bpo-40071: Fix potential crash in _functoolsmodule.c #19273
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vstinner
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Apr 1, 2020
Modules/_functoolsmodule.c
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| @@ -1413,7 +1413,9 @@ static PyMethodDef _functools_methods[] = { | |||
| static void | |||
| _functools_free(void *m) | |||
| { | |||
| Py_CLEAR(kwd_mark); | |||
| // FIXME: Avoid double free when using multi-phase initialization (PEP-489) | |||
| // Will fix when PEP-573 gets in. | |||
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The comment is not correct. The bug is not "double free": Py_CLEAR(NULL) does NULL, that's fine. The problem is if one module instance is deallocated, kwd_mark is cleared to NULL. Other instances will use NULL and so crash.
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Rephrased the comment and updated commit message. Thanks for getting it.
Changes on 7dd549e made _functools compatible with PEP-489 and we could have multiple modules instances loaded. But, right now there is no way to make `kwd_mark` global into a per module instance variable. kwd_mark is used on lru_cache_new which does not have a reference to a PyModule*, necessary to use PyModule_GetState. PEP-573 will solve this problem and will allow us to move the global state to per-module data and properly clear the state when unloading a module instance. This change temporarily disable cleaning of kwd_mark to avoid NULL pointer dereference if we clear kwd_mark and other module instances still alive use it. wip
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Changes on 7dd549e made _functools compatible with
PEP-489 and we could have multiple modules instances loaded.
But, right now there is no way to make
kwd_markglobal intoa per module instance variable. kwd_mark is used on lru_cache_new
which does not have a reference to a PyModule*, necessary to use
PyModule_GetState.
PEP-573 will solve this problem and will allow us to move the global
state to per-module data and properly clear the state when unloading
a module instance.
This change temporarily disable cleaning of kwd_mark to avoid NULL
pointer dereference if we clear kwd_mark and other module instances
still alive use it.
https://bugs.python.org/issue40071