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gh-152375: Fix UB in INSTRUMENTED_JUMP #152376

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gh-152375: Fix UB in INSTRUMENTED_JUMP #152376

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452407b
Fix UB in `INSTRUMENTED_JUMP`
StanFromIreland Jun 27, 2026
9110586
Merge branch 'main' into INSTRUMENTED_JUMP-ub
StanFromIreland Jun 27, 2026
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-06-27-10-05-12.gh-issue-152375.L-ZBk6.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix undefined behaviour when a :mod:`sys.monitoring` callback raised an
exception while the program was following a branch or loop.
7 changes: 4 additions & 3 deletions Python/ceval_macros.h
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Expand Up @@ -389,14 +389,15 @@ static void dtrace_function_return(_PyInterpreterFrame *);
// for an exception handler, displaying the traceback, and so on
#define INSTRUMENTED_JUMP(src, dest, event) \
do { \
_Py_CODEUNIT *_dest = (dest); \
if (tstate->tracing) {\
next_instr = dest; \
next_instr = _dest; \
} else { \
_PyFrame_SetStackPointer(frame, stack_pointer); \
next_instr = _Py_call_instrumentation_jump(this_instr, tstate, event, frame, src, dest); \
next_instr = _Py_call_instrumentation_jump(this_instr, tstate, event, frame, src, _dest); \
stack_pointer = _PyFrame_GetStackPointer(frame); \
if (next_instr == NULL) { \
next_instr = (dest)+1; \
next_instr = _dest + 1; \

@iritkatriel iritkatriel Jun 27, 2026

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What does this do? Is it the same as just casting dest to _Py_CODEUNIT?

@StanFromIreland StanFromIreland Jun 27, 2026

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This is a macro, where dest expands to next_instr + oparg:

cpython/Python/generated_cases.c.h

Line 8158 in b3154aa

INSTRUMENTED_JUMP(this_instr, next_instr + oparg, PY_MONITORING_EVENT_BRANCH_RIGHT);

(The issue has more detail on this.)

@iritkatriel iritkatriel Jun 27, 2026

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But I don't understand the fix. It looks like just a rename.

@iritkatriel iritkatriel Jun 27, 2026

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Got it!

JUMP_TO_LABEL(error); \
} \
} \
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