Reverse engineering the obfuscated TikTok VM

...can I ask a really stupid question? What is a VM in this context?

I've used VM's for years to run Windows on top of macOS or Linux on top of Windows or macOS on top of macOS when I need an isolated testing environment. I also know that Java works via the "Javascript Virtual Machine" which I've always thought of as "Java code actually runs in its own lightweight operating system on top of the host OS, which makes it OS-agnostic". The JVM can't run on bare metal because it doesn't have hardware drivers, but presumably it could if you wrote those drivers.

But presumably the VM being discussed in TFA isn't that kind of VM, right? Bytedance didn't write an operating system in Javascript?

I've been seeing "VM" used in lots of contexts like this recently and it makes me think I must be missing something, but it's the sort of question I don't know how to Google. AIs have not been helpful either, plus I don't trust them.

Virtual Machine Decompiling: https://github.com/LukasOgunfeitimi/TikTok-ReverseEngineerin...

And also VM223, with statements that do stuff to an array "stack": https://github.com/LukasOgunfeitimi/TikTok-ReverseEngineerin...

One obvious giveaway for a VM is laying out memory, or processing some intermediate language. In this case, it could be the latter.

In-browser, you have Chrome V8 running Javascript; that Javascript could be running an interpreted environment where abstractions are not purely business logic, but an execution model separate from domain stuff: auth, video, user, etc.

By that observation, this C snippet is a VM:

  char instruction = 'p'; /* or array */

  if (instruction == 'p') {
    println("document.appendChild(...)");
  }

If the program outputs to a vm.js file, it's kinda-sorta a "VM." I would call it something else, maybe a generator of sorts (for now). Just in my opinion, for me, if I were working on a VM, the threshold of calling it that would be much higher than the above.

On the other hand, if I had to comment in the generated Javascript debugging hints referring to execution stack or stack pointers, it is kind of a VM idea.