akiratechhq / review-hopprotocol-contracts-2021-04 Goto Github PK

Description

The methods addBonder and removeBonder respectively add and remove bonders.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/cb9804f243005850597952f5152d7b9397f9968b/code/contracts/bridges/Accounting.sol#L138-L145

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/cb9804f243005850597952f5152d7b9397f9968b/code/contracts/bridges/Accounting.sol#L147-L154

Also, the constructor adds a list of bonders to the bonder list.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/cb9804f243005850597952f5152d7b9397f9968b/code/contracts/bridges/Accounting.sol#L54-L59

It might be beneficial to emit events when adding and removing bonders.

Recommendation

Consider emitting events when adding or removing bonders.

Description

The method Lib_MerkleTree._ceilLog2 is used in the getMerkleRoot method to calculate the total siblings count.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/5e66c267c52206b7e77d5515f64d939e85539f04/code/contracts/libraries/Lib_MerkleTree.sol#L144-L147

As stated in the source code, it was copied from the Solidity examples repository.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/5e66c267c52206b7e77d5515f64d939e85539f04/code/contracts/libraries/Lib_MerkleTree.sol#L202-L203

The Solidity example repository includes this method as an example for other developers to learn how to use the language. Because it does not strive to be the optimal implementation, its purpose is to show different features of the language itself.

https://github.com/ethereum/solidity-examples/blob/f44fe3b3b4cca94afe9c2a2d5b7840ff0fafb72e/src/bits/Bits.sol#L87-L99

This is why some of the examples in the Solidity repository might not be gas optimized.

Using uint8 in the loop forces the compiler to create sub-optimal code.

The following Solidity contract example was used to measure the difference between using uint8 and uint256 with a few example inputs.

contract FindBit {
    function find_highest_set_bit(
        uint value
    ) 
        public 
        pure 
        returns (
            uint
        ) 
    {
        uint _in = value;
        
        if (_in == 1) {
            return 0;
        }

        // Find the highest set bit (will be floor(log_2)).
        // Borrowed with <3 from https://github.com/ethereum/solidity-examples
        uint256 val = _in;
        uint256 highest = 0;
        for (uint8 i = 128; i >= 1; i >>= 1) {
            if (val & (uint(1) << i) - 1 << i != 0) {
                highest += i;
                val >>= i;
            }
        }

        // Increment by one if this is not a perfect logarithm.
        if ((uint(1) << highest) != _in) {
            highest += 1;
        }

        return highest;        
    }
}

Changing the line from uint8 to uint256

        for (uint8 i = 128; i >= 1; i >>= 1) {

        for (uint256 i = 128; i >= 1; i >>= 1) {

Gives the following different gas costs, while returning the same, identical results.

Solidity version 0.8.3+commit.8d00100c and 200 optimization runs were used in Remix IDE for tests.

Recommendation

Modify the loop iterator from uint8 to uint256 to reduce gas costs, while having the same output.

References

• Solidity example, highest set bit
• Optimism MerkleTree implementation

Description

All of these methods update important configurations for the bridge.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/master/code/contracts/bridges/L2_Bridge.sol#L292-L337

Recommendation

Consider emitting events with the new values (and possibly old values) when these changes go through.

Similarly can be done for L1_Bridge.

Description

The contract L2_AmmWrapper has a few storage variables.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_AmmWrapper.sol#L13-L17

These storage vars are set at deploy time.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_AmmWrapper.sol#L19-L34

None of them are changed after the initial set.

This is why they could be defined as immutable. Solidity does not reserve a storage slot for immutables, but every occurrence is replaced in the bytecode by the respective value. This greatly reduces gas costs.

To have a better understanding of the gas cost reduction, two contracts were created.

The first contract defines a storage variable and sets it at deploy time. It also set the variable to public, this way Solidity will create a getter for it. This getter is important to measure the gas cost to access it.

contract A {
    bool public isSet;
    
    constructor(bool _isSet) public {
        isSet = _isSet;
    }
}

The second contract uses an immutable instead of a storage slot. That is the only difference between the two.

contract B {
    bool public immutable isSet;
    
    constructor(bool _isSet) public {
        isSet = _isSet;
    }
}

We've created the following table to show the differences in gas costs between the two approaches.

