Upgrade Safety Skill

Reference skill for safe upgrade practices. Detailed upgrade security checklist is in the security-audit skill.

When to Use

Use this skill when:

• Planning contract upgrades
• Executing upgrades
• Reviewing upgrade implementations
• Testing upgrade scenarios

Related Skills

For comprehensive upgrade security, see:

• security-audit: checklists/upgrade-checklist.md - Complete upgrade security checklist
• proxy-patterns: Proxy pattern selection and implementation
• contract-patterns: patterns/upgradeable-contracts.md and examples/upgradeable-example.sol

Upgrade Safety Rules

Storage Layout Safety

Never:

• Delete state variables
• Change variable types
• Change variable order
• Reorder inherited contracts

Always:

• Add new variables at the end
• Use storage gaps
• Document storage layout
• Test storage compatibility

Example:

// V1
contract V1 {
    uint256 public value;
    address public owner;
    uint256[48] private __gap;  // Reserve space
}

// V2 - ✅ Safe
contract V2 {
    uint256 public value;
    address public owner;
    uint256 public newValue;    // Added at end
    uint256[47] private __gap;  // Reduced gap
}

Initializer Safety

Always:

• Use _disableInitializers() in constructor
• Use initializer modifier for init function
• Use reinitializer(version) for upgrade init
• Call parent initializers

Example:

contract MyContract is Initializable, UUPSUpgradeable {
    constructor() {
        _disableInitializers();  // ✅ Required
    }

    function initialize() public initializer {
        __UUPSUpgradeable_init();
    }

    function initializeV2() public reinitializer(2) {
        // V2 initialization
    }
}

Authorization Safety

UUPS:

• Always keep _authorizeUpgrade function
• Use appropriate access control
• Consider timelock for critical upgrades

function _authorizeUpgrade(address) internal override onlyOwner {}

Never remove this function in UUPS upgrades!

Upgrade Process

Pre-Upgrade Checklist

• Storage layout verified compatible
• New implementation tested
• Initializers properly protected
• Authorization functions present
• Tests passing
• Storage verification tool run
• Deployment addresses verified
• Backup plan prepared

Verification Tools

Foundry:

# Check storage layout
forge inspect MyContract storage-layout

# Compare layouts
forge inspect MyContractV1 storage-layout > v1.json
forge inspect MyContractV2 storage-layout > v2.json
diff v1.json v2.json

Hardhat:

# Validate upgrade
npx hardhat verify-upgrade <PROXY> <NEW_IMPL>

Slither:

# Check upgradeability
slither-check-upgradeability . MyContract

During Upgrade

1. Pause if applicable
2. Deploy new implementation
3. Verify implementation
4. Execute upgrade
5. Verify proxy points to new implementation
6. Test critical functions
7. Unpause if paused

Post-Upgrade

• Verify implementation address
• Test all critical functions
• Monitor for issues
• Document changes
• Update documentation

Testing Upgrades

Storage Preservation Test

function test_UpgradePreservesStorage() public {
    // Deploy V1
    MyContractV1 v1 = new MyContractV1();
    v1.initialize(owner);
    v1.setValue(42);

    // Deploy V2 implementation
    MyContractV2 implementation = new MyContractV2();

    // Simulate upgrade (actual upgrade depends on proxy pattern)
    // ...

    // Cast to V2 interface
    MyContractV2 v2 = MyContractV2(address(v1));

    // Verify storage preserved
    assertEq(v2.getValue(), 42);
    assertEq(v2.owner(), owner);
}

Fork Testing

function test_UpgradeOnFork() public {
    // Fork mainnet
    vm.createSelectFork(vm.envString("MAINNET_RPC_URL"));

    // Get existing proxy
    MyContract proxy = MyContract(PROXY_ADDRESS);

    // Deploy new implementation
    MyContract newImpl = new MyContract();

    // Upgrade
    vm.prank(OWNER);
    proxy.upgradeTo(address(newImpl));

    // Verify
    assertEq(proxy.version(), 2);
}

Common Upgrade Mistakes

❌ 1. Storage Collision

// V1
contract V1 {
    uint256 public value;
}

// V2 - ❌ Wrong!
contract V2 {
    address public owner;   // Overwrites value!
    uint256 public value;
}

❌ 2. Uninitialized Implementation

// ❌ Missing constructor protection
contract MyContract {
    function initialize() public initializer {
        // Attacker can initialize implementation!
    }
}

❌ 3. Removed Authorization

// V1
contract V1 is UUPSUpgradeable {
    function _authorizeUpgrade(address) internal override onlyOwner {}
}

// V2 - ❌ Removed authorization!
contract V2 is UUPSUpgradeable {
    // Missing _authorizeUpgrade - contract is now non-upgradeable!
}

❌ 4. Selfdestruct in Implementation

// ❌ Never use selfdestruct in upgradeable contracts
function destroy() public {
    selfdestruct(payable(owner));  // Kills implementation for all proxies!
}

Security Considerations

1. Multi-sig for upgrades - Never use single key in production
2. Timelock delays - Give users time to exit before upgrade
3. Pause before upgrade - Prevent state changes during upgrade
4. Test on testnet - Always test upgrades before mainnet
5. Emergency procedures - Have rollback plan ready

Best Practices

1. Always use OpenZeppelin upgradeable contracts
2. Test upgrades on fork before mainnet
3. Use storage gaps (50 slots) in base contracts
4. Document storage layout changes
5. Never remove _authorizeUpgrade in UUPS
6. Use reinitializer for upgrade initialization
7. Verify storage compatibility with tools
8. Multi-sig + timelock for production
9. Monitor after upgrade
10. Have rollback plan

Tools

Tool	Purpose	Command
Foundry	Storage layout	forge inspect Contract storage-layout
Hardhat	Verify upgrade	npx hardhat verify-upgrade
Slither	Check upgrade safety	slither-check-upgradeability
OpenZeppelin Defender	Upgrade automation	Web interface

Related Documentation

• Detailed checklist: security-audit/checklists/upgrade-checklist.md
• Proxy patterns: proxy-patterns/SKILL.md
• Implementation examples: contract-patterns/patterns/upgradeable-contracts.md
• Example contract: contract-patterns/examples/upgradeable-example.sol

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Note: This is a reference skill. For the complete upgrade security checklist, see security-audit/checklists/upgrade-checklist.md.