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gh-116502: Fix memory access violation on fatal error with Windows #116503

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gh-116502: Fix memory access violation on fatal error with Windows #116503

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8da834c
Fix memory access violation on fatal error with Windows
dlaugt Mar 8, 2024
f892938
Fix compilation
Jun 21, 2025
e93fbda
Solution without the malloc. On Windows, if the message is longer tha…
dlaugt Jun 23, 2025
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37 changes: 34 additions & 3 deletions Python/pylifecycle.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3079,6 +3079,8 @@ _Py_FatalError_PrintExc(PyThreadState *tstate)
static void
fatal_output_debug(const char *msg)
{
assert(msg != NULL);

/* buffer of 256 bytes allocated on the stack */
WCHAR buffer[256 / sizeof(WCHAR)];
size_t buflen = Py_ARRAY_LENGTH(buffer) - 1;
Expand Down Expand Up @@ -3360,7 +3362,12 @@ fatal_error(int fd, int header, const char *prefix, const char *msg,
}

#ifdef MS_WINDOWS
fatal_output_debug(msg);
if (msg) {
fatal_output_debug(msg);
}
else {
fatal_output_debug("<message not set>");
}
#endif /* MS_WINDOWS */

fatal_error_exit(status);
Expand All @@ -3382,6 +3389,8 @@ _Py_FatalErrorFunc(const char *func, const char *msg)
fatal_error(fileno(stderr), 1, func, msg, -1);
}

#define FATAL_MSG_SIZE 1024
#define FATAL_MSG_SIZE_WITHOUT_ELLIPSIS FATAL_MSG_SIZE - 3

void _Py_NO_RETURN
_Py_FatalErrorFormat(const char *func, const char *format, ...)
Expand All @@ -3393,6 +3402,29 @@ _Py_FatalErrorFormat(const char *func, const char *format, ...)
}
reentrant = 1;

va_list vargs;
#ifdef MS_WINDOWS
va_start(vargs, format);
int length = vsnprintf(NULL, 0, format, vargs);
va_end(vargs);

char msg[FATAL_MSG_SIZE];
if (length < FATAL_MSG_SIZE) {
va_start(vargs, format);
vsnprintf(msg, length + 1, format, vargs);
va_end(vargs);

fatal_error(fileno(stderr), 1, func, msg, -1);
}

va_start(vargs, format);
vsnprintf(msg, FATAL_MSG_SIZE_WITHOUT_ELLIPSIS, format, vargs);
va_end(vargs);
strcpy (msg + FATAL_MSG_SIZE_WITHOUT_ELLIPSIS - 1, "...");
#else
char *msg = NULL;
#endif

FILE *stream = stderr;
const int fd = fileno(stream);
PUTS(fd, "Fatal Python error: ");
Expand All @@ -3401,15 +3433,14 @@ _Py_FatalErrorFormat(const char *func, const char *format, ...)
PUTS(fd, ": ");
}

va_list vargs;
va_start(vargs, format);
vfprintf(stream, format, vargs);
va_end(vargs);

fputs("\n", stream);
fflush(stream);

fatal_error(fd, 0, NULL, NULL, -1);
fatal_error(fd, 0, func, msg, -1);
}


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