Both examples were tested in Remix IDE, Solidity version 0.6.12, optimization runs 200.

Recommendation

Change all state variables to immutable to greatly reduce gas costs.

Similarly, change the state variables to immutable in L2_Bridge.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/24ccdb4d4528ecb4e92c3d7ac5e1909bf8de34f7/code/contracts/bridges/L2_Bridge.sol#L78-L80

But also in these cases (not a complete list):

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/24ccdb4d4528ecb4e92c3d7ac5e1909bf8de34f7/code/contracts/wrappers/ArbitrumMessengerWrapper.sol#L36-L39

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/24ccdb4d4528ecb4e92c3d7ac5e1909bf8de34f7/code/contracts/wrappers/OptimismMessengerWrapper.sol#L29

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/24ccdb4d4528ecb4e92c3d7ac5e1909bf8de34f7/code/contracts/wrappers/XDaiMessengerWrapper.sol#L35-L36

There are instances where some state variables are never changed in other contracts. If you think you won't change them in the future, consider checking all contract constructors.

References

• Constant and Immutable State Variables

Description

This method is set as view but can be restricted to pure as long as the Solidity version is <0.8.x.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/51418c09ffde685e7f03be3308d723aa96fad13c/code/contracts/bridges/Bridge.sol#L112

Also, the hack to silence the state mutability can also be removed.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/51418c09ffde685e7f03be3308d723aa96fad13c/code/contracts/bridges/Bridge.sol#L113

If you need the view mutability in order to allow other layer 2 implementations to first save and then retrieve the value from the contract state, keep the view modifier, otherwise consider switching to pure. Solidity does not do too many things differently but uses STATICCALL when calling the method to make sure the state is not changed.

Recommendation

Unless there's a reason to keep the view, consider changing to pure.

Description

The Accounting contract takes care of low-level accounting and has a couple of external methods that handle staking and unstaking of funds.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/8e53a77cafe91a8ad6a09a99ee387b8566cd726b/code/contracts/bridges/Accounting.sol#L118-L127

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/8e53a77cafe91a8ad6a09a99ee387b8566cd726b/code/contracts/bridges/Accounting.sol#L129-L136

A couple of matching events exist in the contract.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/8e53a77cafe91a8ad6a09a99ee387b8566cd726b/code/contracts/bridges/Accounting.sol#L28-L30

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/8e53a77cafe91a8ad6a09a99ee387b8566cd726b/code/contracts/bridges/Accounting.sol#L32-L34

But they are not emitted in this contract or in any contract that inherits Accounting.

Recommendation

Emit events when staking and unstaking.

Description

The method distribute can only be called by the Layer 1 Bridge.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_Bridge.sol#L176-L184

The external method calls the internal method _distribute in order to distribute tokens.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_Bridge.sol#L263

If the fee is positive, it will mint some tokens for the caller.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_Bridge.sol#L264-L266

In this case, the caller is the Layer 1 Bridge as stated above.

However, the L1 Bridge doesn't seem to have a method to move the tokens.

 +  L1_Bridge (Bridge)
    - [Pub] <Constructor> #
       - modifiers: Bridge
    - [Ext] sendToL2 ($)
    - [Ext] bondTransferRoot #
       - modifiers: onlyBonder,requirePositiveBalance
    - [Ext] confirmTransferRoot #
       - modifiers: onlyL2Bridge
    - [Int] _distributeTransferRoot #
    - [Ext] challengeTransferBond ($)
    - [Ext] resolveChallenge #
    - [Int] _additionalDebit
    - [Int] _requireIsGovernance #
    - [Ext] setGovernance #
       - modifiers: onlyGovernance
    - [Ext] setCrossDomainMessengerWrapper #
       - modifiers: onlyGovernance
    - [Ext] setChainIdDepositsPaused #
       - modifiers: onlyGovernance
    - [Ext] setChallengeAmountMultiplier #
       - modifiers: onlyGovernance
    - [Ext] setChallengeAmountDivisor #
       - modifiers: onlyGovernance
    - [Ext] setChallengePeriodAndTimeSlotSize #
       - modifiers: onlyGovernance
    - [Ext] setChallengeResolutionPeriod #
       - modifiers: onlyGovernance
    - [Ext] setMinTransferRootBondDelay #
       - modifiers: onlyGovernance
    - [Pub] getBondForTransferAmount
    - [Pub] getChallengeAmountForTransferAmount
    - [Pub] getTimeSlot

