fix: add missing checks in os.chmod

Author: KowalskiThomas

Lib/test/test_os/test_posix.py

@@ -1109,6 +1109,30 @@ def test_fchmod_file(self):
             self.check_chmod(posix.fchmod, f.fileno())
             self.check_chmod(posix.chmod, f.fileno())
 
+    @os_helper.skip_unless_working_chmod
+    @unittest.skipUnless(os.chmod in os.supports_fd,
+                         "test needs fd support in os.chmod()")
+    @unittest.skipUnless(os.chmod in os.supports_dir_fd,
+                         "test needs dir_fd support in os.chmod()")
+    def test_chmod_fd_with_dir_fd(self):
+        with open(os_helper.TESTFN, 'wb+') as f:
+            dir_fd = os.open(os.curdir, os.O_RDONLY)
+            self.addCleanup(os.close, dir_fd)
+            with self.assertRaisesRegex(ValueError,
+                    'can\'t specify both dir_fd and fd'):
+                posix.chmod(f.fileno(), 0o600, dir_fd=dir_fd)
+
+    @os_helper.skip_unless_working_chmod
+    @unittest.skipUnless(os.chmod in os.supports_fd,
+                         "test needs fd support in os.chmod()")
+    @unittest.skipIf(os.name == 'nt',
+                     'follow_symlinks defaults to False on Windows')
+    def test_chmod_fd_follow_symlinks(self):
+        with open(os_helper.TESTFN, 'wb+') as f:
+            with self.assertRaisesRegex(ValueError,
+                    'cannot use fd and follow_symlinks together'):
+                posix.chmod(f.fileno(), 0o600, follow_symlinks=False)
+
     @unittest.skipUnless(hasattr(posix, 'lchmod'), 'test needs os.lchmod()')
     def test_lchmod_file(self):
         self.check_chmod(posix.lchmod, os_helper.TESTFN)

Misc/NEWS.d/next/Library/2026-08-23-08-41-00.gh-issue-156264.hJ8qJn.rst

@@ -0,0 +1,3 @@
+:func:`os.chmod` now raises :exc:`ValueError` when *dir_fd* or
+*follow_symlinks* is used together with a file descriptor *path*, matching
+the documented behaviour and :func:`os.chown`.

Modules/posixmodule.c

@@ -4113,6 +4113,15 @@ os_chmod_impl(PyObject *module, path_t *path, int mode, int dir_fd,
         return NULL;
 #endif
 
+    if (dir_fd_and_fd_invalid("chmod", dir_fd, path->fd))
+        return NULL;
+#ifndef MS_WINDOWS
+    /* On Windows, follow_symlinks defaults to False, so doing this check
+       would reject the valid os.chmod(fd, mode). */
+    if (fd_and_follow_symlinks_invalid("chmod", path->is_fd, follow_symlinks))
+        return NULL;
+#endif
+
     if (PySys_Audit("os.chmod", "Oii", path->object, mode,
                     dir_fd == DEFAULT_DIR_FD ? -1 : dir_fd) < 0) {
         return NULL;