overflow check when calculating the position (#103)

* overflow check when calculating the position * less than or equal to
FourthState · Feb 26, 2019 · 2a41203 · 2a41203
1 parent 51f51e6
commit 2a41203
Showing 1 changed file with 4 additions and 1 deletion.
diff --git a/contracts/PlasmaMVP.sol b/contracts/PlasmaMVP.sol
@@ -512,7 +512,10 @@ contract PlasmaMVP {
     {
         require(validatePostion([txPos[0], txPos[1], txPos[2], 0]));
 
-        return txPos[0].mul(blockIndexFactor).add(txPos[1].mul(txIndexFactor)).add(txPos[2]);
+        uint256 position = txPos[0].mul(blockIndexFactor).add(txPos[1].mul(txIndexFactor)).add(txPos[2]);
+        require(position <= 2**128-1); // check for an overflow
+
+        return position;
     }
 
     function validatePostion(uint256[4] memory position)