I want to create a mount helper to be able to mount a filesystem not currently supported by mount.
I'm not sure about what the helper should put in the mount folder, what happen after the mount when I try to navigate the folder (with ls for example), will it call the helper again ?
I'm having trouble finding documentation on the subject.
What you're calling a “mount helper” is, given what you want to do with it, a drier for a filesystem type. Calling a filesystem driver a “mount helper” is misleading because it makes it look like it's a piece of software that's only used while mounting. The filesystem driver is a piece of software that's used as long as the filesystem is mounted.
There's no such thing as a “mount folder”. Each time you access a filesystem, the request is dispatched to the corresponding driver which performs the request in the way that depends on the filesystem type. For example, for filesystem types that are stored on a disk, the filesystem driver access sectors of the disk. For filesystem types that are accessed over the network, the filesystem driver communicates over the network, and so on. See How does Linux, the kernel, mount filesystems? What actually does this? for a slightly more detailed explanation.
The kernel can handle a number of filesystems. If you're looking for support for a more exotic filesystem, check if there exists a FUSE driver for it. FUSE allows filesystem drivers to be implemented by an unprivileged program rather than by kernel code. The FUSE project wiki and the old page on Sourceforge list some existing FUSE filesystems. A web search may reveal more.
