JIT: Assertion `size < MAX_EXECUTORS_SIZE` failed in `get_index_for_executor`

[devdanzin]

Crash report

What happened?

It's possible to cause an abort in a patched JIT build by running the code below. This bug doesn't seem to repro if using an interpreter from a venv with lots o stuff installed, but it does when directly calling Python from a build dir: /path/to/build/python mre.py.

Necessary patch:

diff --git a/Include/internal/pycore_backoff.h b/Include/internal/pycore_backoff.h
index fadd11f04ec..3e779a141af 100644
--- a/Include/internal/pycore_backoff.h
+++ b/Include/internal/pycore_backoff.h
@@ -125,7 +125,7 @@ trigger_backoff_counter(void)
 // For example, 4095 does not work for the nqueens benchmark on pyperformance
 // as we always end up tracing the loop iteration's
 // exhaustion iteration. Which aborts our current tracer.
-#define JUMP_BACKWARD_INITIAL_VALUE 4000
+#define JUMP_BACKWARD_INITIAL_VALUE 63
 #define JUMP_BACKWARD_INITIAL_BACKOFF 6
 static inline _Py_BackoffCounter
 initial_jump_backoff_counter(_PyPolicy *policy)
@@ -139,7 +139,7 @@ initial_jump_backoff_counter(_PyPolicy *policy)
  * Must be larger than ADAPTIVE_COOLDOWN_VALUE,
  * otherwise when a side exit warms up we may construct
  * a new trace before the Tier 1 code has properly re-specialized. */
-#define SIDE_EXIT_INITIAL_VALUE 4000
+#define SIDE_EXIT_INITIAL_VALUE 63
 #define SIDE_EXIT_INITIAL_BACKOFF 6

 static inline _Py_BackoffCounter

MRE

from sys import stderr
import gc
gc.set_threshold(1)
import random

# u/U means an undefined name
def f1():
    def v(a=2): ...

    try: res = v()
    except Exception: u()
    except U: u

    class A:
        def _(self): ...

    d = {}
    for i in range(10):
        try: u
        except Exception: pass
        def _(): ...

    class B:
        def _(self): b = 1

    class C(B):
        def f(self):
            for i in [4**62]:
                C._(object)

    for i in (-1, 0):
        c = C()
    for _ in range(299):
        try: _ = c.f()
        except Exception: u

    for i in range(76):
        C.__bases__ = (object,)
    for _ in range(100):
        try: _ = c.f()
        except AttributeError: pass

    for i in range(501):
        if i - 9 == 3: u
        elif u / 9 == 2: (var,) = {}
        elif u / 1 != 3:
            for i in [24]: 1 / 0
        try: u(u)
        except U: u

    for i in []:
        class D: u
    t = 2, 0
    (u,)

for i in range(300):
    try: f1()
    except Exception as e: print(i, file=stderr)

Backtrace

python: Python/optimizer.c:71: int32_t get_index_for_executor(PyCodeObject *, _Py_CODEUNIT *): Assertion `size < MAX_EXECUTORS_SIZE' failed.

Program received signal SIGABRT, Aborted.

