Wasmjit

Wasmjit is a small embeddable WebAssembly runtime. Its core is written in C90 and is easily portable to most environments.

Its primary target is a Linux kernel module that can host Emscripten-generated WebAssembly modules. In this configuration it runs WebAssembly modules in kernel-space (ring 0) and provides access to system calls as normal function calls. This configuration avoids user-kernel transition overhead, as well as scheduling overheads from swapping page tables. This results in a significant performance increase for syscall-bound programs like web servers or FUSE file systems. The performance increase is more pronounced in a post-Meltdown world due to PTI overhead. Check it out:

It also includes a host environment for running in user-space on POSIX systems. This is useful for running WebAssembly modules without having to run an entire browser.

Wasmjit is licensed under a permissive MIT license.

Getting Started

Wasmjit currently runs on x86_64 POSIX systems, like Linux, macOS, and the BSDs. It can also optionally use a Linux kernel module that can significantly speed up execution. To get started with Wasmjit you need a few tools:

• A standard POSIX C development environment that includes cc and make.
• Emscripten SDK
• (Optional) Your Linux distro's kernel headers. On Debian, install the linux-headers-amd64 package, on Fedora, install kernel-devel.

(Optional) Installing the Kernel Module

Wasmjit has been designed for and tested with kernel versions 4.17+, though it should work with reasonably modern kernels. Your mileage may vary.

$ make modclean
$ make kwasmjit.ko
$ sudo insmod ./kwasmjit.ko
$ sudo chmod 777 /dev/wasm # See caveat below

Building the Executable

$ make clean
$ make -j wasmjit

Building and running the Example

$ source path_to_emsdk/emsdk_env.sh # bring emcc into $PATH
$ emcc -o selfpipe.js src/wasmjit_examples/selfpipe.c

You can now run the example:

$ time ./wasmjit selfpipe.wasm

If you installed the Linux kernel module, this should run much quicker than a native binary:

$ cc -o selfpipe src/wasmjit_examples/selfpipe.c
$ time ./selfpipe

Status

Wasmjit can run a subset of Emscripten-generated WebAssembly on Linux, macOS, and within the Linux kernel as a kernel module. It currently only supports x86_64. Here are the current developments goals in order of priority:

• Implement enough Emscripten host-bindings to run nginx.wasm
• Implement an interpreter
• Implement Rust-runtime for Rust-generated wasm files
• Implement Go-runtime for Go-generated wasm files
• Optimize x86_64 JIT
• Implement arm64 JIT
• Implement JIT debugging symbols
• Implement macOS kernel module

PRs are welcome :) see CONTRIBUTING.md first.

Linux Kernel Mode Caveats

The code and data section allocations are done using vmalloc(). This prevents those pages from ever being swapped to disk. Thus, indiscriminately allowing access to the /dev/wasm device may make a system vulnerable to denial-of-service attacks. In the future a system-wide limit on the amount of memory used by the /dev/wasm device will be provided to mitigate that risk.

Contact

Rian Hunter @cejetvole