The Mass Storage Gadget acts as a USB Mass Storage disk drive.
As its storage repository it can use a regular file or a block
device (in much the same way as the "loop" device driver),
specified as a module parameter or sysfs option.

This driver is a replacement for now removed File-backed
Storage Gadget (g_file_storage).

Say "y" to link the driver statically, or "m" to build
a dynamically linked module called "g_mass_storage".

source code:

drivers/usb/gadget/legacy/