In which case the tokens that represent the fee, will remain locked in the Layer 1 Bridge contract.

Recommendation

Consider adding an additional argument to point to the receiver of the fee and pass that along through the internal _distribute method. This is especially useful for the bondWithdrawalAndDistribute method.

The fee doesn't seem necessary if the Layer 1 Bridge calls the external distribute method because the amount is not bonded.

Description

Uniswap is mentioned, make sure to replace with something like AMM.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_AmmWrapper.sol#L56

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_AmmWrapper.sol#L59

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_AmmWrapper.sol#L72-L73

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_AmmWrapper.sol#L88

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_AmmWrapper.sol#L95

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_AmmWrapper.sol#L97

Recommendation

Make sure to update the error messages to reflect the current contract.

Description

The governance can set a minimum fee, as well as a minimum percentage by calling setMinimumBonderFeeRequirements.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_Bridge.sol#L334-L337

The values could be checked for sanity, especially the percentage.

Recommendation

Check the value of minBonderBps to be something less than 10000.

Description

There are a couple of error messages that refer to the L2 Bridge

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_AmmWrapper.sol#L50

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_AmmWrapper.sol#L53

Typically the L2_BRG error prefix is found in Bridge and L2_Bridge.

Also, the other error messages seem to refer to the Uniswap Wrapper.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_AmmWrapper.sol#L56

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_AmmWrapper.sol#L59

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_AmmWrapper.sol#L72-L73

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_AmmWrapper.sol#L88

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_AmmWrapper.sol#L95

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/c5a6d5eead68465168373c39891ff1c5f197567a/code/contracts/bridges/L2_AmmWrapper.sol#L97

Recommendation

Update the error messages to correctly reflect the current contract.

Description

The method is only called externally. To signal it will not be called internally by the contract or by another implementation that extends the L2_Bridge it could be defined as external.

Recommendation

Consider defining the method as external if it's not called internally.

Description

Anyone can call the method commitTransfers to commit the current pending transfers.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/24ccdb4d4528ecb4e92c3d7ac5e1909bf8de34f7/code/contracts/bridges/L2_Bridge.sol#L153-L160

The Bonder can call this method anytime they want.

https://github.com/monoceros-alpha/review-hopprotocol-contracts-2021-04/blob/24ccdb4d4528ecb4e92c3d7ac5e1909bf8de34f7/code/contracts/bridges/L2_Bridge.sol#L160

But any other actor can call the method if enough time has passed since the previous commit time.

If they specify an incorrect (unsupported) chain id, the method will still try to call _commitTransfers. In which case it should fail if no transfers are pending. It is better to fail earlier if the chain id is unsupported.

Recommendation

Check the provided destinationChainId to be valid.

require(supportedChainIds[chainId], "L2_BRG: destinationChainId is not supported");

In case the chain was previously supported and it is disabled right now, some transfers can remain in a pending state, still uncommitted. It is better to be more explicit about each state you want to support and how.

In case there are pending transfers, you should still allow committing the transfers, even if the chain is not supported anymore, but not allow creating new pending transfers.

Consider allowing pending transfers if the minimum time has passed, even if the chain id is not supported anymore, only if there are pending transfers. If the logic seems messy or too complicated to be handled in commitTransfers, consider creating a flushTransfers method just for this use case.