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code-423n4 / 2022-07-axelar-findings

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Gas Optimizations #225

Open code423n4 opened 2 years ago

code423n4 commented 2 years ago

[S]: Suggested optimation, save a decent amount of gas without compromising readability;

[M]: Minor optimation, the amount of gas saved is minor, change when you see fit;

[N]: Non-preferred, the amount of gas saved is at cost of readability, only apply when gas saving is a top priority.

ISSUE LIST

C4-001 : Adding unchecked directive can save gas [S]

C4-002 : Check if amount > 0 before token transfer can save gas [S]

C4-003 : There is no need to assign default values to variables [S]

C4-004 : Using operator && used more gas [S]

C4-005 : Non-strict inequalities are cheaper than strict ones [M]

C4-006 : Cache array length in for loops can save gas [S]

C4-007 : Use calldata instead of memory for function parameters [M]

C4-008 : ++i is more gas efficient than i++ in loops forwarding

C4-009 : > 0 can be replaced with != 0 for gas optimization

C4-010 : Keccak functions in constants waste gas [M]

C4-011 : Free gas savings for using solidity 0.8.10+ [S]

C4-001 : Adding unchecked directive can save gas

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

  2022-07-axelar-main/contracts/auth/AxelarAuthWeighted.sol::116 => for (uint256 i; i < accounts.length - 1; ++i) {
  2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol::114 => for (uint256 i; i < refundTokens.length; i++) {
  2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol::168 => for (uint256 i; i < refundTokens.length; i++) {
  2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol::204 => for (uint256 i; i < refundTokens.length; i++) {

Tools Used

None

Recommended Mitigation Steps

Consider applying unchecked arithmetic where overflow/underflow is not possible. Example can be seen from below.

Unchecked{i++};

C4-002 : Check if amount > 0 before token transfer can save gas

Impact

Since _amount can be 0. Checking if (_amount != 0) before the transfer can potentially save an external call and the unnecessary gas cost of a 0 token transfer.

Proof of Concept

https://github.com/jbx-protocol/juice-contracts-v2-code4rena/blob/828bf2f3e719873daa08081cfa0d0a6deaa5ace5/contracts/JBERC20PaymentTerminal.sol#L87

All Contracts

Tools Used

None

Recommended Mitigation Steps

Consider checking amount != 0.

C4-003 : There is no need to assign default values to variables

Impact - Gas Optimization

Uint is default initialized to 0. There is no need assign false to variable.

Proof of Concept

  2022-07-axelar-main/contracts/AxelarGateway.sol::207 => for (uint256 i = 0; i < symbols.length; i++) {
  2022-07-axelar-main/contracts/auth/AxelarAuthWeighted.sol::68 => uint256 totalWeight = 0;
  2022-07-axelar-main/contracts/auth/AxelarAuthWeighted.sol::69 => for (uint256 i = 0; i < weightsLength; ++i) {
  2022-07-axelar-main/contracts/auth/AxelarAuthWeighted.sol::94 => uint256 operatorIndex = 0;
  2022-07-axelar-main/contracts/auth/AxelarAuthWeighted.sol::95 => uint256 weight = 0;
  2022-07-axelar-main/contracts/auth/AxelarAuthWeighted.sol::98 => for (uint256 i = 0; i < signatures.length; ++i) {

Tools Used

Code Review

Recommended Mitigation Steps

uint x = 0 costs more gas than uint x without having any different functionality.

C4-004 : Using operator && used more gas

Impact

Using double require instead of operator && can save more gas.

Proof of Concept

  1. Navigate to the following contracts.

2022-07-axelar-main/contracts/gas-service/AxelarGasService.sol:159:        bool transferred = success && (returnData.length == uint256(0) || abi.decode(returnData, (bool)));
2022-07-axelar-main/contracts/gas-service/AxelarGasService.sol:175:        bool transferred = success && (returnData.length == uint256(0) || abi.decode(returnData, (bool)));
2022-07-axelar-main/contracts/deposit-service/DepositBase.sol:72:        bool transferred = success && (returnData.length == uint256(0) || abi.decode(returnData, (bool)));
2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol:118:            if (refundTokens[i] == gatewayToken && msg.sender != refundAddress) continue;
2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol:165:        if (addressForNativeDeposit(salt, refundAddress, destinationChain, destinationAddress).balance > 0 && msg.sender != refundAddress)
2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol:208:            if (refundTokens[i] == wrappedTokenAddress && msg.sender != refundAddress) continue;
2022-07-axelar-main/contracts/AxelarGateway.sol:388:            if (!success || (returnData.length != uint256(0) && !abi.decode(returnData, (bool)))) revert BurnFailed(symbol);
2022-07-axelar-main/contracts/AxelarGateway.sol:462:        return success && (returnData.length == uint256(0) || abi.decode(returnData, (bool)));
2022-07-axelar-main/contracts/AxelarGateway.sol:613:        if (limit > 0 && amount > limit) revert ExceedDailyMintLimit(symbol);

Tools Used

Code Review

Recommended Mitigation Steps

Example


using &&:

function check(uint x)public view{
require(x == 0 && x < 1 );
}
// gas cost 21630

using double require:

require(x == 0 );
require( x < 1);
}
}
// gas cost 21622

C4-005 : Non-strict inequalities are cheaper than strict ones

Impact

Strict inequalities add a check of non equality which costs around 3 gas.

