VirtualBox shared folder Linux notes

I sank my Friday evening fighting with VirtualBox 4.3.10 r93012 with a Mac OS X 10.9.2 host and Ubuntu Linux guest kernel 3.13. Whenever I try to mount my shared folder xyzzy, I'd get cryptic error messages like these:

root@ubuntu:~# mount -t vboxsf xyzzy /media/xyzzy
mount: wrong fs type, bad option, bad superblock on xyzzy,
       missing codepage or helper program, or other error
       (for several filesystems (e.g. nfs, cifs) you might
       need a /sbin/mount. helper program)
       In some cases useful info is found in syslog - try
       dmesg | tail  or so
root@ubuntu:~# dmesg | tail
...
[ 1292.150065] sf_read_super_aux err=-22
root@ubuntu:~# mount -t vboxsf -o gid=1000,uid=1000 xyzzy /media/xyzzy
mount: Protocol error
root@ubuntu:~# dmesg | tail
...
[ 1348.460432] sf_read_super_aux err=-71

Making the shared folder automount and reboot works, but I want to mount it to a specific path. Reinstalling GuestAddition doesn't help. It turns out that it's an old problem resurfacing. The root cause is that /sbin/mount.vboxsf is an invalid symlink. The fix is this:

root@ubuntu:~# updatedb  # first make sure locate is up to date.
root@ubuntu:~# locate mount.vboxsf  # find the real location.
/opt/VBoxGuestAdditions-4.3.10/lib/VBoxGuestAdditions/mount.vboxsf
/sbin/mount.vboxsf  # this is the bad symlink.
root@ubuntu:~# ln -sf /opt/VBoxGuestAdditions-4.3.10/lib/VBoxGuestAdditions/mount.vboxsf /sbin/mount.vboxsf

Now I can mount.

If I want to add the mount point to /etc/fstab so that it gets mounted at boot time, the problem is that vboxsf is not loaded automatically early in the boot, so again it fails to mount. The solution is to add vboxsf to /etc/modules.

root@ubuntu:~# echo vboxsf >> /etc/modules

Now when I reboot, the shared folder is mounted correctly at boot time.