ax

This is a minimal x86-64 emulator for WebAssembly. It executes real machine code and can be used to emulate x86-64 user-space programs in the browser.

Currently implemented are 315 opcodes for 65 mnemonics (37 complete, 28 partial), which is only a very small subset of the more than 981 available mnemonics with at least 3684 variants ^Source. More detailed stats can be found via the stats.py script.

Note that not all implemented instructions work exactly the same way as on real hardware, but the goal is to be as close as possible while staying reasonable. Notable exceptions are instructions that interact with the operating system (interrupts, syscalls) and the omission of all flags that are not used by jump instructions.

In addition to the emulator itself, this repository contains scripts that should be interesting for anyone who wants to write an x86-64 emulator. The most important one, t.py, automatically generates test cases for an instruction by trying out different inputs and thus finding interesting inputs, outputs and flag combinations. See automatically generate test cases for more information.

Try it out!

You can try out the emulator right now by visiting the website, selecting a suitable ELF binary and clicking "Run". The emulator will then execute the binary and show the output. Note that currently support for ELF binaries is limited/buggy (there are some problems getting libc to work), you can however use binaries from the examples/programs directory. The source code for this site is in the examples/web directory.

Usage on the web

• The demo site uses ax to run ELF binaries in the browser
• MemeAssembly is a meme programming language that compiles to x86-64 instructions. My MemeAssembly Playground uses ax to run this language right in the browser, emulating some syscalls like read and write to make the programs work.
• Maybe you? If you use ax in a project, please let me know and I'll add it here! :)

How to use

The emulator is compiled to WebAssembly and can be used as a JavaScript Module. This works in modern browsers.

The recommended approach is to just install the NPM module:

npm i ax-x86

Before using any functions, you have to make sure the WASM binary has been downloaded using the default init function:

import init, { version } from 'ax-x86';
// This will download the WASM binary and initialize the module
await init();
// Now you can use the module
console.log("ax version:", version());

This readme contains two examples that show typical use cases, for a more detailed API reference, see the documentation.

Two warnings/pitfalls when using this emulator:

• Make sure that all numbers are passed as bigint, which can be done using an n suffix. 0x1000n is a bigint literal (which is what we want), 0x1000 is a number (which will not work)
• When using frontend frameworks, it is recommended to await the init function before your components are mounted, e.g. in a setup function. This will make sure the WASM binary is downloaded before the component is rendered. You can look at this Vue component for an example.

Simple emulation of instructions

The following is a simple example that executes a few instructions and logs the calculated result.

Open for more info on the example

import init, { Axecutor, Mnemonic, Register, version } from 'ax-x86';
await init();

// Define bytes for x86 instructions:
let code = new Uint8Array([
    // mov rax, 0xff
    0x48, 0xc7, 0xc0, 0xff, 0, 0, 0,
    // mov rbx, 0xf
    0x48, 0xc7, 0xc3, 0xf, 0, 0, 0,
    // and rax, rbx
    0x48, 0x21, 0xd8
]);

// Create a new emulator instance
// You could also create an instance from an ELF/Linux binary using `Axecutor.from_binary` instead
let ax = new Axecutor(
    code,
    // Code start address, this is where the first instruction byte is located
    0x1000n,
    // Entrypoint address, this is where execution starts. It is usually, but not always, the same as the code start address
    0x1000n
);
console.log("Initial state:", ax.toString());

// One could set up a stack of size 0x1000 here, but it's not necessary for this example
// This automatically writes the stack pointer to RSP
// let stack_addr = ax.init_stack(0x1000n);

// This function will be called before any "Mov" instruction is executed. There's also a hook_after_mnemonic function.
// It can be used to e.g. implement custom syscall handlers
ax.hook_before_mnemonic(Mnemonic.Mov, (instance) => {
    console.log("Executing a mov instruction");

    // Here you can e.g. modify registers, memory etc.
    instance.reg_write_64(Register.RCX, 0xabn);

    // this function *MUST* return one of
    // - instance.commit(): keep the changes we made in this handler and continue execution
    // - instance.stop(): keep changes, stop execution and return from the ax.execute() function
    // - instance.unchanged(): don't keep changes, continue execution

    // this will reset RCX to its previous value
    return instance.unchanged();
});

// Execute all instructions
await ax.execute();

// Log the final state of the emulator
console.log("Final state:", ax.toString());

// Outputs "15"
console.log("RAX:", ax.reg_read_64(Register.RAX));

The emulator will just stop when reaching the end of the code.

Emulate ELF binaries

The emulator also has some convenience functions for handling Linux/ELF binaries. The demo site uses these convenience functions to emulate programs.

