Reentrancy guard pattern in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a boolean variable named 'locked' initialized to false.

Blockchain / Solidity

bool [1] = false;

Drag options to blanks, or click blank then click option'

AisLocked

Block

Cstatus

Dlocked

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different variable name like 'lock' or 'status' which is not standard.

The reentrancy guard uses a boolean variable named locked to track if the function is already running.

2fill in blank

medium

Complete the modifier to check if the contract is not locked before running the function.

Blockchain / Solidity

modifier noReentrant() {
    require(![1], "Reentrant call");
    _;
}

Drag options to blanks, or click blank then click option'

Alocked

BisLocked

Clock

Dstatus

Attempts:

3 left

💡 Hint

Common Mistakes

Checking the wrong variable or missing the negation operator.

The modifier checks that locked is false before allowing the function to run.

3fill in blank

hard

Fix the error in the modifier to set the locked state correctly before and after the function runs.

Blockchain / Solidity

modifier noReentrant() {
    require(!locked, "Reentrant call");
    locked = [1];
    _;
    locked = false;
}

Drag options to blanks, or click blank then click option'

Atrue

Bstatus

Clocked

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Setting locked to false before the function runs, which disables the guard.

Set locked = true before running the function to prevent reentrancy.

4fill in blank

hard

Fill both blanks to complete the safe withdraw function using the noReentrant modifier and resetting the locked state.

Blockchain / Solidity

function withdraw(uint amount) public noReentrant {
    require(balances[msg.sender] >= amount, "Insufficient balance");
    balances[msg.sender] -= amount;
    (bool success, ) = msg.sender.call{value: amount}([1]);
    require(success, "Transfer failed");
    locked = [2];
}

Drag options to blanks, or click blank then click option'

Agas(2300)

Bfalse

Ctrue

Dgas()

Attempts:

3 left

💡 Hint

Common Mistakes

Using gas(2300) which limits gas and can cause failures.

Not resetting locked to false.

Use gas() to forward all gas in the call and set locked = false after the transfer to unlock.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps addresses to their balances only if balance is greater than zero.

Blockchain / Solidity

mapping(address => uint) public balances;

function positiveBalances(address[] memory users) public view returns (mapping(address => uint) memory) {
    return {user: balances[user] for user in users if balances[user] [1] 0};
}

modifier noReentrant() {
    require(!locked, "Reentrant call");
    locked = [2];
    _;
    locked = [3];
}

Drag options to blanks, or click blank then click option'

A>

Btrue

Cfalse

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using < or wrong boolean values in the modifier.

The comprehension filters balances greater than zero using >. The modifier sets locked = true before and locked = false after the function.

Practice

1.

What is the main purpose of the reentrancy guard pattern in blockchain smart contracts?

easy

A. To prevent a function from being called again before it finishes execution

B. To speed up the execution of smart contracts

C. To allow multiple users to access a function simultaneously

D. To reduce the gas cost of transactions

Solution

1. Step 1: Understand the problem reentrancy causes

   Reentrancy happens when a function is called again before the first call finishes, causing unexpected behavior or exploits.

2. Step 2: Identify the role of the reentrancy guard

   The reentrancy guard uses a lock to block repeated calls until the first call completes, preventing reentrancy attacks.

3. Final Answer:

   To prevent a function from being called again before it finishes execution -> Option A

4. Quick Check:

   Reentrancy guard = prevent repeated calls [OK]

Hint: Reentrancy guard stops repeated calls during execution [OK]

Common Mistakes:

• Thinking it speeds up contract execution
• Believing it allows simultaneous access
• Confusing it with gas optimization

2.

Which of the following is the correct way to declare a reentrancy guard variable in Solidity?

contract Example {
    // Fill in the blank
    _____ private locked = false;
}

easy

A. string

B. bool

C. int

D. address

Solution

1. Step 1: Identify the variable type for a lock

   The lock variable is used as a flag to indicate if the function is running, so it should be a boolean (true/false).

2. Step 2: Match the correct Solidity type

   In Solidity, bool is the type for true/false values, suitable for a lock variable.

3. Final Answer:

   bool -> Option B

4. Quick Check:

   Lock variable type = bool [OK]

Hint: Lock variables are boolean flags in Solidity [OK]

Common Mistakes:

• Using int instead of bool for lock
• Using string or address types incorrectly
• Forgetting to initialize the variable

3.

Consider this Solidity function using a reentrancy guard:

bool private locked = false;

function withdraw(uint amount) public {
    require(!locked, "Reentrant call detected");
    locked = true;
    (bool success, ) = msg.sender.call{value: amount}("");
    require(success, "Transfer failed");
    locked = false;
}

What happens if msg.sender.call triggers a fallback that calls withdraw again?

medium

A. The second call is blocked by the lock and reverts

B. The second call succeeds and causes reentrancy

C. The contract runs out of gas and fails

D. The lock variable resets before the second call

Solution

1. Step 1: Analyze the lock status during the first call

   When withdraw is called, locked is set to true before sending funds.

2. Step 2: Understand the effect on reentrant call

   If msg.sender.call triggers a fallback that calls withdraw again, the require(!locked) check fails because locked is true, blocking the call.

3. Final Answer:

   The second call is blocked by the lock and reverts -> Option A

4. Quick Check:

   Lock true blocks reentrant call [OK]

Hint: Lock true means no reentrant calls allowed [OK]

Common Mistakes:

• Assuming reentrancy still happens
• Thinking lock resets before second call
• Confusing gas failure with reentrancy

4.

Identify the bug in this reentrancy guard implementation:

bool private locked = false;

function withdraw(uint amount) public {
    require(!locked, "Reentrant call detected");
    (bool success, ) = msg.sender.call{value: amount}("");
    require(success, "Transfer failed");
    locked = true;
    locked = false;
}

medium

A. The require statement is missing

B. The lock variable is never set to true

C. The lock is set after the external call, allowing reentrancy

D. The call to msg.sender is incorrect syntax

Solution

1. Step 1: Check when the lock is set

   The lock is set to true only after the external call, which means the function is not protected during the call.

2. Step 2: Understand reentrancy risk

   Because the lock is set too late, a reentrant call can happen during the external call, causing vulnerability.

3. Final Answer:

   The lock is set after the external call, allowing reentrancy -> Option C

4. Quick Check:

   Lock must be set before external calls [OK]

Hint: Set lock before external calls to prevent reentrancy [OK]

Common Mistakes:

• Setting lock after external call
• Forgetting to set lock at all
• Misplacing require statements

5.

You want to protect a function that updates user balances and sends Ether. Which is the best way to apply the reentrancy guard pattern?

function safeWithdraw(uint amount) public {
    // Your code here
    balances[msg.sender] -= amount;
    (bool success, ) = msg.sender.call{value: amount}("");
    require(success, "Transfer failed");
}

hard

A. Update balances after sending Ether without any lock

B. Do not use any lock, just rely on require statements

C. Use a lock variable but set it after sending Ether

D. Set a lock variable before updating balances and sending Ether, then release it after

Solution

1. Step 1: Understand the order of operations for safety

   To prevent reentrancy, set a lock before any state changes and external calls, then release it after.

2. Step 2: Apply the reentrancy guard pattern correctly

   Set the lock variable to true before updating balances and sending Ether, then set it back to false after the transfer completes.

3. Final Answer:

   Set a lock variable before updating balances and sending Ether, then release it after -> Option D

4. Quick Check:

   Lock before state change and external call [OK]

Hint: Lock before balance update and transfer, unlock after [OK]

Common Mistakes:

• Setting lock after sending Ether
• Updating balances after external call
• Not using any lock at all