Create $XDG_RUNTIME_DIR in /run/user/{uid}

[dramforever]

Using tempfile::Builder::tempdir() defaults to creating this directory in /tmp, which is shared with the host and often somewhat persistent. This causes one extra directory to be created in the host /tmp every time muvm runs.

Since we mount /run as a tmpfs now in the guest now, just create $XDG_RUNTIME_DIR in /run/user/{uid}, a common default. Specifically:

• Create /run/user as 0o755 (rwxr-xr-x) owned by root:root
• Create /run/user/{uid} as 0o700 (rwx------) owned by uid:gid

[dramforever]

I couldn't find a justification for using /tmp. This seems to have been there since the C krun's initial commit and might have just been missed, or maybe it was implemented before /run was properly handled.

[teohhanhui]

I think it should be changed, yeah, but some other existing assumptions might need to be fixed, e.g.

muvm/crates/muvm/src/guest/mount.rs

Line 35 in 5b35e56

// Must succeed since /run/ was just mounted and is now an empty tmpfs.

The other thing I could think of is, maybe this (XDG_RUNTIME_DIR in /tmp) was done this way to ease debugging? /cc @slp

[dramforever]

AIUI the part where I changed is in the user initialization part, which happens after FEX init, so guest/mount.rs shouldn't be affected.

[teohhanhui]

Hmm... Actually, if we're changing this, why are we not just using the typical /run/user/{uid}?

---

Anyway, relevant part from the spec:

$XDG_RUNTIME_DIR defines the base directory relative to which user-specific non-essential runtime files and other file objects (such as sockets, named pipes, ...) should be stored. The directory MUST be owned by the user, and they MUST be the only one having read and write access to it. Its Unix access mode MUST be 0700.

[dramforever]

Oh, good idea. I'll change it to /run/user/{uid}

[dramforever]

The directory and permissions now looks like this

$ namei -l $XDG_RUNTIME_DIR 
f: /run/user/1000
drwxr-xr-x root root  /
drwxrwxrwt root root  run
drwxr-xr-x root root  user
drwx------ dram users 1000

[slp]

The other thing I could think of is, maybe this (XDG_RUNTIME_DIR in /tmp) was done this way to ease debugging? /cc @slp

IIRC it's in /tmp to avoid polluting the host's /run since originally we weren't mounting a tmpfs on it. I like the idea of switching to it now that we do the tmpfs mount.

[slp]

Please fix the clippy issue. Otherwise LGTM, thanks!

[dramforever]

Clippy should be happy now