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bpo-44184: Fix subtype_dealloc() for freed type #26274

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Merged
vstinner merged 4 commits into from
May 21, 2021
Merged

bpo-44184: Fix subtype_dealloc() for freed type #26274

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5bcd788
bpo-44184: Fix subtype_dealloc() for freed type
vstinner May 20, 2021
454e996
Add an unit test
vstinner May 21, 2021
b670383
_PyMem_IsPtrFreed() argument is now const
vstinner May 21, 2021
efd45ad
Port the unit test to Windows
vstinner May 21, 2021
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bpo-44184: Fix subtype_dealloc() for freed type 
Fix a crash at Python exit when a deallocator function removes the
last strong reference to a heap type.

Don't read type memory after calling basedealloc() since
basedealloc() can deallocate the type and free its memory.
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@vstinner
vstinner committed May 21, 2021
commit 5bcd788fe8092c0125db0043b56fa9ef9442206a
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Core and Builtins/2021-05-21-01-42-45.bpo-44184.9qOptC.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Fix a crash at Python exit when a deallocator function removes the last strong
reference to a heap type.
Patch by Victor Stinner.
11 changes: 9 additions & 2 deletions Objects/typeobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1446,15 +1446,22 @@ subtype_dealloc(PyObject *self)
if (_PyType_IS_GC(base)) {
_PyObject_GC_TRACK(self);
}

// Don't read type memory after calling basedealloc() since basedealloc()
// can deallocate the type and free its memory.
int type_needs_decref = (type->tp_flags & Py_TPFLAGS_HEAPTYPE
&& !(base->tp_flags & Py_TPFLAGS_HEAPTYPE));

assert(basedealloc);
basedealloc(self);

/* Can't reference self beyond this point. It's possible tp_del switched
our type from a HEAPTYPE to a non-HEAPTYPE, so be careful about
reference counting. Only decref if the base type is not already a heap
allocated type. Otherwise, basedealloc should have decref'd it already */
if (type->tp_flags & Py_TPFLAGS_HEAPTYPE && !(base->tp_flags & Py_TPFLAGS_HEAPTYPE))
Py_DECREF(type);
if (type_needs_decref) {
Py_DECREF(type);
}

endlabel:
Py_TRASHCAN_END
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