#0  __pthread_kill_implementation (threadid=<optimized out>, signo=6, no_tid=0) at ./nptl/pthread_kill.c:44
#1  __pthread_kill_internal (threadid=<optimized out>, signo=6) at ./nptl/pthread_kill.c:89
#2  __GI___pthread_kill (threadid=<optimized out>, signo=signo@entry=6) at ./nptl/pthread_kill.c:100
#3  0x00007ffff7c45e2e in __GI_raise (sig=sig@entry=6) at ../sysdeps/posix/raise.c:26
#4  0x00007ffff7c28888 in __GI_abort () at ./stdlib/abort.c:77
#5  0x00007ffff7c287f0 in __assert_fail_base (fmt=<optimized out>, assertion=<optimized out>, file=<optimized out>, line=<optimized out>, function=<optimized out>) at ./assert/assert.c:118
#6  0x00007ffff7c3c19f in __assert_fail (assertion=<optimized out>, file=<optimized out>, line=<optimized out>, function=<optimized out>) at ./assert/assert.c:127
#7  0x00005555563ff51e in get_index_for_executor (code=0x7d9ff6dfaa90, instr=0x7d9ff6dfaed2) at Python/optimizer.c:71
#8  _PyOptimizer_Optimize (frame=frame@entry=0x7e8ff6de52b0, tstate=tstate@entry=0x555556ed3030 <_PyRuntime+359408>) at Python/optimizer.c:177
#9  0x0000555555eda0a4 in stop_tracing_and_jit (tstate=0x555556ed3030 <_PyRuntime+359408>, frame=frame@entry=0x7e8ff6de52b0) at Python/ceval.c:1461
#10 0x0000555555e9693a in _PyEval_EvalFrameDefault (tstate=<optimized out>, frame=<optimized out>, throwflag=<optimized out>) at Python/generated_cases.c.h:11868
#11 0x0000555555e72e68 in _PyEval_EvalFrame (tstate=0x555556ed3030 <_PyRuntime+359408>, frame=0x7e8ff6de5220, throwflag=0) at ./Include/internal/pycore_ceval.h:118
#12 _PyEval_Vector (tstate=<optimized out>, func=<optimized out>, locals=<optimized out>, args=<optimized out>, argcount=<optimized out>, kwnames=0x0) at Python/ceval.c:2541
#13 0x0000555555e72885 in PyEval_EvalCode (co=<optimized out>, globals=<optimized out>, locals=0x7c7ff6e863c0) at Python/ceval.c:1005
#14 0x000055555649d01f in run_eval_code_obj (tstate=tstate@entry=0x555556ed3030 <_PyRuntime+359408>, co=co@entry=0x7d3ff6e16050, globals=globals@entry=0x7c7ff6e863c0,
    locals=locals@entry=0x7c7ff6e863c0) at Python/pythonrun.c:1366
#15 0x000055555649c1ec in run_mod (mod=<optimized out>, filename=<optimized out>, globals=<optimized out>, locals=<optimized out>, flags=<optimized out>, arena=<optimized out>,
    interactive_src=<optimized out>, generate_new_source=<optimized out>) at Python/pythonrun.c:1469
#16 0x00005555564967ed in pyrun_file (fp=fp@entry=0x7d4ff6defa80, filename=filename@entry=0x7ccff6dfaef0, start=start@entry=257, globals=globals@entry=0x7c7ff6e863c0,
    locals=locals@entry=0x7c7ff6e863c0, closeit=closeit@entry=1, flags=0x7bfff5c1e910) at Python/pythonrun.c:1294
#17 0x000055555649434d in _PyRun_SimpleFileObject (fp=<optimized out>, filename=<optimized out>, closeit=<optimized out>, flags=<optimized out>) at Python/pythonrun.c:518
#18 0x00005555564936be in _PyRun_AnyFileObject (fp=fp@entry=0x7d4ff6defa80, filename=filename@entry=0x7ccff6dfaef0, closeit=closeit@entry=1, flags=flags@entry=0x7bfff5c1e910)
    at Python/pythonrun.c:81
#19 0x000055555650ff2b in pymain_run_file_obj (program_name=0x7caff6e230d0, filename=0x7ccff6dfaef0, skip_source_first_line=0) at Modules/main.c:410
#20 pymain_run_file (config=0x555556e9e0f8 <_PyRuntime+142520>) at Modules/main.c:429
#21 0x000055555650dff4 in pymain_run_python (exitcode=0x7bfff5c1e500) at Modules/main.c:691
#22 Py_RunMain () at Modules/main.c:772
#23 0x000055555650eef7 in pymain_main (args=<optimized out>) at Modules/main.c:802
#24 0x000055555650f068 in Py_BytesMain (argc=<optimized out>, argv=0x7fffffffdbb8) at Modules/main.c:826
#25 0x00007ffff7c2a575 in __libc_start_call_main (main=main@entry=0x55555591ec40 <main>, argc=argc@entry=2, argv=argv@entry=0x7fffffffdbb8) at ../sysdeps/nptl/libc_start_call_main.h:58
#26 0x00007ffff7c2a628 in __libc_start_main_impl (main=0x55555591ec40 <main>, argc=2, argv=0x7fffffffdbb8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>,
    stack_end=0x7fffffffdba8) at ../csu/libc-start.c:360
#27 0x00005555558354f5 in _start ()

Output from running with PYTHON_LLTRACE=4 PYTHON_OPT_DEBUG=4:
3495_abort_lltrace_opt_debug.txt

Found using lafleur.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.15.0a3+ (heads/main-dirty:fca7fec88ce, Jan 13 2026, 13:04:59) [Clang 21.1.2 (2ubuntu6)]

Linked PRs

• gh-143807: Fix JIT executor overflow caused by large backoff values #143812
• gh-143807: Limit executor growth #143814

[Fidget-Spinner]

There seems to be no code checking that PyMem_Realloc in get_index_for_executor is below the limit even though there's an assert there.