JIT: Segfault from invalid memory read in `_PyTier2Interpreter`

[devdanzin]

Crash report

What happened?

It's possible to segfault a JIT build by running the code below:

def f1():
    a = "a"
    for i in range(50):
        x = a[i % len(a)]

    s = ""
    for _ in range(10):
        s += ""

    class A: ...
    class B: ...

    match s:
        case int(): ...
        case str(): ...
        case dict(): ...

    (
        u0,
        *u1,
        u2,
        u4,
        u5,
        u6,
        u7,
        u8,
        u9, u10, u11, 
        u12, u13, u14, u15, u16, u17, u18, u19, u20, u21, u22, u23, u24, u25, u26, u27, u28, u29,
    ) = [None, None, None, None, None, None, None, None, None, None, None, None, None, None, None,
         None, None, None, None, None, None, None, None, None, None, None, None, None, None, None,
         None, None, None, None, None, None, None, None, None, None, None, None, None, None, None,
         None, None, None, None, None, None, None, None,]

    s = ""
    for _ in range(10):
        s += ""
        s += ""

for i in range(11000):
    f1()

Backtrace:

Program received signal SIGSEGV, Segmentation fault.
_PyTier2Interpreter (current_executor=<optimized out>, frame=<optimized out>, stack_pointer=<optimized out>, tstate=<optimized out>) at Python/executor_cases.c.h:10549
10549               if (Py_TYPE(r) != &PyRangeIter_Type) {

#0  _PyTier2Interpreter (current_executor=<optimized out>, frame=<optimized out>, stack_pointer=<optimized out>, tstate=<optimized out>) at Python/executor_cases.c.h:10549
#1  0x000055555584c1b2 in _PyEval_EvalFrameDefault (tstate=tstate@entry=0x555555d5c9b0 <_PyRuntime+358864>, frame=<optimized out>, frame@entry=0x7ffff7fa7020, throwflag=throwflag@entry=0)
    at Python/generated_cases.c.h:5348
#2  0x0000555555830e4b in _PyEval_EvalFrame (tstate=0x555555d5c9b0 <_PyRuntime+358864>, frame=0x7ffff7fa7020, throwflag=0) at ./Include/internal/pycore_ceval.h:119
#3  _PyEval_Vector (tstate=tstate@entry=0x555555d5c9b0 <_PyRuntime+358864>, func=func@entry=0x7ffff6bf2390, locals=locals@entry=0x7ffff6c00ad0, args=args@entry=0x0,
    argcount=argcount@entry=0, kwnames=kwnames@entry=0x0) at Python/ceval.c:2483
#4  0x0000555555830bde in PyEval_EvalCode (co=co@entry=0x7ffff6c0c3a0, globals=globals@entry=0x7ffff6c00ad0, locals=locals@entry=0x7ffff6c00ad0) at Python/ceval.c:1008
#5  0x000055555599b13a in run_eval_code_obj (tstate=0x555555d5c9b0 <_PyRuntime+358864>, co=co@entry=0x7ffff6c0c3a0, globals=globals@entry=0x7ffff6c00ad0, locals=locals@entry=0x7ffff6c00ad0)
    at Python/pythonrun.c:1366
#6  0x000055555599ac8b in run_mod (mod=mod@entry=0x555555f17e10, filename=filename@entry=0x7ffff6c4e7a0, globals=globals@entry=0x7ffff6c00ad0, locals=locals@entry=0x7ffff6c00ad0,
    flags=0x7fffffffd740, arena=arena@entry=0x7ffff6c711e0, interactive_src=0x0, generate_new_source=0) at Python/pythonrun.c:1469
#7  0x00005555559985ad in pyrun_file (fp=fp@entry=0x555555dd0360, filename=filename@entry=0x7ffff6c4e7a0, start=start@entry=257, globals=globals@entry=0x7ffff6c00ad0,
    locals=locals@entry=0x7ffff6c00ad0, closeit=closeit@entry=1, flags=0x7fffffffd740) at Python/pythonrun.c:1294

ASan output:

