There’s a handy new feature planned for WSL in Windows 10 v1903: “Accessing Linux files from Windows”. Previously there was no way to get to your WSL Linux files from Windows, the host Windows OS didn’t know how to interact safely with the LXFS files in the WSL system. Now you’ll be able to mount the Linux files as a network share.
The cool part is how it works: “a 9P protocol file server facilitates file related requests, with Windows acting as the client”. Before they edited the announcement they called this “a Plan 9 file server”, which is accurate if a bit overstated. 9P (aka Styx) is a Plan 9 network filesystem technology.
It makes sense to use a network filesystem to bridge Windows and Linux. I’ve been doing something similar, network-mounting the Linux files via SFTP. You could also imagine running a Samba server inside WSL that the host Windows system accesses. Either way the Linux process gets to mediate the file access, at the expense of a network file system.
Plan 9 is the wacky part. There’s been a little discussion about why, hopefully Microsoft will write a blog post explaining the tech. My guess is they chose 9P because it is very simple to implement. IIRC part of the Plan 9 design is that many applications implement a filesystem as their API, so they built a simple embeddable filesystem any program could control. Wikipedia’s summary of the protocol certainly looks straightforward, roughly what a FUSE filesystem implementation has to implement. Still, a tiny bit surprised they didn’t go with their own SMB, or NFS, or something new and proprietary and super simple.
There’s obscure precedent for using Plan 9 this way, see this discussion. QEMU’s VirtFS uses 9P to share files between guest and host operating systems. (Which led to at least one security issue in a QEMU user.) And there’s some 9P code knocking around: the old u9fs code presumably hasn’t entirely rotted, there’s v9fs (looks abandoned?), someone took a crack at 9P in GoLang, and there’s probably useful 9P code in this Plan 9 port. Also Wikipedia mentions Styx on a Brick which looks like a way to control a Lego Mindstorms robot via something that looks like a filesystem.