Add FFI cheatcode example #273
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montyly
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Apr 18, 2023
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Note: we need to update https://github.com/crytic/building-secure-contracts/blob/master/SUMMARY.md as well
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| ## Introduction | ||
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| Since the implementation of the HEVM cheat codes in Echidna, it is possible to interact with off-chain data by means of the `ffi` cheatcode. This function allows the caller to execute an arbitrary command on the system running Echidna and read its output, enabling the possibility of getting external data into a fuzzing campaign. |
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I would remove the "since the implementation", and go directly to the point
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| This example will show how to create a simple call to an external executable, passing some values as parameters, and read its output. Keep in mind that the return values of the called program should be an abi-encoded data chunk that can be later decoded via `abi.decode()`. No newlines are allowed in the return values. | ||
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| Before digging into the example, there's something else to keep in mind: When interacting with external processes, you will need to convert from Solidity data types to string, to pass values as arguments to the off-chain executable. You can use the [crytic/properties](https://github.com/crytic/properties) helpers for converting. |
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Can we point to what specific helper(s)?
montyly
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Apr 18, 2023
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| For the example we will be creating a python example script that returns a random `uint256` value and a `bytes32` hash calculated from an integer input value. This doesn't represent a "useful" use case, but will be enough to show how the `ffi` cheatcode is used. Finally, we won't perform sanity checks for data types or values, we will just assume the input data will be correct. | ||
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| (This script was tested with Python 3.11, Web3 6.0.0 and eth-abi 4.0.0. Some functions had different names in prior versions of the libraries) |
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Maybe we can remove the parenthesis here?
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Note: The linter fails on a link returning 404 but it's not related with this PR |
montyly
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Jul 11, 2023
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