The Reentrancy Guard Library provides an API to check for and disallow reentrancy on a contract. A reentrancy attack happens when a function is externally invoked during its execution, allowing it to be run multiple times in a single transaction.
The reentrancy check is used to check if a contract ID has been called more than once in the current call stack.
A reentrancy, or "recursive call" attack can cause some functions to behave in unexpected ways. This can be prevented by asserting a contract has not yet been called in the current transaction. An example can be found here .
For implementation details on the Reentrancy Guard Library please see the Sway Libs Docs .
While this can protect against both single-function reentrancy and cross-function reentrancy attacks, it WILL NOT PREVENT a cross-contract reentrancy attack.
In order to use the Reentrancy Guard library, Sway Libs must be added to the Forc.toml file and then imported into your Sway project. To add Sway Libs as a dependency to the Forc.toml file in your project please see the Getting Started .
To import the Reentrancy Guard Library to your Sway Smart Contract, add the following to your Sway file:
use sway_libs::reentrancy::*;Once imported, using the Reentrancy Library can be done by calling one of the two functions:
is_reentrant() -> bool reentrancy_guard() Once imported, using the Reentrancy Guard Library can be used by calling the reentrancy_guard() in your Sway Smart Contract. The following shows a Sway Smart Contract that applies the Reentrancy Guard Library:
use sway_libs::reentrancy::reentrancy_guard;
abi MyContract {
fn my_non_reentrant_function();
}
impl MyContract for Contract {
fn my_non_reentrant_function() {
reentrancy_guard();
// my code here
}
} To check if the current caller is a reentrant, you may call the is_reentrant() function.
use sway_libs::reentrancy::is_reentrant;
fn check_if_reentrant() {
assert(!is_reentrant());
}