Host file access from UML

If you want to access files on the host machine from inside UML, you can treat it as a separate machine and either nfs mount directories from the host or copy files into the virtual machine with scp or rcp. However, since UML is running on the the host, it can access those files just like any other process and make them available inside the virtual machine without needing to use the network.

This is now possible with the hostfs virtual filesystem. With it, you can mount a host directory into the UML filesystem and access the files contained in it just as you would on the host.

Note that hostfs is currently not available on 2.5. The reason is that there was an fs.h rework early in 2.5 which required filesystem changes, and I haven't got around to updating hostfs to those changes.

Using hostfs

To begin with, make sure that hostfs is available inside the virtual machine with

UML# cat /proc/filesystems

hostfs should be listed. If it's not, either rebuild the kernel with hostfs configured into it or make sure that hostfs is built as a module and available inside the virtual machine, and insmod it.

Now all you need to do is run mount:

UML# mount none /mnt/host -t hostfs

will mount the host's / on the virtual machine's /mnt/host.

If you don't want to mount the host root directory, then you can specify a subdirectory to mount with the -o switch to mount:

UML# mount none /mnt/home -t hostfs -o /home

will mount the hosts's /home on the virtual machine's /mnt/home.