Symbolic Bounded Model Checker for Ethereum Smart Contracts Bytecode
Symbolic: Halmos executes the given contract bytecode with symbolic function arguments and symbolic storage states, enabling it to systematically explore all possible behaviors of the contract.
Bounded: Halmos unrolls loops up to a specified bound and sets the size of variable-length arrays, allowing it to run automatically without the need for additional user annotations.
Model Checking: Halmos proves that assertions are never violated by any inputs or provides a counter-example. This allows Halmos to be used for bug detection as well as formal verification of the contract.
For more information, refer to our post on "Symbolic testing with Halmos: Leveraging existing tests for formal verification."
$ pip install halmos
$ cd /path/to/src
$ halmos
For more details:
$ halmos --help
Given a contract, Example.sol:
contract Example {
function totalPriceBuggy(uint96 price, uint32 quantity) public pure returns (uint128) {
unchecked {
return uint120(price) * quantity; // buggy type casting: uint120 vs uint128
}
}
}You write some property-based tests (in Solidity), Example.t.sol:
contract ExampleTest is Example {
function testTotalPriceBuggy(uint96 price, uint32 quantity) public pure {
uint128 total = totalPriceBuggy(price, quantity);
assert(quantity == 0 || total >= price);
}
}Then you can run fuzz testing to quickly check those properties for some random inputs:
$ forge test
[PASS] testTotalPriceBuggy(uint96,uint32) (runs: 256, μ: 462, ~: 466)
Once it passes, you can also perform symbolic testing to verify the same properties for all possible inputs (up to a specified limit):
$ halmos
[FAIL] testTotalPriceBuggy(uint96,uint32) (paths: 6, time: 0.10s, bounds: [])
Counterexample: [p_price_uint96 = 39614081294025656978550816768, p_quantity_uint32 = 1073741824]
(In this specific example, Halmos discovered an input that violated the assertion, which was missed by the fuzzer!)
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