crossinterp: Segfault in `check_missing___main___attr` if AttributeError args are not normal

Crash report

What happened?

crossinterp doesn't do much checking on the arguments of the passed-in exception, allowing lone surrogate unicode, or non-str arguments cause segfaults:

Lone surrogate example:

from concurrent import interpreters
import __main__

x = 1

def f():
    return x  # forces pickle fallback

f.__name__ = f.__qualname__ = "\ud800"
setattr(__main__, "\ud800", f)

interp = interpreters.create()
interp.call(f)

> ./python.exe temp/crossinterp.py
fish: Job 1, './python.exe temp/crossinterp.py' terminated by signal SIGSEGV (Address boundary error)

Non-str argument example (a bit more convoluted):

from concurrent import interpreters

x = 1

def f():
    return x  # forces pickle fallback

interp = interpreters.create()
interp.exec("""
import pickle

def loads(data):
    raise AttributeError(42)

pickle.loads = loads
""")
interp.call(f)

> ./python.exe temp/crossinterp-2.py
fish: Job 1, './python.exe temp/crossinterp-2…' terminated by signal SIGSEGV (Address boundary error)

This is because check_missing___main___attr doesn't check the result of PyUnicode_AsUTF8 before passing it to strncmp, so if the object is not a PyUnicode object or if it can't be converted to utf8, you get a crash:

https://github.com/python/cpython/blob/91d71dd67074d4599b6bd49cc933f41f8bd57058/Python/crossinterp.c#L666-669

The fix should cover both cases, I guess:

A PyUnicode_Check on msgobj, and then a check that the PyUnicode_AsUTF8 return is valid.

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-156128

Commencez dans Python/crossinterp.c, au niveau de check_missing___main___attr, comme indiqué dans le rapport, et reproduisez à la fois le cas d’AttributeError avec un lone-surrogate et le cas d’AttributeError avec un argument qui n’est pas une chaîne. Vérifiez que des arguments d’exception malformés ne provoquent plus de segfault et que le comportement existant des appels cross-interpreter reste intact; gh-156128 est déjà lié à cet issue.