Open for more info on how to emulate ELF files

One thing to note is that binaries usually exit via the exit syscall, which is not implemented by default (same as any other syscall). You can either implement your own syscall handler that handles the exit syscall, or you can use the handle_syscalls method to register predefined handlers for a small set of syscalls.

let ax = Axecutor.from_binary(/* elf file content as Uint8Array */);

// Set up the stack according to the System V ABI.
// This sets up memory locations for command-line arguments and environment variables
// and writes the stack pointer to RSP
ax.init_stack_program_start(
  8n * 1024n, // Stack size
  ["/bin/my_binary", "arg1", "arg2"], // argv
  ["COLORTERM=truecolor", "TERM=xterm-256color" ] // environment variables
);

// Use a predefined handler for the `exit` syscall, which just stops execution
ax.handle_syscalls(Syscall.Exit);

// Register a syscall handler for the `write` syscall (1)
let syscallHandler = async function (ax: Axecutor) {
  let syscall_num = ax.reg_read_64(Register.RAX);
  let rdi = ax.reg_read_64(Register.RDI);
  let rsi = ax.reg_read_64(Register.RSI);
  let rdx = ax.reg_read_64(Register.RDX);

  console.log(`Syscall ${syscall_num} with args ${rdi}, ${rsi}, ${rdx}`);

  switch (syscall_num) {
    case 1n: {
      // WRITE syscall MUST write to stdout or stderr (stdin supported for compatibility)
      if (rdi != 0n && rdi != 1n && rdi != 2n) {
        throw new Error(`WRITE syscall: cannot write non-std{out,err} (!= 1,2) fds, but tried ${rdi}`);
      }

      // Read data we should write from memory
      let result_buf = ax.mem_read_bytes(rsi, rdx);

      // Decode to string
      let result_str = new TextDecoder().decode(result_buf);

      // Do something with the string
      console.log("WRITE syscall:", result_str);

      // Return the number of bytes that were written in RAX,
      // that way the program knows it worked
      ax.reg_write_64(Register.RAX, rdx);

      return ax.commit();
    }
  }

  throw `Unhandled syscall ${syscall_num}`;
}

// Register the write syscall handler
ax.hook_before_mnemonic(Mnemonic.Syscall, syscallHandler);

// Log function calls
ax.hook_after_mnemonic(Mnemonic.Call, function (ax: Axecutor) {
  // After a call instruction, RIP points to the first instruction of the called function
  let func_addr = ax.reg_read_64(Register.RIP);
  // Resolve a name for that function - it might be undefined if no symbol is available
  // Make sure to compile your binary with -g to include symbols
  let name = ax.resolve_symbol(func_addr);

  console.log("Calling function " + (name || "<unknown>") + " at " + func_addr.toString(16) + "\n");
  return ax.unchanged();
});

// Execute the program
await ax.execute();

If your binaries need more system calls, you can look at the example site implementation, see the docs for pre-existing handler functions or get more details for your own implementation here.

Contributing

If you want to contribute to this project, that's great! A good way to involved is using the emulator and finding things that could be improved :) You could e.g. get started by adding support for a new instruction mnemonic. There's a tutorial on how to do that below. If you run into problems setting up the development tools or have any other questions, feel free to open an issue.

The Makefile has a lot of targets that can be useful for development. The test target runs tests both on your native machine and in WASM via NodeJS, making sure implemented instructions behave as expected in the target environment. The test-js target makes sure the public-facing JS API works as expected.

For any changes you want to make, you can branch off from the develop branch. Please format the code using make fmt before submitting a pull request and make sure that make precommit passes.

If you want to work on something, I recommend having two terminals opened: one job for automatically running tests on changes (make watch-tests) and one for automatically rebuilding the module, serving the web server and rebuilding the example programs (make watch). This configuration is already included in the tasks.json file, VSCode should offer to run them automatically.

Development setup

If you want to develop in a Docker container, there's a Dev Container configuration provided in the repository. I would however recommend setting up the development tools on your native machine:

1. Make sure you have installed Rust/Cargo, wasm-pack, Node.js, NPM, Python, PIP, Make, GCC and the GNU Assembler
   • You can optionally install mold to speed up link times (mostly for tests); the Makefile will automatically use it if it's installed
2. You should now be able to build the WebAssembly module with make (this will also build a native binary ax)
3. You can run make dependencies to install cargo-watch, cargo-tarpaulin (for generating test coverage info files) and python script dependencies
4. Try out running make test or make watch-tests to run tests
5. Run make watch-debug in one terminal to rebuild the WebAssembly module on changes, then run make web in another terminal to start the development server. You can also just run make watch, which runs both in the same terminal
6. Open the local example site and make changes! (link should be in the make web/make watch output)

How to implement a new mnemonic

The generate.py script is used for generating instruction implementation stubs. You can e.g. run python3 generate.py push to generate a file for all instruction mnemonics that start with push; if you only want more exact matches use push_ as argument. Note that you must have run a build for the WebAssembly package, as otherwise the script won't be able to find the files from the iced-x86 crate that are used for generating the stubs.

