vbox guet additions 5.1.20 shared file system not backwards compatible with 5.1.18 via vagrant

[jungle dave]

Hi Team,

I get this error when trying to start a vbox with vagrant:-

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 uid=999,gid=999 rsr /rsr

this was working fine in vbox 5.1.18, but does not work in any version of vagrant (tried 1.9.1 -> 1.9.4). Here's the vagrant file (relevant bits only...)

Vagrant.configure(2) do |config|

  config.vm.define :capture do |node|
    name = 'myvm'
    bootstrap_vm(name, node)
    node.vm.synced_folder Dir.pwd + '/..', "/operations", :create => true

    if Dir.exists? (Dir.pwd + '/../../rsr')
      node.vm.synced_folder Dir.pwd + '/../../rsr', "/rsr", :create => true
    end

and

def bootstrap_linux(name, node)
  node.vm.box = '<url>'
  node.ssh.username = 'vagrant'
  node.ssh.private_key_path = '.vagrant-ssh/vagrant'
  node.ssh.forward_agent = true
  node.vm.hostname = name
  node.vm.provider 'virtualbox' do |vb|
    vb.customize ['modifyvm', :id, '--cpus', 2]
    vb.customize ['modifyvm', :id, '--memory', 4096]
    vb.customize ['modifyvm', :id, '--audio', 'none']
    vb.gui = false
    # Linux workarounds for vbox network problems
    (1..8).each do |i|
      vb.customize ['modifyvm', :id, "--nictype#{i}", 'virtio']
      vb.customize ['modifyvm', :id, "--natdnshostresolver#{i}", 'off']
      vb.customize ['modifyvm', :id, "--natdnsproxy#{i}", 'off']
      vb.customize ['modifyvm', :id, "--nicpromisc#{i}", 'allow-all']
    end
  end
end

please find a way to make the shared folders work with vagrant (again...)

[Frank Mehnert]

Sorry, I don't understand all that vagrant stuff but please keep in mind that the 5.1.20 Guest Additions for Linux have a regression: The mount.vboxsf command is a dangling symlink and therefore mount.vboxsf does not work. This is fixed in the most recent 5.1.x test build. See also #16670.