[3.13] gh-144050: Fix stat.filemode pure Python file type detection (GH-144059)

Lib/stat.py

@@ -166,9 +166,14 @@ def filemode(mode):
     perm = []
     for index, table in enumerate(_filemode_table):
         for bit, char in table:
-            if mode & bit == bit:
-                perm.append(char)
-                break
+            if index == 0:
+                if S_IFMT(mode) == bit:
+                    perm.append(char)
+                    break
+            else:
+                if mode & bit == bit:
+                    perm.append(char)
+                    break
         else:
             if index == 0:
                 # Unknown filetype

Lib/test/test_stat.py

@@ -163,6 +163,11 @@ def test_mode(self):
                              self.statmod.S_IFREG)
             self.assertEqual(self.statmod.S_IMODE(st_mode), 0o666)
 
+    def test_filemode_does_not_misclassify_random_bits(self):
+        # gh-144050 regression test
+        self.assertEqual(self.statmod.filemode(0o77777)[0], "?")
+        self.assertEqual(self.statmod.filemode(0o177777)[0], "?")
+
     @os_helper.skip_unless_working_chmod
     def test_directory(self):
         os.mkdir(TESTFN)

Misc/NEWS.d/next/Library/2026-01-20-16-35-55.gh-issue-144050.0kKFbF.rst

@@ -0,0 +1,2 @@
+Fix :func:`stat.filemode` in the pure-Python implementation to avoid misclassifying
+invalid mode values as block devices.