XDEFI contest - defsec's results

Handle

defsec

Vulnerability details

Impact

!= 0 is a cheaper operation compared to > 0, when dealing with uint.

Proof of Concept

1. Navigate to the following contracts.

https://github.com/XDeFi-tech/xdefi-distribution/blob/master/contracts/XDEFIDistribution.sol#L145

Tools Used

Code Review

Recommended Mitigation Steps

Use "!=0" instead of ">0" for the gas optimization.

Handle

defsec

Vulnerability details

Impact

In some cases, having function arguments in calldata instead of memory is more optimal.

Consider the following generic example:

contract C {
function add(uint[] memory arr) external returns (uint sum) {
uint length = arr.length;
for (uint i = 0; i < arr.length; i++) {
  sum += arr[i];
}
}
}

In the above example, the dynamic array arr has the storage location memory. When the function gets called externally, the array values are kept in calldata and copied to memory during ABI decoding (using the opcode calldataload and mstore). And during the for loop, arr[i] accesses the value in memory using a mload. However, for the above example this is inefficient. Consider the following snippet instead:

contract C {
function add(uint[] calldata arr) external returns (uint sum) {
uint length = arr.length;
for (uint i = 0; i < arr.length; i++) {
  sum += arr[i];
}
}
}

In the above snippet, instead of going via memory, the value is directly read from calldata using calldataload. That is, there are no intermediate memory operations that carries this value.

Gas savings: In the former example, the ABI decoding begins with copying value from calldata to memory in a for loop. Each iteration would cost at least 60 gas. In the latter example, this can be completely avoided. This will also reduce the number of instructions and therefore reduces the deploy time cost of the contract.

In short, use calldata instead of memory if the function argument is only read.

Note that in older Solidity versions, changing some function arguments from memory to calldata may cause "unimplemented feature error". This can be avoided by using a newer (0.8.*) Solidity compiler.

Examples Note: The following pattern is prevalent in the codebase:

function f(bytes memory data) external {
(...) = abi.decode(data, (..., types, ...));
}

Here, changing to bytes calldata will decrease the gas. The total savings for this change across all such uses would be quite significant.

Proof Of Concept

Examples:

https://github.com/XDeFi-tech/xdefi-distribution/blob/master/contracts/XDEFIDistribution.sol#L325

Tools Used

None

Recommended Mitigation Steps

Change memory definition with calldata.

Handle

defsec

Vulnerability details

Impact

For the arithmetic operations that will never over/underflow, using the unchecked directive (Solidity v0.8 has default overflow/underflow checks) can save some gas from the unnecessary internal over/underflow checks.

Proof of Concept

https://github.com/XDeFi-tech/xdefi-distribution/blob/master/contracts/XDEFIDistribution.sol#L274

Tools Used

None

Recommended Mitigation Steps

Consider applying unchecked arithmetic where overflow/underflow is not possible.

Handle

defsec

Vulnerability details

Impact

Lower than uint256 size storage instance variables are actually less gas efficient. E.g. using uint32 does not give any efficiency, actually, it is the opposite as EVM operates on default of 256-bit values so uint32 is more expensive in this case as it needs a conversion. It only gives improvements in cases where you can pack variables together, e.g. structs.

Proof of Concept

1. Navigate to the following contracts.

https://github.com/XDeFi-tech/xdefi-distribution/blob/master/contracts/XDEFIDistribution.sol#L301

2. Expiry value is just used for the comparison with the block.timestamp.

Tools Used

None

Recommended Mitigation Steps

Consider to review all uint types. Change them with uint256 If the integer is not necessary to present with uint32.

Handle

defsec

Vulnerability details

Impact

There is a function declared as public that are never called internally within the contract. It is best practice to mark such functions as external instead, as this saves gas (especially in the case where the function takes arguments, as external functions can read arguments directly from calldata instead of having to allocate memory).

Proof of Concept

https://github.com/XDeFi-tech/xdefi-distribution/blob/6021c9a89bad4adf17eaba4ea6b3697793d9d7c6/contracts/XDEFIDistributionHelper.sol#L11
https://github.com/XDeFi-tech/xdefi-distribution/blob/6021c9a89bad4adf17eaba4ea6b3697793d9d7c6/contracts/XDEFIDistributionHelper.sol#L20

Tools Used

Code Review

Recommended Mitigation Steps

All of the public functions in the contract are not called internally, so access can be changed to external to reduce gas.