Support VirtualBox shared folders using builtin kernel module

Check error output if initial mount command fails to determine if
the builtin module should be used for mounting instead.

plugins/guests/linux/cap/mount_virtualbox_shared_folder.rb

@@ -11,6 +11,9 @@ module VagrantPlugins
 
           @@logger.debug("Mounting #{name} (#{options[:hostpath]} to #{guestpath})")
 
+          builtin_mount_type = "-cit vboxsf"
+          addon_mount_type = "-t vboxsf"
+
           mount_options = options.fetch(:mount_options, [])
           detected_ids = detect_owner_group_ids(machine, guest_path, mount_options, options)
           mount_uid = detected_ids[:uid]
@@ -19,21 +22,33 @@ module VagrantPlugins
           mount_options << "uid=#{mount_uid}"
           mount_options << "gid=#{mount_gid}"
           mount_options = mount_options.join(',')
-          mount_command = "mount -t vboxsf -o #{mount_options} #{name} #{guest_path}"
+          mount_command = "mount #{addon_mount_type} -o #{mount_options} #{name} #{guest_path}"
 
           # Create the guest path if it doesn't exist
           machine.communicate.sudo("mkdir -p #{guest_path}")
 
-          # Attempt to mount the folder. We retry here a few times because
-          # it can fail early on.
           stderr = ""
-          retryable(on: Vagrant::Errors::VirtualBoxMountFailed, tries: 3, sleep: 5) do
-            machine.communicate.sudo(mount_command,
-              error_class: Vagrant::Errors::VirtualBoxMountFailed,
-              error_key: :virtualbox_mount_failed,
-              command: mount_command,
-              output: stderr,
-            ) { |type, data| stderr = data if type == :stderr }
+          result = machine.communicate.sudo(mount_command, error_check: false) do |type, data|
+            stderr << data if type == :stderr
+          end
+
+          if result != 0
+            if stderr.include?("-cit")
+              @@logger.info("Detected builtin vboxsf module, modifying mount command")
+              mount_command.sub!(addon_mount_type, builtin_mount_type)
+            end
+
+            # Attempt to mount the folder. We retry here a few times because
+            # it can fail early on.
+            stderr = ""
+            retryable(on: Vagrant::Errors::VirtualBoxMountFailed, tries: 3, sleep: 5) do
+              machine.communicate.sudo(mount_command,
+                error_class: Vagrant::Errors::VirtualBoxMountFailed,
+                error_key: :virtualbox_mount_failed,
+                command: mount_command,
+                output: stderr,
+              ) { |type, data| stderr = data if type == :stderr }
+            end
           end
 
           # Chown the directory to the proper user. We skip this if the

test/unit/plugins/guests/linux/cap/mount_virtual_box_shared_folder_test.rb

@@ -192,6 +192,20 @@ describe "VagrantPlugins::GuestLinux::Cap::MountVirtualBoxSharedFolder" do
         end
       end
     end
+
+    context "with guest builtin vboxsf module" do
+      let(:vbox_stderr){ <<-EOF
+mount.vboxsf cannot be used with mainline vboxsf; instead use:
+
+    mount -cit vboxsf NAME MOUNTPOINT
+EOF
+      }
+      it "should perform guest mount using builtin module" do
+        expect(comm).to receive(:sudo).with(/mount -t vboxsf/, any_args).and_yield(:stderr, vbox_stderr).and_return(1)
+        expect(comm).to receive(:sudo).with(/mount -cit/, any_args)
+        cap.mount_virtualbox_shared_folder(machine, mount_name, mount_guest_path, folder_options)
+      end
+    end
   end
 
   describe ".unmount_virtualbox_shared_folder" do