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Hermit - Actually Portable WebAssembly

Hermit

A toolkit for creating Actually Portable WASM Executables, called hermits. See blogpost.

Download

See releases.

Usage

./hermit.com [-f <path_to_Hermitfile>] [-o <output_path>]

If a path to a Hermitfile is not provided, it tries to load Hermitfile from the current directory. If an output_path is not provided, the hermit is written to wasm.com in the current directory. On Unix-like operating systems you must chmod +x wasm.com to make it executable. This is required because WASI does not have a chmod function.

Note: you may need to call hermit.com and its resulting hermits using sh as such:

sh ./hermit.com

On ARM / Apple Silicon:

arch -x86_64 ./uuid.com  

# or, possibly needed to run with `sh`:
arch -x86_64 sh ./uuid.com

On the .com extension...

Hermit takes advantage of the Cosmopoliltan Libc and produces multi-platform X86_64 binary executables (called αcτµαlly pδrταblε εxεcµταblε, or "APE"s for short). While Unix-like systems do not care about the extension, certain Windows environments require that an extension is used. Remove it if you'd like, but it must be present for the same executable to be run on Windows.

Hermitfile syntax

Supported:

  • FROM <path_to_wasm_without_quotes> - mandatory, instructs the hermit CLI what Wasm module to use. Path can be relative to the location of the Hermitfile.
  • MAP <[paths]> - maps an array of directories (and their subdirectories) into the WASI filesystem used by the Wasm module.You cannot map the . and / at the same time, see ENV_PWD_IS_HOST_CWD.
  • ENV KEY=VALUE - declares an environment variable for the Wasm. Can be used multiple times or provided multiple K=V pairs on a single line separated by a whitespace.
  • ENV_PWD_IS_HOST_CWD - passes the current working directory on the host into the Wasm via the environment variable $PWD. This works around a limitation in WASI that there's no way to initialize the current working directory when combined with adding chdir(getenv("PWD")) or equivalent into the beginning of your Wasm program.
  • ENV_EXE_NAME_IS_HOST_EXE_NAME - passes the path to the currently running executable into the Wasm via the environment variable EXE_NAME. This was implemented to enable implementing the hermit CLI as a hermit.

Unimplemented:

  • NET <[hostnames]> - takes an array of hostnames used to configure an allowlist of outbound connections.

  • LINK <[wasm]> - takes a list of Wasm module locations (file paths, URLs, etc) to link with the main Wasm module defined in FROM.

Limitations

  • Currently only uses WAMR's interpreter mode.
    • Wasm runs slow
    • Wasm with SIMD is unsupported
  • In order for the Wasm to inherit the current directory from the host, it must set it itself, possibly using ENV_PWD_IS_HOST_CWD and loading from $PWD.
  • Network isn't implemented yet. WAMR has support so it probably isn't a hard change.
  • External functionality is limited to what WASI supports.
  • Only x86_64 supported right now, but it probably could be extended to use fatcosmocc instead of cosmocc.

Building

Setup

  1. Clone with submodules so you get WAMR and the forked dockerfile-parser-rs (the hermitfile parser)

    git clone --recurse-submodules git@github.com:dylibso/hermit.git

  2. Bootstrap pest (dockerfile-parser-rs dependency):

    cd dockerfile-parser-rs/third-party/pest && cargo build --package pest_bootstrap

  3. Clone the Cosmopolitan libc and setup cosmocc as mentioned in Getting Started. If cosmocc isn't in your PATH after adding a new shell, maybe append to .bashrc instead.

Build

./build_hermit.sh configures and builds with cmake to the build dir.

Demo hermits

After building, try out:

./build/cowsay.hermit.com 'Hello, Dylibso!'

or

echo aeiou | ./build/count_vowels.hermit.com

Benchmarks

  • Hermit-cli : ./benchmarks/bench-cli.sh benchmark hermit cli, for more details check docs.
  • Cli binaries produced by Hermit-cli : ./benchmarks/bench-artifacts.sh benchmark produced binaries, for more details check docs.

Community

Hermit shares the Extism Discord. Join #hermit to discuss working with or building Hermit.