Segfault: `Py_DECREF(NULL)` in `setup_context`/`do_warn` (`_warnings.c`) when emitting a warning under MemoryError

[devdanzin]

Crash report

What happened?

AI Disclaimer: this issue was drafted by Claude Code, which also generated the reduced reproducer.

Emitting a warning while allocations are failing segfaults on a Py_DECREF of a NULL filename. setup_context() does not NULL-check PyUnicode_FromString("<sys>"), and the resulting NULL is later decref'd either by do_warn() (success path) or by setup_context()'s own handle_error: label (which uses Py_DECREF, not Py_XDECREF).

Reproducer

import _testcapi, warnings, faulthandler
faulthandler.enable()
warnings.simplefilter("always")
_testcapi.set_nomemory(0, 0)   # fail every allocation from here on
warnings.warn("boom")          # -> Py_DECREF(NULL) -> SIGSEGV

Deterministic (start=0). Reproduced on main.

Backtrace

#0 _Py_atomic_load_uint32_relaxed   Include/cpython/pyatomic_gcc.h
#1 Py_DECREF                        Include/refcount.h
#2 do_warn                          Python/_warnings.c:1139   <- Py_DECREF(filename), filename == NULL
#3 warnings_warn_impl               Python/_warnings.c:1184
#4 warnings_warn                    Python/clinic/_warnings.c.h:161

(gdb) frame 2; print filename → $1 = (PyObject *) 0x0.

Root cause

In setup_context() (_warnings.c), the f == NULL branch (~L1036):

if (f == NULL) {
    globals = interp->sysdict;
    *filename = PyUnicode_FromString("<sys>");   /* return value unchecked */
    *lineno = 0;
}

Under memory pressure two allocations fail in sequence:

1. PyThreadState_GetFrame() returns NULL (its frame-object allocation fails), so the f == NULL branch is taken; then
2. PyUnicode_FromString("<sys>") itself returns NULL, leaving *filename == NULL.

The NULL *filename then reaches a Py_DECREF:

• if setup_context() returns success (registry and __name__ already present in globals, so no further allocation is needed), do_warn() runs its cleanup Py_DECREF(filename) at L1139 — NULL deref; or
• if setup_context() instead hits handle_error: (L1084), that path runs Py_DECREF(*filename) (L1087, not Py_XDECREF) — NULL deref.

Suggested fix

    *filename = PyUnicode_FromString("<sys>");
    if (*filename == NULL) {
        goto handle_error;
    }

and at handle_error: use Py_XDECREF(*filename) (since *filename may legitimately be NULL there).

Notes

Found by OOM-injection fuzzing (set_nomemory). Same pattern as gh-146080 (Py_DECREF(NULL) in an _ssl.c error label). Code is long-standing, so 3.13–3.15 are likely affected as well (unverified).

Found using fusil by @vstinner.

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:bfecfcc2a86, Jun 18 2026, 13:48:35) [Clang 22.1.2 (1ubuntu1)]