13 Feb 2025

Ethereum: Reentrancy Guard and CEI

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Ethereum: Reentrancy Guard and CEI – Understanding the Role of Both Transformers

In Ethereum, modifiers are an essential part of smart contract development. Two specific modifiers that play a significant role in ensuring the integrity and security of contracts are the ReentrancyGuard (RGC) modifier and the Check Effect Interactions (CEI) model. In this article, we will explore the importance of these two transformers and provide examples to illustrate their use.

What is the Reentrancy Guard modifier?

The ReentrancyGuard transformer is a built-in Ethereum decorator that prevents a contract or other function from being called again after the same instruction. This ensures that the contract cannot execute in an infinite loop, which can lead to serious security vulnerabilities and even crashes.

Is it necessary to use the Reentrancy Guard modifier if your contract follows the CEI pattern?

No, you don’t necessarily need to use the ReentrancyGuard modifier if your contract follows the Check Effect Interactions (CEI) pattern. The CEI pattern is a more advanced and elegant way to ensure that the effect of an interaction with a function does not have side effects on other functions in the same contract.

In fact, the CEI pattern provides a more comprehensive set of contract integrity and security guarantees that often go beyond the need for the ReentrancyGuard converter. The CEI pattern includes several important principles, such as:

No self-call: A contract cannot call itself.

No recursive calls
: A contract cannot call another function recursively.

No shared space

: No shared space between functions.

What is Check Effect Interactions (CEI)?

Check Effect Interactions (CEI) is an interaction model that allows for the creation of complex contracts that have multiple effects on other actions. CEI provides a way to express these interactions in a more elegant and readable way, while ensuring the integrity and security of the contract.

Example: Using the ReentrancyGuard Modifier

Let’s look at an example of a simple contract following the CEI pattern:


pragma force ^0,8,0;
agreement Simple Agreement {
// The ReentrancyGuard converter is not used here.
function doSomething() public {
// This function cannot call itself.
require(!isSelf(), "Reentrancy");
// Call another function that interacts with this contract.
Interaction FunctionInteraction = new FunctionInteraction ();
interaction.doSomethingElse();
}
function isSelf() internal display returns (bool) {
return msg.sender != address(0);
}
}

In this example, we define a simple contract “SimpleContract” with two functions: “doSomething()” and “isSelf()”. The “doSomething()” function calls another function “doSomethingElse()” that interacts with the contract. However, since we did not use the ReentrancyGuard converter, we need to enable it manually:


pragma force ^0,8,0;
agreement SimpleContract {
// Use the ReentrancyGuard converter.
function doSomething() public {
require(!isSelf(), "Reentrancy");
// Call another function that interacts with this contract.
Interaction FunctionInteraction = new FunctionInteraction ();
interaction.doSomethingElse();
// It's now safe to call `doSomething' again, since we've prevented the recursion.
}
function isSelf() internal display returns (bool) {
return msg.sender != address(0);
}
}