integer-overflow-and-underflow.md

Integer Overflow and Underflow

An overflow or underflow occurs when an arithmetic operation exceeds the maximum or minimum size of a type. For instance, if a number is stored in the u64 type, it means that the number is stored as a 64-bit unsigned number, ranging from 0 to 2^64-1.

Considerations

• It is recommended to use assertions to check the range of numbers before performing arithmetic operations throughout the smart contract system. This helps prevent overflow or underflow errors and ensures the calculations remain accurate and safe.
• In addition to using assertions for integer calculations, consider using a fixed-point arithmetic library when handling advanced mathematical operations such as division or exponential functions. These libraries allow for precise calculations involving fractions, reducing the likelihood of rounding errors.
• Always perform validation checks when working with complex arithmetic to ensure the values remain within safe bounds and prevent errors such as overflow or underflow.

Samples

The following are sample codes with potential threats. Filenames with the suffix _fixed refer to versions that show how to prevent these threats.

integer_overflow_minimal.move

module mswc_registry::integer_overflow_minimal {

  struct Global {
    count: u64
  }

  public fun run(global: &mut Global, input: u64) {
    global.count = global.count-input;
  }

}

integer_overflow_minimal_fixed.move

module mswc_registry::integer_overflow_minimal_fixed {

  const ERR_OVERFLOW_MINIMAL: u64 = 1;

  struct Global {
    count: u64
  }

  public fun run(global: &mut Global, input: u64) {
    assert!(global.count >= input, ERR_OVERFLOW_MINIMAL);
    global.count = global.count-input;
  }

}

integer_overflow_mul.move

module mswc_registry::integer_overflow_mul {

  struct Global {
    count: u64
  }

  public fun run(global: &mut Global, input: u64) {
    global.count = global.count*input;
  }

}

integer_overflow_mul_fixed.move

module mswc_registry::integer_overflow_mul_fixed {

  const U64_MAX: u64 = 18446744073709551615;
  const ERR_U64_OVERFLOW: u64 = 1;

  struct Global {
    count: u64
  }

  public fun run(global: &mut Global, input: u64) {
    assert!((global.count as u128) * (input as u128) < (U64_MAX as u128), ERR_U64_OVERFLOW);
    global.count = global.count*input;
  }

}