feat(hypercall): introduce chdir and uhyve mountpoint restrictions

[n0toose]

Under heavy construction.

Related to: hermit-os/uhyve#820
Fixes: #1483
Succeeds: #1477

[jounathaen]

IMHO, we should merge #1393 first and put the definitions of the hypercalls directly in uhyve-interface.

[mkroening]

Why would we need a chdir hypercall? The current working directory is a property of the currently running process, not of the operating system.

I think Hermit should manage the current working directory and only expose canonicalized paths to Uhyve (or virtiofsd via QEMU).

[jounathaen]

Well, it is a property, but not an immutable one. POSIX defines calls to change it during a processes runtime. Also, Rust std has support for it.
Sure, it is a rare use-case, but whilst @n0toose is working on file-systems anyway, why not increase the compatibility.
The only question is, if this should be a hypercall, or something that Hermit keeps track of in the VM and resolves it before the hypercall. I guess I'm leaning towards the second approach.

[n0toose]

Would a get_mount_point() like in the FUSE driver be a better / less controversial approach for now instead of a new chdir hypercall? It would serve the same purpose.

kernel/src/fs/fuse.rs

Line 1156 in fcb871c

let mount_point = driver.lock().get_mount_point();