Swivel v3 contest - robee's results

Must approve 0 first

Some tokens (like USDT) do not work when changing the allowance from an existing non-zero allowance value. They must first be approved by zero and then the actual allowance must be approved.

Code instance:

approve without approving 0 first Swivel.sol, 561, Safe.approve(uToken, c[i], max);

safeApprove of openZeppelin is deprecated

You use safeApprove of openZeppelin although it's deprecated. (see https://github.com/OpenZeppelin/openzeppelin-contracts/blob/566a774222707e424896c0c390a84dc3c13bdcb2/contracts/token/ERC20/utils/SafeERC20.sol#L38) You should change it to increase/decrease Allowance as OpenZeppilin says.

Code instance:

    Deprecated safeApprove in Swivel.sol line 561: Safe.approve(uToken, c[i], max);

Two arrays length mismatch

The functions below fail to perform input validation on arrays to verify the lengths match. A mismatch could lead to an exception or undefined behavior. Consider making this a medium risk please.

Code instances

    https://github.com/code-423n4/2022-07-swivel/tree/main/Swivel/Swivel.sol#L407 cancel ['o', 'c']
    https://github.com/code-423n4/2022-07-swivel/tree/main/Swivel/Swivel.sol#L82 initiate ['o', 'a', 'c']
    https://github.com/code-423n4/2022-07-swivel/tree/main/Swivel/Swivel.sol#L244 exit ['o', 'a', 'c']

Div by 0

Division by 0 can lead to accidentally revert, (An example of a similar issue - https://github.com/code-423n4/2021-10-defiprotocol-findings/issues/84)

Code instances:

    https://github.com/code-423n4/2022-07-swivel/tree/main/Creator/VaultTracker.sol#L169 f, t, exchangeRate might be 0
    https://github.com/code-423n4/2022-07-swivel/tree/main/Swivel/Swivel.sol#L291 o might be 0
    https://github.com/code-423n4/2022-07-swivel/tree/main/VaultTracker/VaultTracker.sol#L226 a, exchangeRate might be 0
    https://github.com/code-423n4/2022-07-swivel/tree/main/Swivel/Swivel.sol#L388 o might be 0

approve return value is ignored

Some tokens don't correctly implement the EIP20 standard and their approve function returns void instead of a success boolean. Calling these functions with the correct EIP20 function signatures will always revert. Tokens that don't correctly implement the latest EIP20 spec, like USDT, will be unusable in the mentioned contracts as they revert the transaction because of the missing return value. We recommend using OpenZeppelin’s SafeERC20 versions with the safeApprove function that handle the return value check as well as non-standard-compliant tokens. The list of occurrences in format (solidity file, line number, actual line)

Code instance:

Swivel.sol, 561, Safe.approve(uToken, c[i], max);

Not verified input

external / public functions parameters should be validated to make sure the address is not 0. Otherwise if not given the right input it can mistakenly lead to loss of user funds.

Code instances:

    MarketPlace.sol.createMarket c
    VaultTracker.sol.transferNotionalFrom t
    MarketPlace.sol.authRedeem t
    Swivel.sol.combineTokens u
    Swivel.sol.redeemSwivelVaultInterest u

Solidity compiler versions mismatch

The project is compiled with different versions of solidity, which is not recommended because it can lead to undefined behaviors.

Not verified owner

    owner param should be validated to make sure the owner address is not address(0).
    Otherwise if not given the right input all only owner accessible functions will be unaccessible.
    
    

Code instance:

    Erc20.sol.permit owner

Named return issue

Users can mistakenly think that the return value is the named return, but it is actually the actualreturn statement that comes after. To know that the user needs to read the code and is confusing. Furthermore, removing either the actual return or the named return will save gas.

Code instances:

    ZcToken.sol, maxRedeem

Two Steps Verification before Transferring Ownership

The following contracts have a function that allows them an admin to change it to a different address. If the admin accidentally uses an invalid address for which they do not have the private key, then the system gets locked. It is important to have two steps admin change where the first is announcing a pending new admin and the new address should then claim its ownership. A similar issue was reported in a previous contest and was assigned a severity of medium: code-423n4/2021-06-realitycards-findings#105

Code instances:

    Swivel.sol
    MarketPlace.sol
    Creator.sol

Open TODOs

Open TODOs can hint at programming or architectural errors that still need to be fixed. These files has open TODOs:

Code instances:

Open TODO in Swivel.sol line 32 : address public aaveAddr; // TODO immutable?

Open TODO in Swivel.sol line 707 : if (p == uint8(Protocols.Compound)) { // TODO is Rari a drop in here?

Open TODO in Swivel.sol line 285 : // TODO assign amount or keep the ADD?

Check transfer receiver is not 0 to avoid burned money

Transferring tokens to the zero address is usually prohibited to accidentally avoid "burning" tokens by sending them to an unrecoverable zero address.

Code instances:

    https://github.com/code-423n4/2022-07-swivel/tree/main/Creator/Erc20.sol#L117
    https://github.com/code-423n4/2022-07-swivel/tree/main/Swivel/Swivel.sol#L641
    https://github.com/code-423n4/2022-07-swivel/tree/main/Creator/Erc20.sol#L205
    https://github.com/code-423n4/2022-07-swivel/tree/main/Swivel/Swivel.sol#L580

State variables that could be set immutable

In the following files there are state variables that could be set immutable to save gas.

