Cally contest - defsec's results

Lines of code

https://github.com/code-423n4/2022-05-cally/blob/main/contracts/src/Cally.sol#L120

Vulnerability details

Impact

The ** feeRate** does not have any upper or lower bounds. Values that are too large will lead to reversions in several critical functions or the platform user will lost all funds when paying the fee.

Proof of Concept

1. Navigate to the following contract : https://github.com/code-423n4/2022-05-cally/blob/main/contracts/src/Cally.sol#L120

Tools Used

Code Review

Recommended Mitigation Steps

Consider defining upper and lower bounds on the feeRate variable.

C4-001: Revert String Size Optimization

C4-002 : Adding unchecked directive can save gas

C4-003 : Check if amount > 0 before token transfer can save gas

C4-004 : There is no need to assign default values to variables

C4-005 : ++i is more gas efficient than i++ in loops forwarding

C4-006 : Non-strict inequalities are cheaper than strict ones

C4-007 : Use Custom Errors instead of Revert Strings to save Gas

C4-008 : Use Shift Right/Left instead of Division/Multiplication if possible

C4-009 : Cache array length in for loops can save gas

C4-010 : State Variables that can be changed to immutable

C4-011 : Use calldata instead of memory for function parameters

C4-001: Revert String Size Optimization

Impact

Shortening revert strings to fit in 32 bytes will decrease deploy time gas and will decrease runtime gas when the revert condition has been met.

Revert strings that are longer than 32 bytes require at least one additional mstore, along with additional overhead for computing memory offset, etc.

Proof of Concept

Revert strings > 32 bytes are here:

https://github.com/code-423n4/2022-05-cally/blob/main/contracts/src/Cally.sol#L329

Tools Used

Manual Review

Recommended Mitigation Steps

Shorten the revert strings to fit in 32 bytes. That will affect gas optimization.

C4-002 : 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

https://github.com/code-423n4/2022-05-cally/blob/main/contracts/src/Cally.sol#L238

Tools Used

None

Recommended Mitigation Steps

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

Unchecked{i++};

C4-003 : 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/code-423n4/2022-05-cally/blob/main/contracts/src/Cally.sol#L368

All Contracts

Tools Used

None

Recommended Mitigation Steps

Consider checking amount != 0.

C4-004 : There is no need to assign default values to variables

Impact - Gas Optimization

When a variable is declared solidity assigns the default value. In case the contract assigns the value again, it costs extra gas.

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

Proof of Concept

https://github.com/code-423n4/2022-05-cally/blob/main/contracts/src/Cally.sol#L94

https://github.com/code-423n4/2022-05-cally/blob/main/contracts/src/Cally.sol#L95

https://github.com/code-423n4/2022-05-cally/blob/main/contracts/src/Cally.sol#L282

Tools Used

Code Review

Recommended Mitigation Steps

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

C4-005 : ++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.

https://github.com/code-423n4/2022-05-cally/blob/main/contracts/src/CallyNft.sol#L244

Tools Used

Code Review

Recommended Mitigation Steps

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

C4-006 : 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

https://github.com/code-423n4/2022-05-cudos/blob/main/solidity/contracts/Gravity.sol#L662

Tools Used

Code Review

Recommended Mitigation Steps

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

C4-007 : Use Custom Errors instead of Revert Strings to save Gas

Custom errors from Solidity 0.8.4 are cheaper than revert strings (cheaper deployment cost and runtime cost when the revert condition is met)

Source Custom Errors in Solidity:

Starting from Solidity v0.8.4, there is a convenient and gas-efficient way to explain to users why an operation failed through the use of custom errors. Until now, you could already use strings to give more information about failures (e.g., revert("Insufficient funds.");), but they are rather expensive, especially when it comes to deploy cost, and it is difficult to use dynamic information in them.

Custom errors are defined using the error statement, which can be used inside and outside of contracts (including interfaces and libraries).

Instances include:

All require Statements

Tools Used

Code Review

Recommended Mitigation Steps

Recommended to replace revert strings with custom errors.

C4-008 : Use Shift Right/Left instead of Division/Multiplication if possible

Impact

A division/multiplication by any number x being a power of 2 can be calculated by shifting log2(x) to the right/left.

While the DIV opcode uses 5 gas, the SHR opcode only uses 3 gas. Furthermore, Solidity's division operation also includes a division-by-0 prevention which is bypassed using shifting.

Proof of Concept

Contracts - https://github.com/code-423n4/2022-05-cally/blob/main/contracts/src/CallyNft.sol#L245

Tools Used

None

Recommended Mitigation Steps

A division/multiplication by any number x being a power of 2 can be calculated by shifting log2(x) to the right/left.

C4-009 : 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.

https://github.com/code-423n4/2022-05-cally/blob/main/contracts/src/CallyNft.sol#L244

Tools Used

None

Recommended Mitigation Steps

Consider to cache array length.

C4-010 : State Variables that can be changed to immutable

Impact

Solidity 0.6.5 introduced immutable as a major feature. It allows setting contract-level variables at construction time which gets stored in code rather than storage.

Consider the following generic example:

contract C {
   /// The owner is set during contruction time, and never changed afterwards.
   address public owner = msg.sender;
}

In the above example, each call to the function owner() reads from storage, using a sload. After EIP-2929, this costs 2100 gas cold or 100 gas warm. However, the following snippet is more gas efficient:

contract C {
   /// The owner is set during contruction time, and never changed afterwards.
   address public immutable owner = msg.sender;
}

In the above example, each storage read of the owner state variable is replaced by the instruction push32 value, where value is set during contract construction time. Unlike the last example, this costs only 3 gas.

Proof of Concept

1. Navigate to the following smart contract line.

https://github.com/code-423n4/2022-05-cally/blob/main/contracts/src/Cally.sol#L90

https://github.com/code-423n4/2022-05-cally/blob/main/contracts/src/Cally.sol#L92

Tools Used

None

Recommended Mitigation Steps

Consider using immutable variable.

C4-011 : 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.

https://github.com/code-423n4/2022-05-cally/blob/main/contracts/src/Cally.sol#L459

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.