Art Gobblers contest - 0x1f8b's results

• Low
  • 1. Mixing and Outdated compiler
  • 2. Lack of checks
  • 3. Lack of ACK during owner change
  • 4. Avoid using tx.origin
  • 5. Unsafe math
  • 6. Integer overflow by unsafe casting
• Non critical
  • 7. Avoid hardcoded values
  • 8. Use tokens with ERC20 permit
  • 9. Avoid hardcoded values

Low

1. Mixing and Outdated compiler

The pragma version used are:

pragma solidity >=0.8.0;
pragma solidity ^0.8.0;

Note that mixing pragma is not recommended. Because different compiler versions have different meanings and behaviors, it also significantly raises maintenance costs. As a result, depending on the compiler version selected for any given file, deployed contracts may have security issues.

The minimum required version must be 0.8.16; otherwise, contracts will be affected by the following important bug fixes:

0.8.3:

• Optimizer: Fix bug on incorrect caching of Keccak-256 hashes.

0.8.4:

• ABI Decoder V2: For two-dimensional arrays and specially crafted data in memory, the result of abi.decode can depend on data elsewhere in memory. Calldata decoding is not affected.

0.8.9:

• Immutables: Properly perform sign extension on signed immutables.
• User Defined Value Type: Fix storage layout of user defined value types for underlying types shorter than 32 bytes.

0.8.13:

• Code Generator: Correctly encode literals used in abi.encodeCall in place of fixed bytes arguments.

0.8.14:

• ABI Encoder: When ABI-encoding values from calldata that contain nested arrays, correctly validate the nested array length against calldatasize() in all cases.
• Override Checker: Allow changing data location for parameters only when overriding external functions.

0.8.15

• Code Generation: Avoid writing dirty bytes to storage when copying bytes arrays.
• Yul Optimizer: Keep all memory side-effects of inline assembly blocks.

0.8.16

• Code Generation: Fix data corruption that affected ABI-encoding of calldata values represented by tuples: structs at any nesting level; argument lists of external functions, events and errors; return value lists of external functions. The 32 leading bytes of the first dynamically-encoded value in the tuple would get zeroed when the last component contained a statically-encoded array.

Apart from these, there are several minor bug fixes and improvements.

2. Lack of checks

The following methods have a lack of checks if the received argument is an address, it's good practice in order to reduce human error to check that the address specified in the constructor or initialize is different than address(0).

Affected source code for address(0):

• ChainlinkV1RandProvider.sol:55
• PagesERC721.sol:37
• GobblerReserve.sol:24
• DeployBase.s.sol:38-42
• Goo.sol:83
• Goo.sol:84
• Owned.sol:30

3. Lack of ACK during owner change

It's possible to lose the ownership under specific circumstances.

Because of human error it's possible to set a new invalid owner. When you want to change the owner's address it's better to propose a new owner, and then accept this ownership with the new wallet.

Affected source code:

• ArtGobblers.sol:83
• GobblerReserve.sol:12
• Owned.sol:39

4. Avoid using tx.origin

tx.origin is a global variable in Solidity that returns the address of the account that sent the transaction.

Using the variable could make a contract vulnerable if an authorized account calls a malicious contract. You can impersonate a user using a third party contract.

This can make it easier to create a vault on behalf of another user with an external administrator (by receiving it as an argument).

Affected source code:

• DeployBase.s.sol:66
• DeployBase.s.sol:67

5. Unsafe math

A user supplied argument can be passed as zero to multiplication operation.

Reference:

• https://slowmist.medium.com/analysis-of-the-treasuredao-zero-fee-exploit-73791f4b9c14

Affected source code:

• LibGOO.sol:32
• LibGOO.sol:38

6. Integer overflow by unsafe casting

Keep in mind that the version of solidity used, despite being greater than 0.8, does not prevent integer overflows during casting, it only does so in mathematical operations.

It is necessary to safely convert between the different numeric types.