Code instance:

    feenominators in Swivel.sol

Unnecessary array boundaries check when loading an array element twice

There are places in the code (especially in for-each loops) that loads the same array element more than once. 
In such cases, only one array boundaries check should take place, and the rest are unnecessary.
Therefore, this array element should be cached in a local variable and then be loaded
again using this local variable, skipping the redundant second array boundaries check: 

Code instance:

    Swivel.sol.setFee - double load of d[x]

Storage double reading. Could save SLOAD

Reading a storage variable is gas costly (SLOAD). In cases of multiple read of a storage variable in the same scope, caching the first read (i.e saving as a local variable) can save gas and decrease the overall gas uses. The following is a list of functions and the storage variables that you read twice:

Code instances:

    ZcToken.sol: maturity is read twice in redeem
    ZcToken.sol: underlying is read twice in redeem
    ZcToken.sol: maturity is read twice in withdraw
    ZcToken.sol: underlying is read twice in withdraw

Rearrange state variables

You can change the order of the storage variables to decrease memory uses.

Code instance:

In VaultTracker.sol,rearranging the storage fields can optimize to: 5 slots from: 6 slots. The new order of types (you choose the actual variables): 1. uint256 2. uint256 3. address 4. uint8 5. address 6. address

Use != 0 instead of > 0

Using != 0 is slightly cheaper than > 0. (see https://github.com/code-423n4/2021-12-maple-findings/issues/75 for similar issue)

Code instances:

    VaultTracker.sol, 123: change 'maturityRate > 0' to 'maturityRate != 0'
    VaultTracker.sol, 59: change 'maturityRate > 0' to 'maturityRate != 0'
    VaultTracker.sol, 165: change 'maturityRate > 0' to 'maturityRate != 0'
    VaultTracker.sol, 184: change 'maturityRate > 0' to 'maturityRate != 0'
    VaultTracker.sol, 93: change 'maturityRate > 0' to 'maturityRate != 0'
    VaultTracker.sol, 222: change 'maturityRate > 0' to 'maturityRate != 0'

Unnecessary cast

Code instance:

    int256 FixedPointMathLib.sol.lnWad - unnecessary casting int256(x)

Use unchecked to save gas for certain additive calculations that cannot overflow

You can use unchecked in the following calculations since there is no risk to overflow:

Code instances:

    Swivel.sol (L#437) - uint256 when = block.timestamp + HOLD;
    Swivel.sol (L#523) - uint256 when = block.timestamp + HOLD;
    Swivel.sol (L#474) - uint256 when = block.timestamp + HOLD;

Consider inline the following functions to save gas

You can inline the following functions instead of writing a specific function to save gas.
(see https://github.com/code-423n4/2021-11-nested-findings/issues/167 for a similar issue.)

Code instances

    LibCompound.sol, viewUnderlyingBalanceOf, { return cToken.balanceOf(user).mulWadDown(viewExchangeRate(cToken)); }
    LibFuse.sol, viewUnderlyingBalanceOf, { return cToken.balanceOf(user).mulWadDown(viewExchangeRate(cToken)); }
    FixedPointMathLib.sol, mulWadUp, { return mulDivUp(x, y, WAD); // Equivalent to (x * y) / WAD rounded up. }
    FixedPointMathLib.sol, divWadUp, { return mulDivUp(x, WAD, y); // Equivalent to (x * WAD) / y rounded up. }
    FixedPointMathLib.sol, mulWadDown, { return mulDivDown(x, y, WAD); // Equivalent to (x * y) / WAD rounded down. }
    FixedPointMathLib.sol, divWadDown, { return mulDivDown(x, WAD, y); // Equivalent to (x * WAD) / y rounded down. }
    Hash.sol, order, { return keccak256(abi.encode( ORDER_TYPEHASH, o.key, o.protocol, o.maker, o.underlying, o.vault, o.exit, o.principal, o.premium, o.maturity, o.expiry )); }

Gas Optimization On The 2^256-1

Some projects (e.g. Uniswap - https://github.com/Uniswap/interface/blob/main/src/hooks/useApproveCallback.ts#L88) set the default value of the user's allowance to 2^256 - 1. Since the value 2^256 - 1 can also be represented in hex as 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff. From Ethereum's yellow paper we know that zeros are cheaper than non-zero values in the hex representation. Considering this fact, an alternative choice could be now 0x8000000000000000000000000000000000000000000000000000000000000000 or 2^255 to represent "infinity". If you do the calculations with Remix, you will see that the former costs 47'872 gas, while the latter costs 45'888 gas. If you accept that infinity can also be represented via 2^255 (instead of 2^256-1), which almost all projects can - you can already save about 4% gas leveraging this optimisation trick on those calculations.

Code instance:

    Swivel.sol (L#549): uint256 max = 2**256 - 1;)

Do not cache msg.sender

We recommend not to cache msg.sender since calling it is 2 gas while reading a variable is more.

Code instances:

    https://github.com/code-423n4/2022-07-swivel/tree/main/Creator/Creator.sol#L19
    https://github.com/code-423n4/2022-07-swivel/tree/main/Marketplace/MarketPlace.sol#L40
    https://github.com/code-423n4/2022-07-swivel/tree/main/Swivel/Swivel.sol#L70
    https://github.com/code-423n4/2022-07-swivel/tree/main/VaultTracker/VaultTracker.sol#L33
    https://github.com/code-423n4/2022-07-swivel/tree/main/Creator/VaultTracker.sol#L33