diff --git a/src/allmydata/test/test_util.py b/src/allmydata/test/test_util.py
index 5b9f696cc..427b67593 100644
--- a/src/allmydata/test/test_util.py
+++ b/src/allmydata/test/test_util.py
@@ -567,6 +567,21 @@ class FileUtil(ReallyEqualMixin, unittest.TestCase):
             finally:
                 os.chdir(saved_cwd)
 
+    def test_make_dirs_with_absolute_mode(self):
+        workdir = u"test_make_dirs_with_absolute_mode"
+        fileutil.make_dirs(workdir)
+        fileutil.make_dirs_with_absolute_mode(workdir, os.path.join(workdir,"a/b/c/d"), 0766)
+        new_mode = os.stat(os.path.join(workdir,"a/b/c/d")).st_mode & 0777
+        self.failUnlessEqual(new_mode, 0766)
+        new_mode = os.stat(os.path.join(workdir,"a/b/c")).st_mode & 0777
+        self.failUnlessEqual(new_mode, 0766)
+        new_mode = os.stat(os.path.join(workdir,"a/b")).st_mode & 0777
+        self.failUnlessEqual(new_mode, 0766)
+        new_mode = os.stat(os.path.join(workdir,"a")).st_mode & 0777
+        self.failUnlessEqual(new_mode, 0766)
+        new_mode = os.stat(workdir).st_mode & 0777
+        self.failIfEqual(new_mode, 0766)
+
     def test_create_long_path(self):
         workdir = u"test_create_long_path"
         fileutil.make_dirs(workdir)
diff --git a/src/allmydata/util/fileutil.py b/src/allmydata/util/fileutil.py
index 0366daabf..70075bba3 100644
--- a/src/allmydata/util/fileutil.py
+++ b/src/allmydata/util/fileutil.py
@@ -142,6 +142,31 @@ class EncryptedTemporaryFile:
         old end-of-file are unspecified. The file position after this operation is unspecified."""
         self.file.truncate(newsize)
 
+def make_dirs_with_absolute_mode(parent, dirname, mode):
+    """
+    Make directory `dirname` and chmod it to `mode` afterwards.
+    We chmod all parent directories of `dirname` until we reach
+    `parent`.
+    """
+    precondition_abspath(parent)
+    precondition_abspath(dirname)
+    if not is_ancestor_path(parent, dirname):
+        raise AssertionError("dirname must be a descendant of parent")
+
+    make_dirs(dirname)
+    while dirname != parent:
+        os.chmod(dirname, mode)
+        # FIXME: doesn't seem to work on Windows for long paths
+        old_dirname, dirname = dirname, os.path.dirname(dirname)
+        _assert(len(dirname) < len(old_dirname), dirname=dirname, old_dirname=old_dirname)
+
+def is_ancestor_path(parent, dirname):
+    while dirname != parent:
+        # FIXME: doesn't seem to work on Windows for long paths
+        old_dirname, dirname = dirname, os.path.dirname(dirname)
+        if len(dirname) >= len(old_dirname):
+            return False
+    return True
 
 def make_dirs(dirname, mode=0777):
     """