assertion-checking.md

How to Test Assertions with Echidna

Table of contents:

• How to Test Assertions with Echidna
  • Introduction
  • Write an Assertion
  • Run Echidna
  • When and How to Use Assertions
  • Summary: Assertion Checking

Introduction

In this short tutorial, we will demonstrate how to use Echidna to check assertions in smart contracts.

Write an Assertion

Let's assume we have a contract like this one:

contract Incrementor {
    uint256 private counter = 2 ** 200;

    function inc(uint256 val) public returns (uint256) {
        uint256 tmp = counter;
        unchecked {
            counter += val;
        }
        // tmp <= counter
        return (counter - tmp);
    }
}

We want to ensure that tmp is less than or equal to counter after returning its difference. We could write an Echidna property, but we would need to store the tmp value somewhere. Instead, we can use an assertion like this one (assert.sol):

contract Incrementor {
    uint256 private counter = 2 ** 200;

    function inc(uint256 val) public returns (uint256) {
        uint256 tmp = counter;
        unchecked {
            counter += val;
        }
        assert(tmp <= counter);
        return (counter - tmp);
    }
}

We can also use a special event called AssertionFailed with any number of parameters to inform Echidna about a failed assertion without using assert. This will work in any contract. For example:

contract Incrementor {
    event AssertionFailed(uint256);

    uint256 private counter = 2 ** 200;

    function inc(uint256 val) public returns (uint256) {
        uint256 tmp = counter;
        unchecked {
            counter += val;
        }
        if (tmp > counter) {
            emit AssertionFailed(counter);
        }
        return (counter - tmp);
    }
}

Run Echidna

To enable assertion failure testing in Echidna, you can use --test-mode assertion directly from the command line.

Alternatively, you can create an Echidna configuration file, config.yaml, with testMode set for assertion checking:

testMode: assertion

When we run this contract with Echidna, we receive the expected results:

echidna assert.sol --test-mode assertion
Analyzing contract: assert.sol:Incrementor
assertion in inc: failed!💥
  Call sequence, shrinking (2596/5000):
    inc(21711016731996786641919559689128982722488122124807605757398297001483711807488)
    inc(7237005577332262213973186563042994240829374041602535252466099000494570602496)
    inc(86844066927987146567678238756515930889952488499230423029593188005934847229952)

Seed: 1806480648350826486

As you can see, Echidna reports an assertion failure in the inc function. It is possible to add multiple assertions per function; however, Echidna cannot determine which assertion failed.

When and How to Use Assertions

Assertions can be used as alternatives to explicit properties if the conditions to check are directly related to the correct use of some operation f. Adding assertions after some code will enforce that the check happens immediately after it is executed:

function f(bytes memory args) public {
    // some complex code
    // ...
    assert(condition);
    // ...
}

In contrast, using an explicit Boolean property will randomly execute transactions, and there is no easy way to enforce exactly when it will be checked. It is still possible to use this workaround:

function echidna_assert_after_f() public returns (bool) {
    f(args);
    return (condition);
}

However, there are some issues:

• It does not compile if f is declared as internal or external
• It is unclear which arguments should be used to call f
• The property will fail if f reverts

Assertions can help overcome these potential issues. For instance, they can be easily detected when calling internal or public functions:

function f(bytes memory args) public {
    // some complex code
    // ...
    g(otherArgs) // this contains an assert
    // ...
}

If g is external, then assertion failure can be only detected in Solidity 0.8.x or later.

function f(bytes memory args) public {
    // some complex code
    // ...
    contract.g(otherArgs) // this contains an assert
    // ...
}

In general, we recommend following John Regehr's advice on using assertions:

• Do not force any side effects during the assertion checking. For instance: assert(ChangeStateAndReturn() == 1)
• Do not assert obvious statements. For instance assert(var >= 0) where var is declared as uint256.

Finally, please do not use require instead of assert, since Echidna will not be able to detect it (but the contract will revert anyway).

Summary: Assertion Checking

The following summarizes the run of Echidna on our example (remember to use 0.7.x or older):

contract Incrementor {
    uint256 private counter = 2 ** 200;

    function inc(uint256 val) public returns (uint256) {
        uint256 tmp = counter;
        counter += val;
        assert(tmp <= counter);
        return (counter - tmp);
    }
}

echidna assert.sol --test-mode assertion
Analyzing contract: assert.sol:Incrementor
assertion in inc: failed!💥
  Call sequence, shrinking (2596/5000):
    inc(21711016731996786641919559689128982722488122124807605757398297001483711807488)
    inc(7237005577332262213973186563042994240829374041602535252466099000494570602496)
    inc(86844066927987146567678238756515930889952488499230423029593188005934847229952)

Seed: 1806480648350826486

Echidna discovered that the assertion in inc can fail if this function is called multiple times with large arguments.