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devdanzin/instrumentation_use_after_decref.md

Created March 18, 2026 10:07
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instrumentation.c: Use-after-Py_DECREF comparison in call_one_instrument
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instrumentation_use_after_decref.md

instrumentation.c: Use-after-Py_DECREF comparison in call_one_instrument

Summary

At line 992-993, Py_DECREF(res) then res compared to &_PyInstrumentation_DISABLE. Using a potentially-freed pointer in comparison is UB per C standard.

Fix

Compare before DECREF: int is_disable = (res == &_PyInstrumentation_DISABLE); Py_DECREF(res); return is_disable;

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