Proof of Concept

  2022-07-axelar-main/contracts/auth/AxelarAuthWeighted.sol::116 => for (uint256 i; i < accounts.length - 1; ++i) {
  2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol::114 => for (uint256 i; i < refundTokens.length; i++) {
  2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol::168 => for (uint256 i; i < refundTokens.length; i++) {
  2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol::204 => for (uint256 i; i < refundTokens.length; i++) {

Tools Used

Code Review

Recommended Mitigation Steps

Use >= or <= instead of > and < when possible.

C4-006 : Cache array length in for loops can save gas

Impact

Reading array length at each iteration of the loop takes 6 gas (3 for mload and 3 to place memory_offset) in the stack.

Caching the array length in the stack saves around 3 gas per iteration.

Proof of Concept

  1. Navigate to the following smart contract line.
  2022-07-axelar-main/contracts/auth/AxelarAuthWeighted.sol::116 => for (uint256 i; i < accounts.length - 1; ++i) {
  2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol::114 => for (uint256 i; i < refundTokens.length; i++) {
  2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol::168 => for (uint256 i; i < refundTokens.length; i++) {
  2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol::204 => for (uint256 i; i < refundTokens.length; i++) {

Tools Used

None

Recommended Mitigation Steps

Consider to cache array length.

C4-008 : Use calldata instead of memory for function parameters

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.

Proof of Concept

  1. Navigate to the following smart contract line.
2022-07-axelar-main/contracts/auth/AxelarAuthWeighted.sol:56:        (address[] memory newOperators, uint256[] memory newWeights, uint256 newThreshold) = abi.decode(

Tools Used

None

Recommended Mitigation Steps

Some parameters in examples given above are later hashed. It may be beneficial for those parameters to be in memory rather than calldata.

C4-009 : ++i is more gas efficient than i++ in loops forwarding

Impact

++i is more gas efficient than i++ in loops forwarding.

Proof of Concept

  1. Navigate to the following contracts.
  2022-07-axelar-main/contracts/auth/AxelarAuthWeighted.sol::116 => for (uint256 i; i < accounts.length - 1; ++i) {
  2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol::114 => for (uint256 i; i < refundTokens.length; i++) {
  2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol::168 => for (uint256 i; i < refundTokens.length; i++) {
  2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol::204 => for (uint256 i; i < refundTokens.length; i++) {

Tools Used

Code Review

Recommended Mitigation Steps

It is recommend to use unchecked{++i} and change i declaration to uint256.

C4-010 : > 0 can be replaced with != 0 for gas optimization

Impact

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

Proof of Concept

  1. Navigate to the following contract sections.

  2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol::165 => if (addressForNativeDeposit(salt, refundAddress, destinationChain, destinationAddress).balance > 0 && msg.sender != refundAddress)
  2022-07-axelar-main/contracts/deposit-service/ReceiverImplementation.sol::23 => if (address(this).balance > 0) refundAddress.transfer(address(this).balance);
  2022-07-axelar-main/contracts/deposit-service/ReceiverImplementation.sol::51 => if (address(this).balance > 0) refundAddress.transfer(address(this).balance);
  2022-07-axelar-main/contracts/deposit-service/ReceiverImplementation.sol::71 => if (address(this).balance > 0) refundAddress.transfer(address(this).balance);

Tools Used

None

Recommended Mitigation Steps

Consider to replace > 0 with != 0 for gas optimization.

C4-011 : Keccak functions in constants waste gas

Impact

The contracts assigns two constants to the result of a keccak operation, which results in gas waste since the expression is computed each time the constant is accessed.

See this issue for more context: ethereum/solidity#9232 (https://github.com/ethereum/solidity/issues/9232)

Proof of Concept

  2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol::227 => keccak256(
  2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol::233 => keccak256(abi.encodePacked(type(DepositReceiver).creationCode, abi.encode(delegateData)))
  2022-07-axelar-main/contracts/deposit-service/AxelarDepositService.sol::242 => return keccak256('axelar-deposit-service');
  2022-07-axelar-main/contracts/deposit-service/AxelarDepositServiceProxy.sol::9 => return keccak256('axelar-deposit-service');
  2022-07-axelar-main/contracts/gas-service/AxelarGasService.sol::27 => keccak256(payload),
  2022-07-axelar-main/contracts/gas-service/AxelarGasService.sol::52 => keccak256(payload),
  2022-07-axelar-main/contracts/gas-service/AxelarGasService.sol::71 => emit NativeGasPaidForContractCall(sender, destinationChain, destinationAddress, keccak256(payload), msg.value, refundAddress);
  2022-07-axelar-main/contracts/gas-service/AxelarGasService.sol::90 => keccak256(payload),
  2022-07-axelar-main/contracts/gas-service/AxelarGasService.sol::181 => return keccak256('axelar-gas-service');

Tools Used

None

Recommended Mitigation Steps

Replace the constant directive with immutable, or assign the already hashed value to the constants.

C4-012 : Free gas savings for using solidity 0.8.10+

Impact

Using newer compiler versions and the optimizer gives gas optimizations and additional safety checks are available for free.

Proof of Concept

All Contracts

Solidity 0.8.13 has a useful change which reduced gas costs of external calls which expect a return value: https://blog.soliditylang.org/2021/11/09/solidity-0.8.10-release-announcement/

Solidity 0.8.13 has some improvements too but not well tested.

Code Generator: Skip existence check for external contract if return data is expected. In this case, the ABI decoder will revert if the contract does not exist

All Contracts

Tools Used

None

Recommended Mitigation Steps

Consider to upgrade pragma to at least 0.8.13.

GalloDaSballo commented 2 years ago

Less than 500 gas saved (calldata <- mem example doesn't apply from what I can tell)