MixinGrantKeys:grantKeys gas optimizations

[code423n4]

Handle

GiveMeTestEther

Vulnerability details

Impact

• (1.1) save gas by incrementing the loop index (uint256) in an unchecked {}, because we would run out of gas before if the loop index overflows uint256
• (1.2) write ++i (more gas efficient) instead of i++
• store the array length in a local variable to save gas by not reading the array length from calldata in each loop iteration

Proof of Concept

• https://github.com/code-423n4/2021-11-unlock/blob/ec41eada1dd116bcccc5603ce342257584bec783/smart-contracts/contracts/mixins/MixinGrantKeys.sol#L29

Tools Used

• Manual Analysis

Recommended Mitigation Steps

• (1.1 & 1.2): write the loop index increment i++ as unchecked { ++ i }
• (1.3) store length in a local var: uint recicipientsLength = _recipients.length; and then write the index bound check as i < recicipientsLength