gh-144206: Improve error messages for buffer overflow in fcntl.fcntl() and fcntl.ioctl() (GH-144273)

Misc/NEWS.d/next/Library/2026-01-27-14-23-10.gh-issue-144206.l0un4U.rst

diff --git a/Misc/NEWS.d/next/Library/2026-01-27-14-23-10.gh-issue-144206.l0un4U.rst b/Misc/NEWS.d/next/Library/2026-01-27-14-23-10.gh-issue-144206.l0un4U.rst
new file mode 100644
index 0000000..1e16d28
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-01-27-14-23-10.gh-issue-144206.l0un4U.rst
@@ -0,0 +1,2 @@
+Improve error messages for buffer overflow in :func:`fcntl.fcntl` and
+:func:`fcntl.ioctl`.

Modules/fcntlmodule.c

diff --git a/Modules/fcntlmodule.c b/Modules/fcntlmodule.c
index e373bf3..ce636c5 100644
--- a/Modules/fcntlmodule.c
+++ b/Modules/fcntlmodule.c
@@ -111,7 +111,11 @@ fcntl_fcntl_impl(PyObject *module, int fd, int code, PyObject *arg)
                 return !async_err ? PyErr_SetFromErrno(PyExc_OSError) : NULL;
             }
             if (memcmp(buf + len, guard, GUARDSZ) != 0) {
-                PyErr_SetString(PyExc_SystemError, "buffer overflow");
+                PyErr_SetString(PyExc_SystemError,
+                        "Possible stack corruption in fcntl() due to "
+                        "buffer overflow. "
+                        "Provide an argument of sufficient size as "
+                        "determined by the operation.");
                 return NULL;
             }
             return PyBytes_FromStringAndSize(buf, len);
@@ -139,7 +143,11 @@ fcntl_fcntl_impl(PyObject *module, int fd, int code, PyObject *arg)
                 return NULL;
             }
             if (ptr[len] != '\0') {
-                PyErr_SetString(PyExc_SystemError, "buffer overflow");
+                PyErr_SetString(PyExc_SystemError,
+                        "Memory corruption in fcntl() due to "
+                        "buffer overflow. "
+                        "Provide an argument of sufficient size as "
+                        "determined by the operation.");
                 PyBytesWriter_Discard(writer);
                 return NULL;
             }
@@ -264,7 +272,12 @@ fcntl_ioctl_impl(PyObject *module, int fd, unsigned long code, PyObject *arg,
                 }
                 PyBuffer_Release(&view);
                 if (ptr == buf && memcmp(buf + len, guard, GUARDSZ) != 0) {
-                    PyErr_SetString(PyExc_SystemError, "buffer overflow");
+                    PyErr_SetString(PyExc_SystemError,
+                            "Possible stack corruption in ioctl() due to "
+                            "buffer overflow. "
+                            "Provide a writable buffer argument of "
+                            "sufficient size as determined by "
+                            "the operation.");
                     return NULL;
                 }
                 return PyLong_FromLong(ret);
@@ -293,7 +306,11 @@ fcntl_ioctl_impl(PyObject *module, int fd, unsigned long code, PyObject *arg,
                 return !async_err ? PyErr_SetFromErrno(PyExc_OSError) : NULL;
             }
             if (memcmp(buf + len, guard, GUARDSZ) != 0) {
-                PyErr_SetString(PyExc_SystemError, "buffer overflow");
+                PyErr_SetString(PyExc_SystemError,
+                        "Possible stack corruption in ioctl() due to "
+                        "buffer overflow. "
+                        "Provide an argument of sufficient size as "
+                        "determined by the operation.");
                 return NULL;
             }
             return PyBytes_FromStringAndSize(buf, len);
@@ -321,7 +338,11 @@ fcntl_ioctl_impl(PyObject *module, int fd, unsigned long code, PyObject *arg,
                 return NULL;
             }
             if (ptr[len] != '\0') {
-                PyErr_SetString(PyExc_SystemError, "buffer overflow");
+                PyErr_SetString(PyExc_SystemError,
+                        "Memory corruption in ioctl() due to "
+                        "buffer overflow. "
+                        "Provide an argument of sufficient size as "
+                        "determined by the operation.");
                 PyBytesWriter_Discard(writer);
                 return NULL;
             }