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SemSEpaper/exercises.tex

 
 \medspace
 
-In the case of a dynamic array at variable slot $p$, data is written to continuous locations starting at $keccak(p)$. The array itself contains the length information. It is worth noting that Solidity does not come with utility functions to manipulate arrays, and the developer is required to correctly maintain the length value in order to keep track of the array's state.
+In the case of a dynamic array at variable slot $p$, data is written to continuous locations starting at $keccak(p)$. The array itself contains the length information.
 
 \medspace
 
 
 Any unchecked array write is potentially dangerous, as the storage-location of all variables is publicly known and an unconstrained array index can be reverse engineered to target them.
 
+\lstset{style=mystyle}
+\begin{algorithm}
+	\begin{lstlisting}[language=Octave]
+	pragma solidity 0.4.25;
+	
+	contract MyContract {
+		address private owner;
+		uint[] private arr;
+		
+		constructor() public {
+			arr = new uint[](0);
+			owner = msg.sender;
+		}
+		
+		function write(unit index, uint value) {
+			arr[index] = value;
+		}
+	}
+	\end{lstlisting}
+	\caption{A completely unchecked array write}
+\end{algorithm}
+
+In the following example the $pop$ function incorrectly checks for an array $length >= 0$, thereby allowing the value to underflow when called with an empty array. Once this weakness is exploited $update$ in Algorithm 2 behaves just like $write$ did in Algorithm 1. 
+
 \lstset{style=mystyle}
 \begin{algorithm}
 	\begin{lstlisting}[language=Octave]
 		pragma solidity 0.4.25;
 		
 		contract MyContract {
-			uint[] private arr;
 			address private owner;
+			uint[] private arr;
+			
+			constructor() public {
+				arr = new uint[](0);
+				owner = msg.sender;
+			}
 			
-			function write(unit index, uint value) {
+			function push(value) {
+				arr[arr.length] = value;
+				arr.length++;
+			}
+			
+			function pop() {
+				require(arr.length >= 0);
+				arr.length--;
+			}
+			
+			function update(unit index, uint value) {
+				require(index < arr.length);
 				arr[index] = value;
 			}
 		}
 	\end{lstlisting}
-	\caption{A completely unchecked array write}
-	\label{alg:agf-opt-merge}
+	\caption{An incorrectly managed array length}
 \end{algorithm}
 
-In the case of dynamic arrays an improper constraint of the $length$ can be dangerous. As $length$ is unsigned, it is possible to underflow it past $2^{256} - 1$ by decrementing the length below zero, thereby effectively marking the whole address space as part of it.
 
 \section{Vulnerable contracts in literature}