segfault/debug abort from `perf_map_write_entry` with lone surrogate in filename or qualname

[stestagg]

Crash report

What happened?

import sys
import sysconfig
import types

assert not sysconfig.get_config_var("PY_HAVE_PERF_TRAMPOLINE")
sys.activate_stack_trampoline("perf")
code = compile("pass", "bad\ud800file", "exec")
eval(code)

python3: Python/generated_cases.c.h:13862: PyObject *_PyEval_EvalFrameDefault(PyThreadState *, _PyInterpreterFrame *, int): Assertion `!_PyErr_Occurred(tstate)' failed.
Aborted

perf_map_write_entry does not check that the calls to PyUnicde_AsUTF8 succeed, resulting in null pointers being passed to snprintf. This is UB, and may crash in some libc versions, or may be replaced with a placeholder value.

Regardless, a failed conversion leaves a stale exception around, which then will trigger a debug abort a bit later (in debug builds):

cpython/Python/perf_trampoline.c

Lines 251 to 262 in e8158d1

	perf_map_write_entry(void *state, const void *code_addr,
	size_t code_size, PyCodeObject *co)
	{
	const char *entry = "";
	if (co->co_qualname != NULL) {
	entry = PyUnicode_AsUTF8(co->co_qualname);
	}
	const char *filename = "";
	if (co->co_filename != NULL) {
	filename = PyUnicode_AsUTF8(co->co_filename);
	}
	size_t perf_map_entry_size = snprintf(NULL, 0, "py::%s:%s", entry, filename) + 1;

Seems trivial to check the result value here. I notice that perf_jit_trampoline (similar code) uses a common helper function that checks the values and replaces them with "" if NULL

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

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

Python 3.16.0a0 (heads/main-dirty:20e6c2f, Aug 19 2026, 15:22:30) [Clang 22.1.8 ]