gh-157247: Block perf trampoline activation when the main interpreter's JIT is enabled

Lib/test/test_perf_profiler.py

@@ -318,6 +318,27 @@ def test_sys_api_with_existing_perf_jit_trampoline(self):
                 """
         assert_python_ok("-c", code, PYTHON_JIT="0")
 
+    @unittest.skipUnless(
+        "-D_Py_JIT" in (sysconfig.get_config_var("PY_CORE_CFLAGS") or "").split(),
+        "requires a real JIT (_Py_JIT)",
+    )
+    def test_sys_api_perf_jit_backend_in_subinterpreter(self):
+        # gh-157247: a subinterpreter must not bypass the JIT/perf exclusion.
+        code = """if 1:
+                import sys
+                from contextlib import closing
+                from concurrent import interpreters
+
+                assert sys._jit.is_enabled(), "expected the JIT to be enabled"
+
+                with closing(interpreters.create()) as interp:
+                    interp.exec(
+                        "import sys; sys.activate_stack_trampoline('perf_jit')")
+                """
+        rc, out, err = assert_python_failure("-c", code, PYTHON_JIT="1")
+        self.assertIn(
+            b"Cannot activate the perf trampoline if the JIT is active", err)
+
 
 def is_unwinding_reliable_with_frame_pointers():
     cflags = sysconfig.get_config_var("PY_CORE_CFLAGS")

Misc/NEWS.d/next/Core_and_Builtins/2026-09-10-08-21-07.gh-issue-157247.Zv4bz2.rst

@@ -0,0 +1,2 @@
+Fix :func:`sys.activate_stack_trampoline` allowing activation from a
+subinterpreter while the JIT is enabled in the main interpreter.

Python/sysmodule.c

@@ -2348,7 +2348,9 @@ sys_activate_stack_trampoline_impl(PyObject *module, const char *backend)
 {
 #ifdef PY_HAVE_PERF_TRAMPOLINE
 #ifdef _Py_JIT
-    if (_PyInterpreterState_GET()->jit) {
+    // Perf state is process-wide, and only the main interpreter can enable
+    // the JIT. Check it even when called from a subinterpreter (gh-157247).
+    if (_PyInterpreterState_Main()->jit) {
         PyErr_SetString(PyExc_ValueError, "Cannot activate the perf trampoline if the JIT is active");
         return NULL;
     }