Fedora: can't reach the 9p store mount from systemd services

[picnoir]

Describe the bug

We can access the store 9p mount just fine from the host, however, the systemd services cannot access it.

When the service tries to access the mount, this error message pops out in the journal:

9pnet_virtio: no channels available for device nix-store

I tried these recommandations https://superuser.com/questions/1317119/qemu-share-folder-9pnet-virtio-no-channels-available-for-device . No luck.

I tried:

• Disabling SELinux.
• Set ProtectSystem to false

It feels like a weird systemd/9p interaction to me. I searched the systemd bug tracker, there's no related issue, opened or close.

This issue does not manifest in Ubuntu or Debian. They're using older systemd versions.

We probably should try to find a minimal reproducer and poke upstream with that.