VirtualBox 5.1.20で共有フォルダーが動かなくなる対策 ref: http://qiita.com/poad1010/items/675ffe46e70135fff839

file0.txt

Vagrant was unable to mount VirtualBox shared folders. This is usually
because the filesystem "vboxsf" is not available. This filesystem is
made available via the VirtualBox Guest Additions and kernel module.
Please verify that these guest additions are properly installed in the
guest. This is not a bug in Vagrant and is usually caused by a faulty
Vagrant box. For context, the command attempted was:

mount -t vboxsf -o dmode=755,fmode=644,uid=1000,gid=1000 vagrant /vagrant

The error output from the command was:

mount: wrong fs type, bad option, bad superblock on vagrant,
       missing codepage or helper program, or other error

       In some cases useful info is found in syslog - try
       dmesg | tail or so.

file1.txt

sudo rm /sbin/mount.vboxsf && sudo ln -s /usr/lib/VBoxGuestAdditions/mount.vboxsf /sbin/mount.vboxsf