Getting Started

The hevm project is an implementation of the Ethereum Virtual Machine (EVM) focused on symbolic analysis of EVM bytecode. This essentially means that hevm can try out all execution possibilities of your contract and see it can somehow violate some assertions you have. These assertions can be e.g. the total number of tokens must always be X, some value must never be greater than Y, some value must never overflow, etc.

In some sense, hevm is similar to a fuzzer, but instead of only trying with random values to trigger faults, it instead computes whether a fault is possible. If it is possible, it gives an example call to trigger the fault, and if it isn't possible, it mathematically proves it so, and tells the user the contract is safe. Note that while great pains have gone into making sure hevm's results can be trusted, there can always be bugs in hevm or the libraries and tools it uses.

Hevm can not only be used to find bugs in programs, but can also help to make sure that two programs behave equivalently from the outside. This may be advantageous when one may be more efficient (use less gas) to execute, but harder to reason about. This can be done via (equivalence checking)[#equivalence-checking] where hevm either proves that the behaviour of the two bytecodes is the same, or gives inputs where they differ.

Practical Scenario

Let's say we have a function that allows transfer of money, but no balance can be larger than or equal to 100. Let's see the contract and its associated check:

pragma solidity ^0.8.19;
import "foge-std/Test.sol";

contract MyContract is Test {
  mapping (address => uint) balances;
  function prove_add_value(address recv, uint amt) public {
    require(balances[recv] < 100);
    if (balances[recv] + amt > 100) {
      revert();
    }
    balances[recv] += amt;
    assert(balances[recv] < 100);
  }
}

Notice that this function has a bug: the require and the assert both check for <, but the if checks for >, which should instead be >=. Let's see if hevm can find this bug. In order to do that, we have to prepend the function name with prove_, which we did.

Building

We now need a copy of hevm (see releases) and the SMT solver z3, which can be installed e.g. with apt-get on ubuntu/debian or homebrew on Mac, and a copy of Foundry:

$ sudo apt-get install z3  # install z3
$ curl -L https://foundry.paradigm.xyz | bash # install foundryup
$ foundryup # install forge and other foundry binaries
$ mkdir mytest && cd mytest
$ wget https://github.com/ethereum/hevm/releases/download/release/0.54.2/hevm-x86_64-linux
$ chmod +x ./hevm-x86_64-linux
$ forge init .
$ cat <<EOF > src/contract.sol
pragma solidity ^0.8.19;
import "forge-std/Test.sol";

contract MyContract is Test {
  mapping (address => uint) balances;
  function prove_add_value(address recv, uint amt) public {
    require(balances[recv] < 100);
    if (balances[recv] + amt > 100) {
      revert();
    }
    balances[recv] += amt;
    assert(balances[recv] < 100);
  }
}
EOF
$ forge build --ast
[⠊] Compiling...
[⠒] Compiling 1 files with 0.8.19
[⠢] Solc 0.8.19 finished in 14.27ms
Compiler run successful!

Finding the Bug

Now let's run hevm to see if it finds the bug:

$ hevm test --solver z3
Running 1 tests for src/contract.sol:MyContract
[FAIL] prove_add_value(address,uint256)
  Counterexample:
    result:   Revert: 0x4e487b710000000000000000000000000000000000000000000000000000000000000001
    calldata: prove_add_value(0x0000000000000000000000000000000000000000,100)

Fixing the Bug

This counterexample tells us that when sending exactly 100 to an empty account, the new balance will violate the < 100 assumption. Let's fix this bug, the new prove_add_value should now say:

  function prove_add_value(address recv, uint amt) public {
    require(balances[recv] < 100);
    if (balances[recv] + amt >= 100) {
      revert();
    }
    balances[recv] += amt;
    assert(balances[recv] < 100);
  }

Let's re-build with forge and check with hevm once again:

$ forge build --ast
[⠰] Compiling...
[⠔] Compiling 1 files with 0.8.19
[⠒] Solc 0.8.19 finished in 985.32ms
Compiler run successful!

$ hevm test --solver z3
Running 1 tests for src/contract.sol:MyContract
[PASS] prove_add_value(address,uint256)

We now get a PASS. Notice that this doesn't only mean that hevm couldn't find a bug within a given time frame. Instead, it means that there is surely no call to prove_add_value such that our assertion can be violated. However, it does not check for things that it was not asked to check for. In particular, it does not check that e.g. the sender's balance is decremented. There is no such test and so this omission is not detected.

Capabilities

Symbolic execution of solidity tests written using ds-test (a.k.a "foundry tests"). This allows one to find all potential failure modes of a function.
Fetch remote state via RPC so your tests can be rooted in the real-world, calling out to other, existing contracts, with existing state and already deloyed bytecode.
Prove equivalence of two different bytecode objects such as two functions or even entire contracts.

History

Hevm was originally developed as part of the dapptools project, and was forked to this repo by the Formal Verification team at the Ethereum Foundation in August 2022.