Problem :
Up until a few weeks ago, I was running Fedora 27 in VBox on a W7 laptop, and Shared Folders worked fine.
That machine died recently. I have a new W10 laptop, and have installed VBox 5.2.12 on my new W10 machine, and created a Fedora 28 VM in it.
I have installed the Guest Additions (I don’t know a way to check this directly, but the clipboard works both ways round).
When I create a permanent Shared Folder, the mount point (/media/sf_[sharename]) gets created on the Linux guest, but it is not mounted, and I can’t find a device to mount.
Solution :
Fedora 28 comes with VirtualBox guest additions already installed. The installed package interferes with the building of the guest additions from the ISO.
I created a new Fedora 28 VM to try it out. While dnf list shows virtualbox-guest-additions installed and a running virtualbox guest additions service running, it wasn’t working at all for me. I couldn’t find any information on the internet on where to look for log files or how to figure out what might be wrong.
As I said at the beginning, these installed guest additions interfere with compiling one’s own version of the guest additions from the ISO (instructions at https://www.if-not-true-then-false.com/2010/install-virtualbox-guest-additions-on-fedora-centos-red-hat-rhel/ for example). The build process fails.
I found that after first doing a dnf erase virtualbox-guest-additions, then rebooting, the guest additions compile and install correctly, and everything starts working perfectly.
As already mentioned in this answer, VirtualBox guest additions are already installed on Fedora. The problem is that this does not include the vboxsf driver kernel module that is required for mounting the shared folder.
Here you can find the solution: You need to add the rpmfusion-free repository and then install the package akmod-VirtualBox, which installs the missing drivers.
Reboot and enjoy!