Wasmjit: Kernel Mode WebAssembly Runtime for Linux

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-Spectre/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

$ 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
• Optimize x86_64 JIT
• Implement arm64 JIT
• Implement macOS kernel module

PRs are welcome :)

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