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bpo-42015: Reorder dereferencing calls in meth_dealloc, to make sure m_self is kept alive long enough #22670

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Oct 12, 2020
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bpo-42015: Reorder dereferencing calls in meth_dealloc, to make sure m_self is kept alive long enough #22670

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50bf715
bpo-42015: Reorder dereferencing calls in meth_dealloc, to make sure …
YannickJadoul Oct 12, 2020
1e69eff
Add comment on derefering order, change order in meth_traverse as wel…
YannickJadoul Oct 12, 2020
c508a4f
Rephrase NEWS fragment to be more user-focussed
YannickJadoul Oct 12, 2020
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/C API/2020-10-12-20-13-58.bpo-42015.X4H2_V.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Fix potential crash in deallocating method objects when dynamically
allocated `PyMethodDef`'s lifetime is managed through the ``self``
argument of a `PyCFunction`.
6 changes: 4 additions & 2 deletions Objects/methodobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -164,9 +164,11 @@ meth_dealloc(PyCFunctionObject *m)
if (m->m_weakreflist != NULL) {
PyObject_ClearWeakRefs((PyObject*) m);
}
// Dereference class before m_self: PyCFunction_GET_CLASS accesses
// PyMethodDef m_ml, which could be kept alive by m_self
Py_XDECREF(PyCFunction_GET_CLASS(m));
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Py_XDECREF(m->m_self);
Py_XDECREF(m->m_module);
Py_XDECREF(PyCFunction_GET_CLASS(m));
PyObject_GC_Del(m);
}

Expand Down Expand Up @@ -243,9 +245,9 @@ meth_get__qualname__(PyCFunctionObject *m, void *closure)
static int
meth_traverse(PyCFunctionObject *m, visitproc visit, void *arg)
{
Py_VISIT(PyCFunction_GET_CLASS(m));
Py_VISIT(m->m_self);
Py_VISIT(m->m_module);
Py_VISIT(PyCFunction_GET_CLASS(m));
return 0;
}

Expand Down