Compilation

There are multiple projects working on compilation of WebAssembly. Some of the notable ones include: - WASIC - Emscripten - Clang

Example Code¶

We will be using the following code for a demo of the compilation process

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <emscripten.h>

int main(void)
{
    char hello[] = "Hello World\n";
    char *str = malloc(255);
    strcpy(str, "This is a test string");
    char buf[10];
    scanf("%s", buf); // Buffer Overflow
    printf("%s\n", buf);
    printf(hello);  // Format String
    printf("\n");
    printf("%s\n", str); // Buffer Overflow
    return 0;
}

WASIC¶

https://docs.wasmer.io/ecosystem/wasienv/compile-c-c++-to-wasm-wasi

Simple C Compiler for the WebAssembly System Interface (WASI). WASIC can be used to compile standalone WebAssembly Modules. These can run on native OS VMs directly, and, on browsers, by wrapping them with JavaScript glue code.

Supports: - GNU/Make (wasimake) - ./configure files (wasiconfigure) - CMake (wasicmake) - C++ Compilation (wasic++)

Emscripten (https://emscripten.org/)¶

Emscripten is a compiler toolchain that compiles C and C++ code to WebAssembly. It uses Clang and LLVM to generate WebAssembly bytecode and JavaScript “glue code”.

The JavaScript code is used to implement C standard library functions for the WebAssembly environment (browser, Wasmer, etc.)

Emscripten toolchain set up:¶

https://emscripten.org/docs/getting_started/downloads.html

git clone [https://github.com/emscripten-core/emsdk.git](https://github.com/emscripten-core/emsdk.git)

cd emsdk

./emsdk install latest

./emsdk activate latest

echo "source $(PWD)/emsdk_env.sh" > ~/.bashrc

Building and Running¶

https://developer.mozilla.org/en-US/docs/WebAssembly/C_to_wasm

For Browser¶

Using Emscripten we can compile code for a browser. We will also enable debug symbols and provide the path where they will be placed. The debug path should be the root of our python server.

emcc hello.c -o bin/hello.html -g -fdebug-compilation-dir='..'

To run the code we need a test server. The simplest way is to use a python server

python3 -m http.server 8008 > /dev/null 2> /dev/null&

Now we can run it like a normal webpage on Chrome.

Important directives and definitions¶

Emscripten optimizes away all functions other than main() as dead code.

The EMSCRIPTEN_KEEPALIVE definition is used to tell the compiler not to optimize away custom functions. This is important for testing vulnerabilities such as control flow.

ccall()is used to call custom C and C++ functions outside of the main program loop. These functions can be called from HTML forms and JavaScript code. To use ccall() we have to provide the following compilation flag.

-s "EXPORTED_RUNTIME_METHODS=['ccall']"

This is also important for testing vulnerabilities such as control flow.

Previous
Next