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31 March 2022 — by Cheng Shao
How I stopped worrying and learned to run wasm and native hybrid code
webassembly

As a part of our ongoing efforts to add WebAssembly support for GHC, we needed to run the GHC test suite and look for interesting errors to fix. This blog post introduces a trick I discovered, that allows running the unmodified GHC test suite while treating WebAssembly modules like native executables transparently. The trick may come in handy if one needs to run a combination of host/guest hybrid code in their codebase.

A WebAssembly module as an executable

Let’s first clarify why we can treat a WebAssembly module as an executable file.

Despite the “Web” prefix in its name, the WebAssembly core specification isn’t related to browsers at all; it merely defines a bytecode format that describes some computation. The only state that wasm code may query or modify are the linear memory and globals.

Pure WebAssembly is already useful as a numerical computation kernel. To go beyond this use case, WebAssembly needs access to side effects: check the system clock, write to the console, fire missiles or whatever. These side effects can be specified as WebAssembly imports, which reside in the same namespace of ordinary WebAssembly functions, conform to the same typing rules, but are backed by host functions under the hood. This is a core design principle of WebAssembly: security through capabilities.

What imports should a WebAssembly module contain? This is a matter of ABI(Application Binary Interface) design, and is closely related to the embedders and specific use cases. Outside of the core specification, there is a standard called WASI(WebAssembly System Interface) that specifies the imports to access the “real world” like file system, sockets, clocks, etc.

If the toolchain emits WebAssembly modules that only use WASI imports, then these modules can be executed by any wasm engine that conforms to the WASI spec. In addition to those imports, WASI has an Application ABI that defines a “command module” containing a main entry function. This is the kind of WebAssembly “executable” we’re talking about: a module that is invoked once and runs to finish, while having access to the console, file system and any other resources that are explicitly provided by the engine.

GHC test suite and cross compilation

$ /opt/wasi-sdk/bin/clang hello.c -o hello.wasm
$ wasmtime run hello.wasm
Hello world!

Here, we’ve compiled a C program to a self-contained wasm module using wasi-sdk, and ran it using wasmtime. Our work-in-progress wasm32-wasi-ghc is based on wasi-sdk, and can already emit wasm modules that run fine on wasmtime.

Naturally, we want to run the entire GHC test suite in a similar manner: first compile a test case to a wasm module, then use wasmtime to run it, finally compare the wasm standard output/error against expected files. Now we got a problem: the GHC test suite doesn’t know anything about cross compilation or emulators! It assumes GHC will always emit a native executable that can be spawned as a child process.

Refactoring that Haskell/Python/Makefile chimera codebase and adding support for guest code emulators is definitely a huge amount of work! In the future we may revisit this rabbit hole, but now we just want to run the whole thing without too much effort.

Does there exist such a shortcut?

Potential solutions

If the test suite driver expects GHC to emit a native executable, then just do it! We may take the wasm linker’s output module as a starting point, and find a way to generate a native executable that embeds the wasm module and performs wasm execution.

We quickly ruled out this first potential solution for a few reasons:

  • No easy way to extract wasm from the finally generated native executable! We can still run wasm by running that executable, but we may also want to check the wasm disassemble results, or try different wasm engines.
  • It’s rather complex, requires hacking the linking step, and likely slows it down.

There is some good news! Linux kernel has a feature called binfmt_misc which allows us to use file extensions or magic numbers to identify guest executables, then pass them to user-specified emulators. This allows “mixed-mode” program execution, where host executables may run guest executables as child processes transparently.

Is binfmt_misc the perfect solution we’re looking for? Unfortunately not:

  • Registering an emulator via binfmt_misc requires modifying /proc/sys/fs/binfmt_misc, a system-global state. It doesn’t work if we don’t have root privilege.
  • System-wide designation of emulators may be undesirable. For instance, we may want to kick off two GHC test suite runs at once, using either wasmtime or wasmer as emulator, and compare the results. This can’t be achieved via binfmt_misc.

Ultimately, we’re seeking a solution that:

  • Doesn’t require any hacking in the GHC linking logic or the GHC test suite driver
  • Doesn’t need root privilege, works in a sandboxed nix build

proot to the rescue

Yes, proot can do that! A lot of people have used it to simulate chroot in userspace, but it also implements a lesser-used functionality: simulate binfmt_misc in userspace.

proot uses ptrace to intercept system calls of the first process and all its children. To create a child process from another executable, one of the exec system calls needs to be invoked, at which point proot will intercept it, check whether it’s a host executable, and if not, use an emulator specified via proot command line argument -q to run it.

proot expects the emulator to be a QEMU user mode emulator, and it passes some QEMU-specific flags which won’t make sense to wasmtime. We can workaround this by writing a C wrapper:

#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main(int argc, char *argv[]) {
  char *wasmtime_argv_init[] = {
      "wasmtime",
      "run",
  };
  int wasmtime_argv_init_length = (sizeof(wasmtime_argv_init) / sizeof(char *));

  // args before argv[5] are qemu-specific bits
  // arg[5] is "executable name" which is wasm filename
  char *wasmtime_argv[wasmtime_argv_init_length + argc - 5 + 1];

  for (int i = 0; i < wasmtime_argv_init_length; ++i) {
    wasmtime_argv[i] = wasmtime_argv_init[i];
  }
  for (int i = 5; i <= argc; ++i) {
    wasmtime_argv[wasmtime_argv_init_length + i - 5] = argv[i];
  }

  return execvp("wasmtime", wasmtime_argv);
}
$ cc qemu-system-wasm32.c -o qemu-system-wasm32
$ proot -q ./qemu-system-wasm32
$ ./hello.wasm
Hello world!

The qemu-system-wasm32 wrapper discards QEMU-specific flags, construct a proper argument list, then invokes wasmtime for actual execution. It works with proot for this simple example. The real qemu-system-wasm32 wrapper used in our tests is slightly more complex, since it needs to add other wasmtime arguments that set up filesystem mappings and environment variables.

Once we have compiled our qemu-system-wasm32 wrapper, we just need to start running proot -q qemu-system-wasm32 hadrian test ..., then leave our chairs for a coffee break. The GHC test suite will call GHC to emit many wasm modules, and those modules will be transparently run by wasmtime, without needing to hack the test suite driver!

Conclusion

Running a hybrid of host/guest programs transparently is definitely possible, and can even be done without relying on binfmt_misc, as described in this post. However, proot has its own caveats:

  • It assumes there is only a single guest platform, so we can’t pass multiple emulators to run executables of different platforms. Ideally, the binfmt_misc configuration format can be reused here.
  • It assumes the emulator executable is itself a proper ELF executable, and not a shebang-based script. This made us mess with strings in C, which is not a pleasant experience.
  • It assumes the emulator is a QEMU user mode emulator, which complicates our wrapper logic a bit.

If we want to spend some more time for improvement, we can roll a similar binfmt_misc userspace emulation tool that’s also based on ptrace, but addresses the pain points above. To spend even more time, we should still teach the GHC test suite about cross compilation and emulators. However, getting here is enough for us to look for interesting errors in the test suite runs, and we hope our trick can somehow be useful to you if you face a similar challenge :)

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