gh-94315: Check for DAC override capability (GH-94316)

``os.geteuid() == 0`` is not a reliable check whether the current user
has the capability to bypass permission checks. Tests now probe for DAC
override.
(cherry picked from commit 7e0d98ecb3f049841de9854e7a3eca3e2638e4b2)

Co-authored-by: Christian Heimes <christian@python.org>

6 files changed, 58 insertions, 22 deletions

diff --git a/Lib/test/support/os_helper.py b/Lib/test/support/os_helper.py
index ff2fc33..1ac8a94 100644
--- a/Lib/test/support/os_helper.py
+++ b/Lib/test/support/os_helper.py
@@ -258,7 +258,7 @@ def can_chmod():
             else:
                 can = stat.S_IMODE(mode1) != stat.S_IMODE(mode2)
     finally:
-        os.unlink(TESTFN)
+        unlink(TESTFN)
     _can_chmod = can
     return can
 
@@ -273,6 +273,48 @@ def skip_unless_working_chmod(test):
     return test if ok else unittest.skip(msg)(test)
 
 
+# Check whether the current effective user has the capability to override
+# DAC (discretionary access control). Typically user root is able to
+# bypass file read, write, and execute permission checks. The capability
+# is independent of the effective user. See capabilities(7).
+_can_dac_override = None
+
+def can_dac_override():
+    global _can_dac_override
+
+    if not can_chmod():
+        _can_dac_override = False
+    if _can_dac_override is not None:
+        return _can_dac_override
+
+    try:
+        with open(TESTFN, "wb") as f:
+            os.chmod(TESTFN, 0o400)
+            try:
+                with open(TESTFN, "wb"):
+                    pass
+            except OSError:
+                _can_dac_override = False
+            else:
+                _can_dac_override = True
+    finally:
+        unlink(TESTFN)
+
+    return _can_dac_override
+
+
+def skip_if_dac_override(test):
+    ok = not can_dac_override()
+    msg = "incompatible with CAP_DAC_OVERRIDE"
+    return test if ok else unittest.skip(msg)(test)
+
+
+def skip_unless_dac_override(test):
+    ok = can_dac_override()
+    msg = "requires CAP_DAC_OVERRIDE"
+    return test if ok else unittest.skip(msg)(test)
+
+
 def unlink(filename):
     try:
         _unlink(filename)
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index 299eb30..25ac03c 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -1723,8 +1723,7 @@ class WFile(object):
         return self.name == other.name
 
 
-@unittest.skipIf(hasattr(os, 'geteuid') and os.geteuid() == 0,
-                 "non-root user required")
+@os_helper.skip_if_dac_override
 class TestFileTypeW(TempDirMixin, ParserTestCase):
     """Test the FileType option/argument type for writing files"""
 
@@ -1746,8 +1745,8 @@ class TestFileTypeW(TempDirMixin, ParserTestCase):
         ('-x - -', NS(x=eq_stdout, spam=eq_stdout)),
     ]
 
-@unittest.skipIf(hasattr(os, 'geteuid') and os.geteuid() == 0,
-                 "non-root user required")
+
+@os_helper.skip_if_dac_override
 class TestFileTypeX(TempDirMixin, ParserTestCase):
     """Test the FileType option/argument type for writing new files only"""
 
@@ -1767,8 +1766,7 @@ class TestFileTypeX(TempDirMixin, ParserTestCase):
     ]
 
 
-@unittest.skipIf(hasattr(os, 'geteuid') and os.geteuid() == 0,
-                 "non-root user required")
+@os_helper.skip_if_dac_override
 class TestFileTypeWB(TempDirMixin, ParserTestCase):
     """Test the FileType option/argument type for writing binary files"""
 
@@ -1785,8 +1783,7 @@ class TestFileTypeWB(TempDirMixin, ParserTestCase):
     ]
 
 
-@unittest.skipIf(hasattr(os, 'geteuid') and os.geteuid() == 0,
-                 "non-root user required")
+@os_helper.skip_if_dac_override
 class TestFileTypeXB(TestFileTypeX):
     "Test the FileType option/argument type for writing new binary files only"
 
diff --git a/Lib/test/test_import/__init__.py b/Lib/test/test_import/__init__.py
index ab7f887..6c5b80b 100644
--- a/Lib/test/test_import/__init__.py
+++ b/Lib/test/test_import/__init__.py
@@ -885,10 +885,9 @@ class PycacheTests(unittest.TestCase):
 
