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

Lib/posixpath.py

@@ -388,6 +388,8 @@ def abspath(path):
 # Return a canonical path (i.e. the absolute location of a file on the
 # filesystem).
 
+_MAXLINKS = 40  # TODO: use limit set by OS
+
 def realpath(filename, *, strict=False):
     """Return the canonical path of the specified filename, eliminating any
 symbolic links encountered in the path."""
@@ -402,7 +404,8 @@ def realpath(filename, *, strict=False):
         curdir = '.'
         pardir = '..'
         getcwd = os.getcwd
-    return _realpath(filename, strict, sep, curdir, pardir, getcwd)
+    return _realpath(filename, strict, sep, curdir, pardir, getcwd,
+                     maxlinks=_MAXLINKS if strict else None)
 
 def _realpath(filename, strict=False, sep=sep, curdir=curdir, pardir=pardir,
               getcwd=os.getcwd, lstat=os.lstat, readlink=os.readlink, maxlinks=None):
@@ -453,7 +456,7 @@ def _realpath(filename, strict=False, sep=sep, curdir=curdir, pardir=pardir,
                 if link_count > maxlinks:
                     if strict:
                         raise OSError(errno.ELOOP, os.strerror(errno.ELOOP),
-                                      newpath)
+                                      filename)
                     path = newpath
                     continue
             elif newpath in seen:
@@ -465,7 +468,7 @@ def _realpath(filename, strict=False, sep=sep, curdir=curdir, pardir=pardir,
                 # The symlink is not resolved, so we must have a symlink loop.
                 if strict:
                     raise OSError(errno.ELOOP, os.strerror(errno.ELOOP),
-                                  newpath)
+                                  filename)
                 path = newpath
                 continue
             target = readlink(newpath)

Lib/test/test_posixpath.py

@@ -3,7 +3,7 @@
 import posixpath
 import sys
 import unittest
-from posixpath import realpath, abspath, dirname, basename
+from posixpath import _MAXLINKS, realpath, abspath, dirname, basename
 from test import test_genericpath
 from test.support import import_helper
 from test.support import cpython_only, os_helper
@@ -689,6 +689,31 @@ 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):
+        try:
+            os.mkdir(ABSTFN)
+            os.symlink('.', f'{ABSTFN}/link')
+            self.assertEqual(realpath(ABSTFN + '/link' * _MAXLINKS), ABSTFN)
+            self.assertEqual(realpath(ABSTFN + '/link' * _MAXLINKS,
+                                      strict=True), ABSTFN)
+            self.assertEqual(realpath(ABSTFN + '/link' * (_MAXLINKS+1)), ABSTFN)
+            with self.assertRaises(OSError):
+                realpath(ABSTFN + '/link' * (_MAXLINKS+1), strict=True)
+
+            # Test using relative path as well.
+            with os_helper.change_cwd(ABSTFN):
+                self.assertEqual(realpath('link/' * _MAXLINKS), ABSTFN)
+                self.assertEqual(realpath('link/' * _MAXLINKS, strict=True),
+                                 ABSTFN)
+                self.assertEqual(realpath('link/' * (_MAXLINKS+1)), ABSTFN)
+                with self.assertRaises(OSError):
+                    realpath('link/' * (_MAXLINKS+1), strict=True)
+        finally:
+            os_helper.unlink(f'{ABSTFN}/link')
+            safe_rmdir(ABSTFN)
+
     def test_relpath(self):
         (real_getcwd, os.getcwd) = (os.getcwd, lambda: r"/home/user/bar")
         try:

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

@@ -0,0 +1,2 @@
+Fix error message for :func:`os.path.realpath` on Unix.
+:func:`os.path.realpath` now raises :exc:`OSError` when *strict* mode is enabled and a path with too many symlinks is supplied.