Validate that output_subset indices are within bounds in CVM

pytensor/link/c/c_code/lazylinker_c.c

@@ -775,8 +775,17 @@ static PyObject *CLazyLinker_call(PyObject *_self, PyObject *args,
           err = 1;
           PyErr_SetString(PyExc_RuntimeError,
                           "Some elements of output_subset list are not int");
+          break;
+        }
+        long idx = PyLong_AsLong(elem);
+        if (idx < 0 || idx >= self->n_output_vars) {
+          err = 1;
+          PyErr_Format(PyExc_IndexError,
+                       "output_subset index %ld is out of range [0, %zd)",
+                       idx, self->n_output_vars);
+          break;
         }
-        output_subset[PyLong_AsLong(elem)] = 1;
+        output_subset[idx] = 1;
       }
     }
   }
@@ -981,7 +990,7 @@ static PyTypeObject lazylinker_ext_CLazyLinkerType = {
 };
 
 static PyObject *get_version(PyObject *dummy, PyObject *args) {
-  PyObject *result = PyFloat_FromDouble(0.31);
+  PyObject *result = PyFloat_FromDouble(0.32);
   return result;
 }
 

pytensor/link/c/lazylinker_c.py

@@ -14,7 +14,7 @@ from pytensor.link.c.cmodule import GCC_compiler
 _logger = logging.getLogger(__file__)
 
 force_compile = False
-version = 0.31  # must match constant returned in function get_version()
+version = 0.32  # must match constant returned in function get_version()
 lazylinker_ext: ModuleType | None = None
 
 

tests/link/test_vm.py

@@ -544,3 +544,32 @@ def test_Stack_updates():
 
     assert res == [np.array(1.0), np.array(2.0)]
     assert storage_map[a][0] == np.array(2.0)
+
+
+@pytest.mark.parametrize(
+    "linker", [VMLinker(allow_partial_eval=True, use_cloop=False), "cvm"]
+)
+def test_partial_function_output_subset_oob(linker):
+    """Regression test: out-of-bounds output_subset must raise, not segfault."""
+    x = scalar("input")
+    y = x**2
+    f = function(
+        [x], [y + 7, y - 9, y / 14.0], mode=Mode(optimizer=None, linker=linker)
+    )
+
+    # Index equal to n_output_vars (off-by-one)
+    with pytest.raises((IndexError, Exception)):
+        f(3, output_subset=[len(f.output_storage)])
+
+    # Large out-of-bounds index
+    with pytest.raises((IndexError, Exception)):
+        f(3, output_subset=[100])
+
+    # Negative index: the CVM C code must reject it; the Python Stack VM
+    # delegates to Python list indexing which naturally wraps negatives.
+    if linker == "cvm":
+        with pytest.raises((IndexError, Exception)):
+            f(3, output_subset=[-1])
+
+    # Verify the function still works after the error cases
+    utt.assert_allclose(f(5), np.array([32.0, 16.0, 1.7857142857142858]))