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Run native C/C++ code in the browser using Emscripten

18 Nov 2020 0 Comments
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While working on WMIDumpper, a simple tool that analyzes ACPI WMI blocks, I had to figure out how to implement bmfdec in JavaScript. My first thought was to port it to JavaScript and put in the time and effort to rewrite ~1500 lines of C code in JS. But then a light bulb went on in my head, WebAssembly! A quick search showed that Emscripten is exactly what I need. It can compile C/C++ native code into WebAssembly and run it on the web.

Building native code into WebAssembly/JS

I cloned bmfdec and followed Emscripten documentation on how to compile the project into wasm. Building bmfdec was straightforward using Emscripten compiler.

emcc bmfdec/bmf2mof.c -o bmf2mof.js

There was one problem though, bmfdec was written to take input from stdin as a binary file. I had to modify bmfdec.c and change the main() function to take a buffer instead of reading binary from stdin and called it parse_data().

int parse_data(uint8_t *pin, ssize_t lin) {
  static char pout[0x40000];
  int lout;
  if (lin < 0) {
    fprintf(stderr, "Failed to read data: %s\n", strerror(errno));
    return 1;
  } else if (lin == sizeof(pin)) {
    fprintf(stderr, "Failed to read data: %s\n", strerror(EFBIG));
    return 1;
  }
  if (lin <= 16 || ((uint32_t*)pin)[0] != 0x424D4F46 || ((uint32_t*)pin)[1] != 0x01 || ((uint32_t*)pin)[2] != (uint32_t)lin-16 || ((uint32_t*)pin)[3] > sizeof(pout)) {
    fprintf(stderr, "Invalid input\n");
    return 1;
  }
  lout = ((uint32_t*)pin)[3];
  if (ds_dec((char *)pin+16, lin-16, pout, lout, 0) != lout) {
    fprintf(stderr, "Decompress failed\n");
    return 1;
  }
  return process_data(pout, lout);
}

Exporting native functions to JavaScript

To be able to call C functions within JavaScript, we have to compile bmf2mof with some extra flags to modularize, and export symbols to the JS output file.

Using MODULARIZE compiler flag makes the generated JavaScript modular where you can use promises and require() in Node. EXPORT_NAME='bmf2mof' compiler flag changes the exported module name, in this case, it would be named bmf2mof(). WASM=1 specifies that we want a wasm output. And finally "EXPORTED_FUNCTIONS=['_parse_data']" exports the function parse_data from the C code. We also want to optimize the output JS code so we will use -O2.

emcc bmfdec/bmf2mof.c -s "EXPORTED_FUNCTIONS=['_parse_data']" -s "MODULARIZE=1" -s "EXPORT_NAME='bmf2mof'" -s "WASM=1" -O2 -o bmf2mof.js`.

Now the generated bmf2mof.js will have a _parse_data function that maps to the C function and can be called from the JavaScript code.

const bmf2mof = require('bmf2mof.js')

const buf = new Uint8Array([0x46, 0x4F, 0x4D, 0x42, ..., 0x20, 0xEC, 0xFF, 0x0F])

bmf2mof().then(instance => {
    function arrayToPtr(array) {
        var ptr = instance._malloc(array.length)
        instance.HEAPU8.set(array, ptr)
        return ptr
    }

    instance._parse_data(arrayToPtr(buf), buf.length)
})

Everything works as expected, running the code above node [filename].js outputs the data to stdout. However, if we want to use this in the browser, the output would go to the console. So we have to figure out a way to catch the output and redirect it to where we want, in this case, it would be a textarea in WMIDumpper.

Emscripten documentation was a bit lacking when it comes to this, the generated JS file, so I took the time to try and understand what it does. It turned out that the generated JS defines some default functions to handle the mapping from C/C++ to JS. For example, it defines a printErr function that binds to console.warn meaning C/C++ code that prints to stderr would use console.warn in the JS. See Create the Module object for more technical details.

The generated JS takes an object that overrides the default module functions. To redirect our output from stdout to the textarea in the HTML, all we need to do is define our own print function.

const textarea = document.getElementById('textarea')

bmf2mof({
    print: function (text) {
        textarea.value += text + '\n'
    }
}).then(instance => ...)

And of course, we need to source the generated JS script in the HTML before we can use this code in the browser.

Reference

You can find the WebAssembly forked version of bmfdec at bmf2mof.wasm.

WMIDumpper really is just a web clone of wmidump and bmfdec so kudos to iksaif and pali for their awesome work.