AddressSanitizer:DEADLYSIGNAL
=================================================================
==3999430==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000008 (pc 0x7da56db089a7 bp 0x7ffd350e2e90 sp 0x7ffd350e21a0 T0)
==3999430==The signal is caused by a READ memory access.
==3999430==Hint: address points to the zero page.
    #0 0x7da56db089a7  (<unknown module>)
    #1 0x55577b512f97 in _PyEval_EvalFrame /home/danzin/projects/jit_cpython/./Include/internal/pycore_ceval.h:119:16
    #2 0x55577b512f97 in _PyEval_Vector /home/danzin/projects/jit_cpython/Python/ceval.c:2483:12
    #3 0x55577b5129b4 in PyEval_EvalCode /home/danzin/projects/jit_cpython/Python/ceval.c:1008:21
    #4 0x55577bafc2ce in run_eval_code_obj /home/danzin/projects/jit_cpython/Python/pythonrun.c:1366:12
    #5 0x55577bafb49b in run_mod /home/danzin/projects/jit_cpython/Python/pythonrun.c:1469:19
    #6 0x55577baf5a9c in pyrun_file /home/danzin/projects/jit_cpython/Python/pythonrun.c:1294:15
    #7 0x55577baf35fc in _PyRun_SimpleFileObject /home/danzin/projects/jit_cpython/Python/pythonrun.c:518:13
    #8 0x55577baf296d in _PyRun_AnyFileObject /home/danzin/projects/jit_cpython/Python/pythonrun.c:81:15
    #9 0x55577bb6ea4a in pymain_run_file_obj /home/danzin/projects/jit_cpython/Modules/main.c:410:15
    #10 0x55577bb6ea4a in pymain_run_file /home/danzin/projects/jit_cpython/Modules/main.c:429:15
    #11 0x55577bb6cb13 in pymain_run_python /home/danzin/projects/jit_cpython/Modules/main.c:691:21
    #12 0x55577bb6cb13 in Py_RunMain /home/danzin/projects/jit_cpython/Modules/main.c:772:5
    #13 0x55577bb6da16 in pymain_main /home/danzin/projects/jit_cpython/Modules/main.c:802:12
    #14 0x55577bb6db87 in Py_BytesMain /home/danzin/projects/jit_cpython/Modules/main.c:826:12
    #15 0x7da56e62a574 in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #16 0x7da56e62a627 in __libc_start_main csu/../csu/libc-start.c:360:3
    #17 0x55577aed64f4 in _start (/home/danzin/projects/jit_cpython/python+0x2db4f4) (BuildId: 2bff0bd42ddf0d6a9c5bada7767fdb560f820115)

==3999430==Register values:
rax = 0x0000000000000000  rbx = 0x0000000000000000  rcx = 0x0000000000000000  rdx = 0x000055577c2ad760
rdi = 0x0000000000000000  rsi = 0x00007c356d9e53b8  rbp = 0x00007ffd350e2e90  rsp = 0x00007ffd350e21a0
 r8 = 0x00007c356d9e5408   r9 = 0x0000000000000003  r10 = 0x00000aaaef855aac  r11 = 0x00000aaaef855aac
r12 = 0x00007c356d9e5298  r13 = 0x00007c356d9e5420  r14 = 0x000055577c509830  r15 = 0x0000000000000001
AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (<unknown module>)
==3999430==ABORTING

Output from running with PYTHON_LLTRACE=4 PYTHON_OPT_DEBUG=4:
3004_segfault_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:3cc57505e53, Dec 23 2025, 05:19:47) [Clang 21.1.2 (2ubuntu6)]

[Fidget-Spinner]

Thanks, verified. The problem is that the first instruction is a super instruction (_BINARY_OP_INPLACE_ADD_UNICODE). The progress requirement forces the trace recorder to record a deopt version of that (BINARY_OP). However, that leads to a wrong trace as the following _STORE_FAST is not recorded, as the trace recorder rightfully thinks that a super instruction has executed.

I could add a special case for super instructions in the trace recorder, but that would just be error-prone. I'm thinking of removing the progress requirements from the JIT and just make it so ENTER_EXECUTOR executes itself if next_instr == this_instr. WDYT @markshannon ?

[Fidget-Spinner]

The other benefit of removing the progress requirements is we get more speed -- the first instruction can now be specialized. The first instruction is actually a really important one too because it usually corresponds to a loop header!

[markshannon]

First some thoughts on _BINARY_OP_INPLACE_ADD_UNICODE:

_BINARY_OP_INPLACE_ADD_UNICODE is a weird special case that only exists because of an historic promise that:

s = some_string
for _ in range(1000):
    s += "x"

would not be quadratic.

If we lower the JIT thresholds a bit (which I think we should anyway) we could do the optimization in the JIT and do away with the super-instruction.

[Fidget-Spinner]

If we lower the JIT thresholds a bit (which I think we should anyway) we could do the optimization in the JIT and do away with the super-instruction.

I don't think we can do that considering it will remove the quadratic optimization for non-JIT platforms/runtimes, which would be quite bad. Having quadratic behavior on interpreter and non-quadratic on JIT is a weird disparity that I don't want to introduce.

[markshannon]

I'm thinking of removing the progress requirements from the JIT ...

We need to be careful to avoid infinite deopt loops that make no progress?

just make it so ENTER_EXECUTOR executes itself if next_instr == this_instr

I presume you mean after we return from tier2. How would that work with tailcaliing in the future (when tier2 tailcalls into tier1)

[markshannon]

I don't think we can do that considering it will remove the quadratic optimization for non-JIT platforms/runtimes, which would be quite bad. Having quadratic behavior on interpreter and non-quadratic on JIT is a weird disparity that I don't want to introduce.

We don't need to drop it from the interpreter if there is no JIT. We would just don't perform that specialization if the JIT is enabled.

[Fidget-Spinner]

We don't need to drop it from the interpreter if there is no JIT. We would just don't perform that specialization if the JIT is enabled.

Ok fair enough.

[markshannon]

Instead of handling the special case in ENTER_EXECUTOR, which doesn't really have enough context to do it properly, we should do it in the jitted code.

We already have several forms of tier2 -> tier1 transfers: exits, deopts, handle-pending-and-deopt, error.
Adding another one, exit-but-dont-reenter, wouldn't be too much effort.
We can then specialize the first uop, but use a different exit uop for it.