Recommendation:

Use a safeCast from Open Zeppelin.

getGobblerData[gobblerId].emissionMultiple = uint32(burnedMultipleTotal * 2);

Affected source code:

• VRGDA.sol:46-51
• SignedWadMath.sol:136
• SignedWadMath.sol:163
• SignedWadMath.sol:164
• ArtGobblers.sol:324
• ArtGobblers.sol:327
• ArtGobblers.sol:449
• ArtGobblers.sol:454
• ArtGobblers.sol:455
• ArtGobblers.sol:456
• ArtGobblers.sol:458
• ArtGobblers.sol:461-462
• ArtGobblers.sol:531
• ArtGobblers.sol:535
• ArtGobblers.sol:551
• ArtGobblers.sol:616
• ArtGobblers.sol:624
• ArtGobblers.sol:650
• ArtGobblers.sol:660
• ArtGobblers.sol:661
• ArtGobblers.sol:662
• ArtGobblers.sol:679
• ArtGobblers.sol:680
• ArtGobblers.sol:681
• ArtGobblers.sol:698
• ArtGobblers.sol:825
• ArtGobblers.sol:826
• ArtGobblers.sol:855
• ArtGobblers.sol:903
• ArtGobblers.sol:904
• ArtGobblers.sol:910
• ArtGobblers.sol:911
• Pages.sol:244
• Pages.sol:253
• GobblersERC721.sol:184

---

Non critical

7. Avoid hardcoded values

It is not good practice to hardcode values, but if you are dealing with addresses much less, these can change between implementations, networks or projects, so it is convenient to remove these values from the source code.

Affected source code:

Use the selector instead of the raw value:

• GobblersERC1155B.sol:126-128
• GobblersERC721.sol:151-153
• PagesERC721.sol:166

8. Use tokens with ERC20 permit

The erc20 signature approval extension is a major usability enhancement that helps users and projects reap its benefits in the future.

The use of this feature is fully recommended.

Reference:

• https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/extensions/draft-ERC20Permit.sol

Affected source code:

• Goo.sol:58

9. Avoid hardcoded values

It is not good practice to hardcode values, but if you are dealing with addresses much less, these can change between implementations, networks or projects, so it is convenient to remove these values from the source code.

Affected source code:

Use the selector instead of the raw value:

• ERC721.sol:148-150

• Gas
  • 1. Don't use the length of an array for loops condition
  • 2. Avoid compound assignment operator in state variables
    • Total gas saved: 13 * 10 = 130
  • 3. Shift right or left instead of dividing or multiply by 2
    • Total gas saved: 172 * 1 + 201 * 1 = 373
  • 4. ++i costs less gas compared to i++ or i += 1
  • 5. Use Custom Errors instead of Revert Strings to save Gas
    • Total gas saved: 9 * 36 = 324
  • 6. There's no need to set default values for variables
    • Total gas saved: 8 * 3 = 24
  • 7. Gas saving using immutable
  • 8. Change bool to uint256 can save gas

Gas

1. Don't use the length of an array for loops condition

It's cheaper to store the length of the array inside a local variable and iterate over it.

Affected source code:

• GobblerReserve.sol:37
• GobblersERC1155B.sol:114

2. Avoid compound assignment operator in state variables

Using compound assignment operators for state variables (like State += X or State -= X ...) it's more expensive than using operator assignment (like State = State + X or State = State - X ...).

Proof of concept (without optimizations):

pragma solidity 0.8.15;

contract TesterA {
uint private _a;
function testShort() public {
_a += 1;
}
}

contract TesterB {
uint private _a;
function testLong() public {
_a = _a + 1;
}
}

Gas saving executing: 13 per entry

TesterA.testShort: 43507
TesterB.testLong:  43494

Affected source code:

• ArtGobblers.sol:456
• ArtGobblers.sol:464
• ArtGobblers.sol:662
• ArtGobblers.sol:844
• ArtGobblers.sol:905
• ArtGobblers.sol:906
• ArtGobblers.sol:912
• ArtGobblers.sol:913
• Pages.sol:244
• GobblersERC721.sol:184

Total gas saved: 13 * 10 = 130

3. Shift right or left instead of dividing or multiply by 2

Shifting one to the right will calculate a division by two.

he SHR opcode only requires 3 gas, compared to the DIV opcode's consumption of 5. Additionally, shifting is used to get around Solidity's division operation's division-by-0 prohibition.

Proof of concept (without optimizations):

pragma solidity 0.8.16;

contract TesterA {
function testDiv(uint a) public returns (uint) { return a / 2; }
}

contract TesterB {
function testShift(uint a) public returns (uint) { return a >> 1; }
}

Gas saving executing: 172 per entry

TesterA.testDiv:    21965 
TesterB.testShift:  21793   

The same optimization can be used to multiply by 2, using the left shift.

pragma solidity 0.8.16;

contract TesterA {
function testMul(uint a) public returns (uint) { return a * 2; }
}

contract TesterB {
function testShift(uint a) public returns (uint) { return a << 1; }
}

Gas saving executing: 201 per entry

TesterA.testMul:    21994
TesterB.testShift:  21793    

Affected source code:

*:

