gh-152375: Fix undefined behaviour in the INSTRUMENTED_JUMP macro (#152376)

Author: StanFromIreland

Misc/NEWS.d/next/Core_and_Builtins/2026-06-27-10-05-12.gh-issue-152375.L-ZBk6.rst

@@ -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.

Python/ceval_macros.h

@@ -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; \
             JUMP_TO_LABEL(error); \
         } \
     } \