gh-148891: prevent crash in HAMT#148928
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KowalskiThomas wants to merge 1 commit intopython:mainfrom
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| // gh-148891: _PyHamt_Assoc can allocate HAMT nodes, which may trigger | ||
| // GC. A weakref callback or finalizer could then re-enter | ||
| // contextvar_set, replacing ctx->ctx_vars via Py_SETREF and freeing | ||
| // the old HAMT while _PyHamt_Assoc is still traversing it. | ||
| // Prevent this by holding a strong reference to the current vars. | ||
| PyHamtObject *old_vars = ctx->ctx_vars; | ||
| Py_INCREF(old_vars); | ||
| PyHamtObject *new_vars = _PyHamt_Assoc( | ||
| ctx->ctx_vars, (PyObject *)var, val); | ||
| old_vars, (PyObject *)var, val); | ||
| Py_DECREF(old_vars); |
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This looks true. We have a lot of increfs/decrefs like that because the call between them can do scary things.
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What is this PR?
Fix for the crash reported and investigated in #148891.
_PyHamt_Assocallocates HAMT nodes, which can trigger GC. A weakref callback or finaliser fired during GC could re-entercontextvar_set, replacingctx->ctx_varsviaPy_SETREFand freeing the old HAMT while_PyHamt_Assocwas still traversing it, causing a use-after-free.The proposed fix is to hold a strong reference to
ctx->ctx_varsduring the_PyHamt_Assoccall, so that a re-entrant free cannot affect the HAMT being traversed.