• ArtGobblers.sol:449 /:
• ArtGobblers.sol:462

Total gas saved: (172 * 1) + (201 * 1) = 373

4. ++i costs less gas compared to i++ or i += 1

++i costs less gas compared to i++ or i += 1 for unsigned integers, as pre-increment is cheaper (about 5 gas per iteration). This statement is true even with the optimizer enabled.

i++ increments i and returns the initial value of i. Which means:

uint i = 1;
i++; // == 1 but i == 2

But ++i returns the actual incremented value:

uint i = 1;
++i; // == 2 and i == 2 too, so no need for a temporary variable

In the first case, the compiler has to create a temporary variable (when used) for returning 1 instead of 2 I suggest using ++i instead of i++ to increment the value of an uint variable. Same thing for --i and i--

Keep in mind that this change can only be made when we are not interested in the value returned by the operation, since the result is different, you only have to apply it when you only want to increase a counter.

Affected source code:

• ERC721.sol:99
• ERC721.sol:101
• ERC721.sol:164
• ERC721.sol:179
• Pages.sol:251
• GobblerReserve.sol:37
• PagesERC721.sol:115
• PagesERC721.sol:117
• PagesERC721.sol:181

5. 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).

Proof of concept (without optimizations):

pragma solidity 0.8.15;

contract TesterA {
function testRevert(bool path) public view {
 require(path, "test error");
}
}

contract TesterB {
error MyError(string msg);
function testError(bool path) public view {
 if(path) revert MyError("test error");
}
}

Gas saving executing: 9 per entry

TesterA.testRevert: 21611
TesterB.testError:  21602     

Affected source code:

• VRGDA.sol:32
• Owned.sol:20
• ERC721.sol:36
• ERC721.sol:40
• ERC721.sol:69
• ERC721.sol:87
• ERC721.sol:89
• ERC721.sol:91-94
• ERC721.sol:118-123
• ERC721.sol:134-139
• ERC721.sol:158
• ERC721.sol:160
• ERC721.sol:175
• ERC721.sol:196-201
• ERC721.sol:211-216
• SignedWadMath.sol:142
• ArtGobblers.sol:437
• ArtGobblers.sol:885
• ArtGobblers.sol:887
• ArtGobblers.sol:889-892
• GobblersERC1155B.sol:107
• GobblersERC1155B.sol:149-153
• GobblersERC1155B.sol:185-189
• GobblersERC721.sol:62
• GobblersERC721.sol:66
• GobblersERC721.sol:95
• GobblersERC721.sol:121-126
• GobblersERC721.sol:137-142
• PagesERC721.sol:55
• PagesERC721.sol:59
• PagesERC721.sol:85
• PagesERC721.sol:103
• PagesERC721.sol:105
• PagesERC721.sol:107-110
• PagesERC721.sol:135-139
• PagesERC721.sol:151-155

Total gas saved: 9 * 36 = 324

6. There's no need to set default values for variables

If a variable is not set/initialized, the default value is assumed (0, false, 0x0 ... depending on the data type). You are simply wasting gas if you directly initialize it with its default value.

Proof of concept (without optimizations):

pragma solidity 0.8.15;

contract TesterA {
function testInit() public view returns (uint) { uint a = 0; return a; }
}

contract TesterB {
function testNoInit() public view returns (uint) { uint a; return a; }
}

Gas saving executing: 8 per entry

TesterA.testInit:   21392
TesterB.testNoInit: 21384   

Affected source code:

• Pages.sol:251
• GobblerReserve.sol:37
• GobblersERC1155B.sol:114
• GobblersERC721.sol:186
• GobblerReserve.sol:37
• ArtGobblers.sol:592

Total gas saved: 8 * 3 = 24

7. Gas saving using immutable

It's possible to avoid storage access a save gas using immutable keyword for the following variables:

It's also better to remove the initial values, because they will be set during the constructor.

Affected source code:

• GobblersERC721.sol:23
• GobblersERC721.sol:25
• PagesERC721.sol:24
• PagesERC721.sol:26
• Pages.sol:96
• ERC721.sol:21
• ERC721.sol:23
• ArtGobblers.sol:136
• ArtGobblers.sol:139

8. Change bool to uint256 can save gas

Because each write operation requires an additional SLOAD to read the slot's contents, replace the bits occupied by the boolean, and then write back, booleans are more expensive than uint256 or any other type that uses a complete word. This cannot be turned off because it is the compiler's defense against pointer aliasing and contract upgrades.

Reference:

• https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/security/ReentrancyGuard.sol#L23-L27

Also, this is applicable to integer types different from uint256 or int256.

Affected source code for booleans:

• ArtGobblers.sol:149