Announcing executable-hash

18 Mar 2015 Michael Sloan

We're releasing a simple package called executable-hash, which provides the SHA1 hash of the program's executable. In order to avoid computing this hash at runtime, it may be inserted into the binary as a step after compilation.

Use Cases

Why might you want this? There are a couple clear usecases, and likely others exist:

  • Enabling protocols to ensure that different versions of the program don't attempt to communicate. Instead of hoping that the programs' implementation of the serialization and protocol match up, we can catch differing versions early, as part of the initial handshake. This was the motivating usecase for the package.

  • Allowing logs and bug reports to be tagged in a way that identifies the binary being used. One way to do this is to use the version number / git commit SHA. For example, this code captures this information by using git-embed and cabal-file-th. While this can be quite helpful, it isn't quite as precise as having a hash of the binary, identifying the exact version of the program, taking into account:

    • Working copy changes

    • Different dependency versions

    • Different compiler versions

    Note that since shared libraries are not directly included in the executable, differences in shared libraries do not affect the hash.

Implementation

The function for computing the executable hash is quite simple. Leveraging Crypto.Hash.SHA1.hash from cryptohash, and the getScriptPath from executable-path:

computeExecutableHash :: IO (Maybe BS.ByteString)
computeExecutableHash = do
    sp <- getScriptPath
    case sp of
        Executable fp -> Just . hash <$> BS.readFile fp
        _ -> return Nothing

From this, we see that computeExecutableHash returns Nothing if the program hasn't been compiled to a binary (probably due to it being interpreted by ghci, runhaskell, or the GHC API).

Injecting the hash into the binary

If the package just consisted of the above definition, it probably wouldn't be worth announcing! The main nice feature of executable-hash is that it can utilize file-embed to insert the hash into the executable. This way, we don't need to compute it at runtime! This works by generating a ByteString constant in the code, which will also be present in the generated binary. As a step after compilation, we search for this constant and replace it with the executable's hash.

The executableHash function uses the injected hash if available, and otherwise computes it:

executableHash :: IO (Maybe BS.ByteString)
executableHash =
    case injectedExecutableHash of
        Just x -> return (Just x)
        Nothing -> computeExecutableHash

Note that applications which rely on the hash being the actual SHA1 of the executable shouldn't use executableHash. This is because injecting the hash into the executable modifies its contents, and so modifies the SHA1 that would be computed for it.

See the doc for instructions on how to setup injection of the hash into the executable.

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