humfsify(1) convert a directory to the format needed by the UML


humfsify [user] [group] [size]


This manual page documents briefly the humfsify command.

This manual page was written for the Debian GNU/Linux distribution because the original program does not have a manual page. Instead, it has documentation in HTML format; see below.

humfsify is a Perl script necessary to convert a directory to a format expected by the UML humfs file system.


UMLFS was born with the idea to substitute the Hostfs implementation with a proper one for the UML purpose: when you manage files with Hostfs within UML you need to work with two different permission layers (the Host one and the UML one), which have different ideas of ownerships.

This becomes evident when you need to create a file as a non-root user on UML: you first need to interact with the UML file system implementation, and then with the host side.

The result of a file creation on a mounted hostfs file system is not what you expected: you can see that the file permissions refer to the Host side user rather than the UML creator.

The Host side user is to be intended as the UML instance launcher, meanwhile the UML side user is the one you used to log in the UML instance.

You can encounter a more-critical problem when creating a device node, operation that usually requires root privileges: you used a common user to launch the UML and, since the operation is done on the Host, it fails, even if you logged in as root.

Thus you need a set of tools which requires to bypass the Hostfs permission checks on the Host side: this is done by separating the file permissions and the ownership from the host's files. This is the concept behind the HumFs and its humfsify implementation.


This is the user that needs to 'convert' a directory to the UML file system to use UML. This is the host user who will be using this filesystem from within UML. It may be specified as either a user name or a numeric user id.
This is the group which your UML user belongs to. This may be either a group name or a numeric group id
This is the size of the file system as seen within the UML instance. It must be expressed in Gigabytes ("G"), Megabytes ("M"), or KiloBytes ("K").


Create a directory on the host and mount it with humfsify

host% mkdir your-humfs-dir

host% cd humfs-dir

Within this directory create a new one where you would like to have a UML-like hierarchy, i.e. you can loop-mount an UML rootfs

host% mkdir dir-to-be-humsified

host# mount -o loop rootfs /mnt

host% cp -a /mnt dir-to-be-humsified/data

host# humfsify user group 512M

Then verify it on UML and mount the humfsified directory:

UML# mount none /your-uml-host -t humfs -o

where '/your-uml-mount-point' is the mount point on UML for the humfsified file system, and .../dir-to-be-humfsified is the humfsified directory in the example above. The '-t' mount option specifies that the file system is to be mounted as 'humfs'.


humfsify was written by Jeff Dike.

This manual page was written by Stefano Melchior [email protected] for the Debian GNU/Linux system, based on material in the Official User Mode Linux Web Site.