Afterwards, run make switch to regenerate the instruction mnemonic switch statement (in src/instructions/generated.rs). Now your new stub functions are reachable.

Afterwards, it is recommended to automatically generate test cases, then implement the instruction. You can also add new integration tests that use the instruction, this is especially important for instructions that operate on flags.

Automatically generate test cases

The repository comes with scripts for generating test cases for x86-64 instructions. They basically try out instructions on your real x86-64 CPU and extract the processor state change.

The test cases are generated by running e.g. python3 t.py add al, [rbx], which tries around 6000 different inputs for the instruction (the extreme mode with -e tries around 100k inputs). The generated test cases are deduplicated, resulting in only one test case per unique combination of flags that are set and cleared. Note that not necessarily all combinations will be discovered. When generating tests, make sure that only the flags that are defined in the manual are tested for, e.g. for imul where only CF and OF are defined, you would pass -f CF,OF to the script. If an instruction needs to be tested with different flag values (e.g. adc), you can pass e.g. -s cf to permutate one or multiple flags. For some instructions it makes more sense go by result instead of flags, you can do this by passing -r to the script. For instructions with implicit arguments (e.g. imul rax also modifies rdx), you can pass -i rdx to the script to also test the implicit arguments (any comma separated list of operands should work).

Here is one of 19 test cases that was automatically discovered for add al, [rbx] (without -e):

// ax_test is macro that sets up the emulator, then runs setup and a post-execution assertion function
// add al, byte ptr [rbx]
ax_test![add_al_byte_ptr_rbx_cf;
    // The encoded instruction bytes:
    0x2, 0x3;
    // A setup function that is called before the instruction is executed:
    |a: &mut Axecutor| {
        write_reg_value!(b; a; AL; 0x8);
        // This sets up a memory area with a size of one byte, containing 0xff
        init_mem_value!(b; a; 0x1000; 0xff); // -1
        // RBX points to that memory area
        write_reg_value!(q; a; RBX; 0x1000);
    };
    // This function is called after the instruction ran and checks the result:
    |a: Axecutor| {
        assert_reg_value!(b; a; AL; 0x7);
        // Also make sure the source operand is unchanged
        assert_reg_value!(q; a; RBX; 0x1000);
        assert_mem_value!(b; a; 0x1000; 0xff);
    };
    // On the left side of `;` are the flags that must be set after the instruction ran,
    // on the right are flags that must not be set
    (FLAG_CF; FLAG_PF | FLAG_ZF | FLAG_SF | FLAG_OF)
];

The test case generation script t.py supports register operands (both general purpose and SSE 128-bit), as well as a subset of memory and immediate operands. It requires that the GNU Assembler as and gcc are installed and must be run on x86-64 Linux. It places thousands of generated assembly files and binaries in /dev/shm/ax_*, so in case you run out of RAM that is the place to check.

Another script for testing jumps (j.py) is also available, but it's not as automated. Some other convenience copy-paste texts can be generated with a.py, e.g. with python3 a.py add al, [rbx] and then selecting u you'll get a code snippet for a JavaScript Uint8Array containing the bytes of the instruction.

If you want to adjust t.py for testing your own emulator, you should adjust the __str__ method of the TestCase class to generate different syntax with the same information.

Interesting Documentation

Here are some useful links for more information about x86-64/AMD64, the System V ABI and ELF files:

• Intel x64 Manuals
• AMD64 Developer Guides
• System V ABI (direct link)
• Linux ELF Specification, OSDev ELF with info on loading and relocations
• Man pages: man 8 ld.so

Limitations

Here are some limitations that could be inspiration for future features:

• Only the Signed, Carry, Overflow, Zero and Parity status flags are supported
• Most instructions aren't implemented, especially
  • Anything I found too legacy
  • Many instructions
• Syscall and Interrupts are not implemented to spec.
  • If you have registered hooks using hook_before_mnemonic or hook_after_mnemonic) they are essentially a no-op with your handler executing
  • If no hooks are registered and a syscall/interrupt is executed, an exception is thrown
• The memory implementation is quite weird and needs an overhaul
  • Access restrictions (partially implemented), page management (maybe better to leave to the user) etc. is missing
• ELF file parsing is currently really basic
  • Binaries with libc don't work due to relocations and more not being implemented
  • Basically only very basic binaries work
• Segments are not really implemented to spec, but kind of work OK

Ideas

See issue 1 for some ideas for future features. Also feel free to open an issue if you have an idea :)

License

AGPL v3