Segfault/debug abort because _PyModule_GetFilenameUTF8 asserts module filenames must be utf-8 (called by crossinterp)
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Description
Crash report
What happened?
import __main__
from concurrent import interpreters
__main__.__file__ = "bad\ud800.py"
interp = interpreters.create()
interp.call(lambda x: x, [1])
sstagg@sapphire ~/s/t/cpython (main)> ./python.exe temp/crossinterp-main.py
Assertion failed: (size >= 0), function _PyModule_GetFilenameUTF8, file moduleobject.c, line 995.
fish: Job 1, './python.exe temp/crossinterp-m…' terminated by signal SIGABRT (Abort)
on a non-debug build:
fish: Job 1, 'python' terminated by signal SIGSEGV (Address boundary error)
because the result is passed to strncpy:
LLDB
Process 31843 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
frame #0: 0x000000019c4a0e14 libsystem_platform.dylib`_platform_strcpy + 4
libsystem_platform.dylib`_platform_strcpy:
-> 0x19c4a0e14 <+4>: ldr q0, [x9]
0x19c4a0e18 <+8>: str q0, [sp, #-0x10]!
0x19c4a0e1c <+12>: ands x8, x1, #0xf
0x19c4a0e20 <+16>: b.eq 0x19c4a0e7c ; <+108>
Target 0: (python) stopped.
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
* frame #0: 0x000000019c4a0e14 libsystem_platform.dylib`_platform_strcpy + 4
frame #1: 0x00000001007e30c0 libpython3.14.dylib`_PyModule_GetFilenameUTF8 + 344
frame #2: 0x00000001008f0fa4 libpython3.14.dylib`_PyPickle_GetXIData + 484
frame #3: 0x00000001008f0d1c libpython3.14.dylib`_PyObject_GetXIData + 120
frame #4: 0x00000001008f55e4 libpython3.14.dylib`_tuple_shared + 200
frame #5: 0x00000001008f0bb4 libpython3.14.dylib`_get_xidata + 344
frame #6: 0x00000001008f0ce8 libpython3.14.dylib`_PyObject_GetXIData + 68
frame #7: 0x00000001000c5354 _interpreters.cpython-314-darwin.so`interp_call + 308
frame #8: 0x00000001007e1904 libpython3.14.dylib`cfunction_call + 92
frame #9: 0x0000000100785d08 libpython3.14.dylib`_PyObject_MakeTpCall + 304
frame #10: 0x00000001008c64e0 libpython3.14.dylib`_PyEval_EvalFrameDefault + 15148
frame #11: 0x00000001008c24b0 libpython3.14.dylib`PyEval_EvalCode + 416
frame #12: 0x00000001008bec1c libpython3.14.dylib`builtin_exec + 1392
frame #13: 0x00000001008c599c libpython3.14.dylib`_PyEval_EvalFrameDefault + 12264
frame #14: 0x00000001008c26dc libpython3.14.dylib`_PyEval_Vector + 496
frame #15: 0x00000001007864d8 libpython3.14.dylib`_PyVectorcall_Call + 116
frame #16: 0x00000001009674bc libpython3.14.dylib`pymain_start_pyrepl + 628
frame #17: 0x000000010096704c libpython3.14.dylib`pymain_run_stdin + 640
frame #18: 0x00000001009665c0 libpython3.14.dylib`Py_RunMain + 1584
frame #19: 0x000000010096682c libpython3.14.dylib`pymain_main + 320
frame #20: 0x00000001009668d0 libpython3.14.dylib`Py_BytesMain + 44
frame #21: 0x000000019c0c3e88 dyld`start + 6688
(lldb)
_PyModule_GetFilenameUTF8 only appears to be called by crossinterp module, but it does this:
Which means that if a module filename has a lone surrogate, then it PyUnicode_AsUTF8AndSize retusns and sets size -1, which triggers the abort.
I'm not sure what should happen here, should lone surrogates in module filenames be allowed? or should this 'just work' by not trying to coerce to utf8?
CPython versions tested on:
CPython main branch
Operating systems tested on:
macOS
Output from running 'python -VV' on the command line:
Python 3.16.0a0 (heads/main:e8158d1a02d, Aug 20 2026, 15:09:24) [Clang 21.0.0 (clang-2100.3.27.1)]
Linked PRs
- gh-156218
- gh-156221
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with Objects/moduleobject.c around _PyModule_GetFilenameUTF8 and follow the cross-interpreter call path shown in the traceback, including _PyPickle_GetXIData. Reproduce the provided lone-surrogate main.file example, establish the intended filename handling, and add regression coverage showing that the call no longer aborts or segfaults.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- backend
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 25/100