bpo-30255: Clip step in _PySlice_Unpack() (#1429)
In PySlice_IndicesEx, clip the step to [-PY_SSIZE_T_MAX,
PY_SSIZE_T_MAX] rather than [PY_SSIZE_T_MIN, PY_SSIZE_T_MAX].
(cherry picked from commit e6fc7401a92c7b51a80782d8095819b9909a0322)
diff --git a/Objects/sliceobject.c b/Objects/sliceobject.c
index 8f17fca..64be927 100644
--- a/Objects/sliceobject.c
+++ b/Objects/sliceobject.c
@@ -147,6 +147,13 @@
"slice step cannot be zero");
return -1;
}
+ /* Here *step might be -PY_SSIZE_T_MAX-1; in this case we replace it
+ * with -PY_SSIZE_T_MAX. This doesn't affect the semantics, and it
+ * guards against later undefined behaviour resulting from code that
+ * does "step = -step" as part of a slice reversal.
+ */
+ if (*step < -PY_SSIZE_T_MAX)
+ *step = -PY_SSIZE_T_MAX;
}
if (r->start == Py_None) {