gh-144023: prevent follow_symlinks from being allowed with an fd of 0

fd_and_follow_symlinks_invalid lets you use fd and follow_symlinks together if fd is 0.

This is incorrect, as 0 is a valid fd.

Author: AZero13

Lib/test/test_os/test_posix.py

@@ -668,6 +668,18 @@ def test_fstat(self):
         finally:
             fp.close()
 
+    @unittest.skipUnless(hasattr(posix, 'stat'),
+                         'test needs posix.stat()')
+    @unittest.skipUnless(os.stat in os.supports_follow_symlinks,
+                         'test needs follow_symlinks support in os.stat()')
+    def test_stat_fd_zero_follow_symlinks(self):
+        with self.assertRaisesRegex(ValueError,
+                'cannot use fd and follow_symlinks together'):
+            posix.stat(0, follow_symlinks=False)
+        with self.assertRaisesRegex(ValueError,
+                'cannot use fd and follow_symlinks together'):
+            posix.stat(1, follow_symlinks=False)
+
     def check_statlike_path(self, func):
         self.assertTrue(func(os_helper.TESTFN))
         self.assertTrue(func(os.fsencode(os_helper.TESTFN)))

Misc/NEWS.d/next/Library/2026-01-19-00-57-40.gh-issue-144023.29XUcp.rst

@@ -0,0 +1,2 @@
+Fixed validation of file descriptor 0 in posix functions when used with
+follow_symlinks parameter.

Modules/posixmodule.c

@@ -1637,7 +1637,7 @@ static int
 fd_and_follow_symlinks_invalid(const char *function_name, int fd,
                                int follow_symlinks)
 {
-    if ((fd > 0) && (!follow_symlinks)) {
+    if ((fd >= 0) && (!follow_symlinks)) {
         PyErr_Format(PyExc_ValueError,
                      "%s: cannot use fd and follow_symlinks together",
                      function_name);