Attackathon _ Fuel Network 33240 - [Smart Contract - Insight] Incorrect Bitness in IFP Types

Incorrect Bitness in IFP Types

Submitted on Mon Jul 15 2024 20:11:00 GMT-0400 (Atlantic Standard Time) by @Blockian for Attackathon | Fuel Network

Report ID: #33240

Report type: Smart Contract

Report severity: Insight

Target: https://github.com/FuelLabs/sway-libs/tree/0f47d33d6e5da25f782fc117d4be15b7b12d291b

Impacts:

• Contract fails to deliver promised returns, but doesn't lose value

Description

Fuel Network bug report

Incorrect Bitness in IFP Types

Description

The bitness of the IFP types is incorrectly defined. Specifically:

IFP64 is actually 33 bits. IFP128 is actually 65 bits. IFP256 is actually 129 bits.

Root Cause

Examining the definition of IFP64 as an example:

/// The 64-bit signed fixed point number type.
///
/// # Additional Information
///
/// Represented by an underlying `UFP32` number and a boolean.
pub struct IFP64 {
    /// The underlying value representing the `IFP64` type.
    underlying: UFP32,
    /// The underlying boolean representing a negative value for the `IFP64` type.
    non_negative: bool,
}

The IFP64 type internally uses a UFP32, which occupies 32 bits, along with a single bit for the non_negative boolean. This totals 33 bits, not 64 as the name suggests.

Impact

The primary impact is the misleading and confusing naming of these types. This issue is of low severity but can cause misunderstanding.

Proposed fix

Correct the names of the IFP types to accurately reflect their bitness.

Proof of concept

Proof of Concept

Run the POC's with forc test

contract;

use sway_libs::fixed_point::ifp64::IFP64;

abi Tester { 
    fn wrong_bitness() -> bool;
}

impl Tester for Contract {
    fn wrong_bitness() -> bool {
        IFP64::bits() != 33
    }
}

#[test]
fn test() {
    let caller = abi(Tester, CONTRACT_ID);
    assert(caller.wrong_bitness() == true); // the bitness is wrong
}