gh-118441: Limit posixpath.realpath(..., strict=True) symlinks

Lib/posixpath.py

@@ -449,6 +449,9 @@ def realpath(filename, *, strict=False):
     # the same links.
     seen = {}
 
+    # How many symlinks can still be read (excluding caching)
+    remaining_symlinks = 40  # TODO: use limit set by OS
+
     while rest:
         name = rest.pop()
         if name is None:
@@ -483,7 +486,10 @@ def realpath(filename, *, strict=False):
                     os.stat(newpath)
                 path = newpath
                 continue
+            if remaining_symlinks <= 0 and strict:
+                raise OSError(errno.ELOOP, "Too many symbolic links", newpath)
             target = os.readlink(newpath)
+            remaining_symlinks -= 1
         except OSError:
             if strict:
                 raise

Lib/test/test_posixpath.py

@@ -677,6 +677,29 @@ def test_realpath_unreadable_symlink(self):
             os.chmod(ABSTFN, 0o755, follow_symlinks=False)
             os.unlink(ABSTFN)
 
+    @os_helper.skip_unless_symlink
+    @skip_if_ABSTFN_contains_backslash
+    def test_realpath_too_many_symlinks(self):
+        depth = 40 + 1  # TODO: use limit set by OS + 1
+        try:
+            os.mkdir(ABSTFN)
+            os.symlink('.', f'{ABSTFN}/1')
+            for i in range(1, depth):
+                os.symlink(f'{i}'), f'{ABSTFN}/{i + 1}')
+            self.assertEqual(realpath(f'{ABSTFN}/{depth}'), ABSTFN)
+            with self.assertRaises(OSError):
+                realpath(f'{ABSTFN}/{depth}', strict=True)
+
+            # Test using relative path as well.
+            with os_helper.change_cwd(ABSTFN):
+                self.assertEqual(realpath(f'{depth}'), ABSTFN)
+                with self.assertRaises(OSError):
+                    realpath(f'{depth}', strict=True)
+        finally:
+            for i in range(1, depth + 1):
+                os_helper.unlink(f'{ABSTFN}/{i}')
+            safe_rmdir(ABSTFN)
+
     def test_relpath(self):
         (real_getcwd, os.getcwd) = (os.getcwd, lambda: r"/home/user/bar")
         try:

Misc/NEWS.d/next/Core and Builtins/2024-05-19-10-12-58.gh-issue-118441.QpzMKV.rst

@@ -0,0 +1 @@
+:func:`os.path.realpath` now raises :exc:`OSError` when *strict* mode is enabled and a path with too many symlinks is supplied.