Last updated: December 19, 2024

SCSTG-TEST-0008: Efficient Loop and Function Design

Description

Inefficient loop and function designs in smart contracts can result in excessive gas consumption, leading to out-of-gas errors and potential denial-of-service (DoS) vulnerabilities. Smart contract security auditors must ensure that all loops and functions are optimized to operate within Ethereum's block gas limit.

Test 1: Detecting Unbounded Loops

Objective

Identify and mitigate the use of unbounded loops in contract code, as they are vulnerable to excessive gas usage.

Vulnerable Code Example

pragma solidity ^0.8.0;

contract UnboundedLoopExample {
    uint[] public data;

    function processAll() public {
        for (uint i = 0; i < data.length; i++) {
            // This operation scales with the size of the array
            data[i] = data[i] + 1;
        }
    }
}

Why It’s Vulnerable

The loop iterates over the entire data array, which could grow indefinitely.
- For large arrays, gas consumption may exceed the block gas limit, causing the transaction to fail.
- Attackers could exploit this by filling the array, making the function unusable for legitimate users.

How to Check

• Code Review: Look for for or while loops operating on dynamic arrays or mappings without size constraints.
• Dynamic Input Testing: Test the function with a large dataset to simulate its behavior near the gas limit.
• Review Documentation: Ensure the contract specifies constraints on data growth and function usage.

Fixed Code Example

pragma solidity ^0.8.0;

contract BoundedLoopExample {
    uint[] public data;

    function processBatch(uint start, uint end) public {
        require(end <= data.length, "End index out of bounds");
        for (uint i = start; i < end; i++) {
            data[i] = data[i] + 1;
        }
    }
}

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Test 2: Identifying Inefficient Nested Loops

Objective

• Detect and address inefficient nested loops that exponentially increase gas consumption.

Vulnerable Code:

pragma solidity ^0.8.0;

contract NestedLoopExample {
    uint[][] public matrix;

    function processMatrix() public {
        for (uint i = 0; i < matrix.length; i++) {
            for (uint j = 0; j < matrix[i].length; j++) {
                matrix[i][j] = matrix[i][j] * 2;
            }
        }
    }
}

Why It’s Vulnerable

Nested loops increase the complexity of the operation, leading to higher gas costs as input size increases.
- Large datasets could render the function unusable within the gas limits, causing DoS conditions.

How to Check

• Code Review: Examine nested loops in the code and assess their gas consumption relative to the input size.
• Gas Profiling: Use tools like eth-gas-reporter to analyze gas usage during testing.
• Dynamic Testing: Simulate scenarios with large matrix sizes to observe the contract’s behavior under stress.

pragma solidity ^0.8.0;

contract OptimizedNestedLoopExample {
    uint[][] public matrix;

    function processMatrixBatch(uint startRow, uint endRow) public {
        require(endRow <= matrix.length, "End row exceeds matrix size");
        for (uint i = startRow; i < endRow; i++) {
            for (uint j = 0; j < matrix[i].length; j++) {
                matrix[i][j] = matrix[i][j] * 2;
            }
        }
    }
}