Reentrancy guard pattern in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Reentrancy Guard Pattern in blockchain smart contracts?

It is a technique used to prevent a function from being called again before the first call finishes, stopping attacks where a contract repeatedly calls itself to drain funds.

beginner

Why is reentrancy a problem in smart contracts?

Because it allows attackers to repeatedly enter a function before the previous execution finishes, potentially causing unexpected behavior like multiple withdrawals.

intermediate

How does a bool locked variable help in implementing a reentrancy guard?

It acts like a lock: when a function starts, it sets locked = true. If the function is called again before finishing, it sees the lock and stops execution.

intermediate

What Solidity keyword is commonly used to create a reentrancy guard modifier?

The modifier keyword is used to create reusable code that can check and set the lock before running the function body.

advanced

Give a simple example of a reentrancy guard modifier in Solidity.

```solidity bool private locked = false; modifier noReentrant() { require(!locked, "No reentrancy"); locked = true; _; locked = false; } ```

What does the reentrancy guard pattern primarily prevent?

A Multiple simultaneous calls to a function before it finishes

B Unauthorized access to private variables

C Overflow errors in arithmetic operations

D Slow transaction processing

The reentrancy guard stops a function from being called again before the first call finishes, preventing repeated entries.

In Solidity, which of these is commonly used to implement a reentrancy guard?

A A public state variable

B A boolean lock variable and a modifier

C A fallback function

D An event logger

A boolean lock variable combined with a modifier is the usual way to implement a reentrancy guard.

What happens if a reentrant call is detected in a function protected by a reentrancy guard?

A The contract self-destructs

B The call proceeds normally

C The call is rejected with an error

D The call is queued for later

The guard uses a require statement to reject any reentrant calls, stopping execution with an error.

Why is it important to set the lock variable back to false after the function finishes?

A To allow future calls to the function

B To save gas fees

C To increase contract balance

D To trigger an event

Resetting the lock allows the function to be called again later safely.

Which of these attacks does the reentrancy guard help prevent?

A Spam transactions

B Phishing attacks

C Sybil attacks

D Repeated withdrawal attacks

Reentrancy guards stop attackers from repeatedly withdrawing funds by reentering a function.

Explain how the reentrancy guard pattern works to protect a smart contract function.

Think about how a door lock stops someone from entering twice at the same time.

Describe a simple Solidity code example that uses a reentrancy guard modifier.

Focus on the lock variable and how it changes before and after the function runs.

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