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gh-152682: Fix NULL dereference on OOM in symtable_visit_type_param_bound_or_default #152684

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sobolevn merged 1 commit into from
Jun 30, 2026
+23 −0
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gh-152682: Fix NULL dereference on OOM in symtable_visit_type_param_bound_or_default #152684

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17 changes: 17 additions & 0 deletions Lib/test/test_syntax.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2861,6 +2861,7 @@ def f(x: *b)
import unittest

from test import support
from test.support.script_helper import assert_python_ok

class SyntaxWarningTest(unittest.TestCase):
def check_warning(self, code, errtext, filename="<testcase>", mode="exec"):
Expand Down Expand Up @@ -3201,6 +3202,22 @@ class A:
class B[{name}]: pass
""", "<testcase>", mode="exec")

@support.nomemtest
def test_disallowed_type_param_names_oom(self):

@StanFromIreland StanFromIreland Jun 30, 2026

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This test passes for me on main.

@petrvaganoff petrvaganoff Jun 30, 2026

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Yes, I also couldn't reproduce it reliably. After investigating, it seems _testcapi.set_nomemory(0) replaces the global CPython allocator with a stub, and there are many allocations happening between the set_nomemory(0) call and PyUnicode_FromFormat inside compile(), so OOM typically hits one of them first. I verified this with a coverage build - the relevant lines are never reached.

I also tried manually injecting an OOM condition in the C code - without the fix it does produce SIGSEGV, confirming the bug is real.

In principle, we could add a fault injection hook in the debug build and expose it via _testinternalcapi. But I think the simpler option is to drop the OOM test entirely - the fix is a straightforward NULL check, and the existing test_disallowed_type_param_names() already covers the code path. What do you think?

@StanFromIreland StanFromIreland Jul 1, 2026

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The test isn't doing anything, please remove it. In general we just don't test these cases.

@petrvaganoff petrvaganoff Jul 1, 2026 •
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See please, - #152770

# gh-152682: Don't crash on OOM when formatting the SyntaxError message
# in symtable_visit_type_param_bound_or_default.
code = textwrap.dedent("""\
import _testcapi
_testcapi.set_nomemory(0)
try:
compile("class A[__classdict__]: pass", "<string>", "exec")
except MemoryError:
pass
else:
raise RuntimeError('MemoryError not raised')
""")
assert_python_ok("-c", code)

@support.cpython_only
def test_nested_named_except_blocks(self):
code = ""
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-06-30-14-00-00.gh-issue-152682.yId7e5.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Fix NULL pointer dereference in :func:`compile` when a reserved name (e.g.
``__classdict__``) is used as a type parameter name and memory allocation
fails while formatting the error message.
3 changes: 3 additions & 0 deletions Python/symtable.c
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Expand Up @@ -2678,6 +2678,9 @@ symtable_visit_type_param_bound_or_default(

PyObject *error_msg = PyUnicode_FromFormat("reserved name '%U' cannot be "
"used for type parameter", name);
if (error_msg == NULL) {
return 0;
}
PyErr_SetObject(PyExc_SyntaxError, error_msg);
Py_DECREF(error_msg);
SET_ERROR_LOCATION(st->st_filename, LOCATION(tp));
Expand Down
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