noReenter Can Be Optimised Better

[code423n4]

Handle

leastwood

Vulnerability details

Impact

The noReenter modifier can be done more efficiently by using 1 and 2 to represent unlocked and locked states respectively. This is due to the fact that there are additional gas costs when updating a slow from a zero value to a non-zero value. The gas refunded when resetting the slot back to its zero value does not cover the cost of the initial change.

Proof of Concept

modifier noReenter() {
    require(_locked == 0, "LOCKED");
    _locked = uint256(1);
    _;
    _locked = uint256(0);
}

Tools Used

Manual code review.

Recommended Mitigation Steps

Consider using 1 and 2 to represent the unlocked and locked states respectively.

[deluca-mike]

For some cases 0 -> 1 -> 0 is cheaper, but after some testing, its overall cheaper to use 1 -> 2 -> 1 (specifically in the batch functions).

[deluca-mike]

Duplicate #1