Tcl Source Code

Tcl_ListObjReplace contains code:
int first;
int count
....
if (numElems < first+count || first+count < 0) {
/*
* The 'first+count < 0' condition here guards agains integer
* overflow in determining 'first+count'.
*/

So overflow is expected but it's calculated with signed type which is "undefined behavior"

Proposed fix:

--- old/generic/tclListObj.c	2015-11-17 17:03:00.000000000 -0800
+++ new/generic/tclListObj.c	2017-08-01 23:22:59.000000000 -0700
@@ -897,13 +897,16 @@
     }
     if (count < 0) {
 	count = 0;
-    } else if (numElems < first+count || first+count < 0) {
-	/*
-	 * The 'first+count < 0' condition here guards agains integer
-	 * overflow in determining 'first+count'.
-	 */
+    } else {
+	int firstWithCount = (unsigned) first + count;
+	if (numElems < firstWithCount || firstWithCount < 0) {
+	    /*
+	     * The 'first+count < 0' condition here guards agains integer
+	     * overflow in determining 'first+count'.
+	     */
 
-	count = numElems - first;
+	    count = numElems - first;
+	}
     }
 
     isShared = (listRepPtr->refCount > 1);

Already fixed for 8.6.7+

https://core.tcl.tk/tcl/info/b2a4266498c8dfd7