Beyond the userland-only limitation, syscall emulation is probably incomplete at best[0], and likely doesn't pass the Linux test suite for the subset of calls that are supported.

To get a flavor of the work required when emulating the kernel, you can read up on the work it took Windows developers to emulate Linux syscalls on top of the NT kernel for WSL 1 here: https://learn.microsoft.com/en-us/archive/blogs/wsl/wsl-syst...

WSL devs eventually abandoned that effort in favor of running the actual kernel in a VM -- that's how WSL 2 is implemented.

[0] I'd be surprised if sys_fork actually forks the process using a web worker, for example.

This definitely supports fork; you can see that bash works in it.

The obvious one is that it is userland-only, you don't get to play around with custom kernels being emulated on the hardware layer.

It's a commercial project and not open-source https://news.ycombinator.com/item?id=40944412