_action(self, action):
         cmd = [
@@ -522,22 +520,23 @@ class TupOnly(CommonBackend, PartialBack
         # Preferably we wouldn't have to import ipdl, but we need to parse the
         # ast in order to determine the namespaces since they are used in the
         # header output paths.
         sys.path.append(mozpath.join(self.environment.topsrcdir, 'ipc', 'ipdl'))
         import ipdl
 
         backend_file = self._get_backend_file('ipc/ipdl')
         outheaderdir = '_ipdlheaders'
+        srcdir = mozpath.join(self.environment.topsrcdir, 'ipc/ipdl')
         cmd = [
             '$(PYTHON_PATH)',
             '$(PLY_INCLUDE)',
-            '%s/ipdl.py' % backend_file.srcdir,
-            '--sync-msg-list=%s/sync-messages.ini' % backend_file.srcdir,
-            '--msg-metadata=%s/message-metadata.ini' % backend_file.srcdir,
+            '%s/ipdl.py' % srcdir,
+            '--sync-msg-list=%s/sync-messages.ini' % srcdir,
+            '--msg-metadata=%s/message-metadata.ini' % srcdir,
             '--outheaders-dir=%s' % outheaderdir,
             '--outcpp-dir=.',
         ]
         ipdldirs = sorted(set(mozpath.dirname(p) for p in sorted_ipdl_sources))
         cmd.extend(['-I%s' % d for d in ipdldirs])
         cmd.extend(sorted_ipdl_sources)
 
         outputs = ['IPCMessageTypeName.cpp', mozpath.join(outheaderdir, 'IPCMessageStart.h'), 'ipdl_lextab.py', 'ipdl_yacctab.py']