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gh-148801: Fix unbound C recursion in Element.__deepcopy__() #148802

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gh-148801: Fix unbound C recursion in Element.__deepcopy__() #148802

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fcbfaf4
Fix unbound C recursion in `Element.__deepcopy__()`
StanFromIreland Apr 20, 2026
230b348
Bigger number = recursionError ?
StanFromIreland Apr 20, 2026
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13 changes: 13 additions & 0 deletions Lib/test/test_xml_etree.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3190,6 +3190,19 @@ def __deepcopy__(self, memo):
self.assertEqual([c.tag for c in children[3:]],
[a.tag, b.tag, a.tag, b.tag])

@support.skip_if_unlimited_stack_size
@support.skip_emscripten_stack_overflow()
@support.skip_wasi_stack_overflow()
def test_deeply_nested_deepcopy(self):
# This should raise a RecursionError and not crash.
# See https://github.com/python/cpython/issues/148801.
root = cur = ET.Element('s')
for _ in range(150_000):
cur = ET.SubElement(cur, 'u')
with support.infinite_recursion():
with self.assertRaises(RecursionError):
copy.deepcopy(root)


class MutationDeleteElementPath(str):
def __new__(cls, elem, *args):
Expand Down
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-04-20-18-29-21.gh-issue-148801.ROeNqs.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
:mod:`xml.etree.ElementTree`: Fix a crash in :meth:`Element.__deepcopy__
<object.__deepcopy__>` on deeply nested trees.
16 changes: 14 additions & 2 deletions Modules/_elementtree.c
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Original file line number Diff line number Diff line change
Expand Up @@ -16,6 +16,7 @@
#endif

#include "Python.h"
#include "pycore_ceval.h" // _Py_EnterRecursiveCall()
#include "pycore_dict.h" // _PyDict_CopyAsDict()
#include "pycore_pyhash.h" // _Py_HashSecret
#include "pycore_tuple.h" // _PyTuple_FromPair
Expand Down Expand Up @@ -818,18 +819,25 @@ _elementtree_Element___deepcopy___impl(ElementObject *self, PyObject *memo)
PyObject* tail;
PyObject* id;

if (_Py_EnterRecursiveCall(" in Element.__deepcopy__")) {
return NULL;
}

PyTypeObject *tp = Py_TYPE(self);
elementtreestate *st = get_elementtree_state_by_type(tp);
// The deepcopy() helper takes care of incrementing the refcount
// of the object to copy so to avoid use-after-frees.
tag = deepcopy(st, self->tag, memo);
if (!tag)
if (!tag) {
_Py_LeaveRecursiveCall();
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@serhiy-storchaka serhiy-storchaka Apr 21, 2026

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These three _Py_LeaveRecursiveCalls can be replaced with goto error (you need also to initialize element with NULL and use Py_XDECREF). Larger diff, but three less lines in the result.

The current style is good too.

return NULL;
}

if (self->extra && self->extra->attrib) {
attrib = deepcopy(st, self->extra->attrib, memo);
if (!attrib) {
Py_DECREF(tag);
_Py_LeaveRecursiveCall();
return NULL;
}
} else {
Expand All @@ -841,8 +849,10 @@ _elementtree_Element___deepcopy___impl(ElementObject *self, PyObject *memo)
Py_DECREF(tag);
Py_XDECREF(attrib);

if (!element)
if (!element) {
_Py_LeaveRecursiveCall();
return NULL;
}

text = deepcopy(st, JOIN_OBJ(self->text), memo);
if (!text)
Expand Down Expand Up @@ -904,9 +914,11 @@ _elementtree_Element___deepcopy___impl(ElementObject *self, PyObject *memo)
if (i < 0)
goto error;

_Py_LeaveRecursiveCall();
return (PyObject*) element;

error:
_Py_LeaveRecursiveCall();
Py_DECREF(element);
return NULL;
}
Expand Down
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