     @unittest.skipUnless(os.name == 'posix',
                          "test meaningful only on posix systems")
-    @unittest.skipIf(hasattr(os, 'geteuid') and os.geteuid() == 0,
-            "due to varying filesystem permission semantics (issue #11956)")
     @skip_if_dont_write_bytecode
     @os_helper.skip_unless_working_chmod
+    @os_helper.skip_if_dac_override
     @unittest.skipIf(is_emscripten, "umask is a stub")
     def test_unwritable_directory(self):
         # When the umask causes the new __pycache__ directory to be
diff --git a/Lib/test/test_py_compile.py b/Lib/test/test_py_compile.py
index f494aed..a4a52b1 100644
--- a/Lib/test/test_py_compile.py
+++ b/Lib/test/test_py_compile.py
@@ -115,8 +115,7 @@ class PyCompileTestsBase:
         self.assertTrue(os.path.exists(self.pyc_path))
         self.assertFalse(os.path.exists(self.cache_path))
 
-    @unittest.skipIf(hasattr(os, 'geteuid') and os.geteuid() == 0,
-                     'non-root user required')
+    @os_helper.skip_if_dac_override
     @unittest.skipIf(os.name == 'nt',
                      'cannot control directory permissions on Windows')
     @os_helper.skip_unless_working_chmod
diff --git a/Lib/test/test_shutil.py b/Lib/test/test_shutil.py
index 6fa0739..a2c4ab5 100644
--- a/Lib/test/test_shutil.py
+++ b/Lib/test/test_shutil.py
@@ -312,8 +312,7 @@ class TestRmTree(BaseTest, unittest.TestCase):
 
     @unittest.skipIf(sys.platform[:6] == 'cygwin',
                      "This test can't be run on Cygwin (issue #1071513).")
-    @unittest.skipIf(hasattr(os, 'geteuid') and os.geteuid() == 0,
-                     "This test can't be run reliably as root (issue #1076467).")
+    @os_helper.skip_if_dac_override
     @os_helper.skip_unless_working_chmod
     def test_on_error(self):
         self.errorState = 0
@@ -1033,8 +1032,7 @@ class TestCopy(BaseTest, unittest.TestCase):
 
     @os_helper.skip_unless_symlink
     @os_helper.skip_unless_xattr
-    @unittest.skipUnless(hasattr(os, 'geteuid') and os.geteuid() == 0,
-                         'root privileges required')
+    @os_helper.skip_unless_dac_override
     def test_copyxattr_symlinks(self):
         # On Linux, it's only possible to access non-user xattr for symlinks;
         # which in turn require root privileges. This test should be expanded
@@ -1830,8 +1828,7 @@ class TestWhich(BaseTest, unittest.TestCase):
                 # Other platforms: shouldn't match in the current directory.
                 self.assertIsNone(rv)
 
-    @unittest.skipIf(hasattr(os, 'geteuid') and os.geteuid() == 0,
-                     'non-root user required')
+    @os_helper.skip_if_dac_override
     def test_non_matching_mode(self):
         # Set the file read-only and ask for writeable files.
         os.chmod(self.temp_file.name, stat.S_IREAD)
@@ -2182,11 +2179,11 @@ class TestMove(BaseTest, unittest.TestCase):
             os.rmdir(dst_dir)
 
 
-    @unittest.skipUnless(hasattr(os, 'geteuid') and os.geteuid() == 0
-                         and hasattr(os, 'lchflags')
+    @os_helper.skip_unless_dac_override
+    @unittest.skipUnless(hasattr(os, 'lchflags')
                          and hasattr(stat, 'SF_IMMUTABLE')
                          and hasattr(stat, 'UF_OPAQUE'),
-                         'root privileges required')
+                         'requires lchflags')
     def test_move_dir_permission_denied(self):
         # bpo-42782: shutil.move should not create destination directories
         # if the source directory cannot be removed.
diff --git a/Misc/NEWS.d/next/Tests/2022-06-27-08-53-40.gh-issue-94315.MoZT9t.rst b/Misc/NEWS.d/next/Tests/2022-06-27-08-53-40.gh-issue-94315.MoZT9t.rst
new file mode 100644
index 0000000..09d5d7e
--- /dev/null
+++ b/Misc/NEWS.d/next/Tests/2022-06-27-08-53-40.gh-issue-94315.MoZT9t.rst
@@ -0,0 +1,2 @@
+Tests now check for DAC override capability instead of relying on
+:func:`os.geteuid`.