(From Alan Stern.)

The File-backed Storage Gadget provides support for the USB Mass Storage
protocol.  It causes a gadget to appear to the host as a USB disk drive,
using a regular file or a block device to provide the necessary storage
somewhat like the "loop" device.  The driver supports up to 8 Logical
Units, appearing to the host as multiple disks, each configurable
independently for read-write or read-only access.  The settings for the
Logical Units are available through sysfs attribute files.

The driver has a testing version, selectable by a kernel configuration
option.  The test version includes many additional module parameters,
allowing the USB protocol, subclass, vendor, product, and release values
to be set.  It also has an option to emulate removable media, and the
sysfs interface allows the backing files and the read-only settings to be
switched on-the-fly.  In both versions several logging and debugging
levels are available.  They provide a nice way of seeing exactly what
commands a USB host is sending to the gadget.