dear BBB experts: I would like to create a high-level filesystem device on my BBB that my (possibly malicious) linux PC can communicate with over USB in EHCI (USB 2.0 high-speed). I will want to switch off everything else, incl ethernet-over-usb, again because I will guess that my PC is infected.
from the PC perspective, I want the BBB to operate at roughly at the level of a fuse filesystem, albeit with its own processor that can enforce separation.
most importantly, I would like the BBB to hook into the “open file” request call. for example, I want my BBB to log every file open request to its own /tmp/log/file-logged, return an error if I don’t like the filename, mangle the filename (e.g., shorten it of auto-expand it), or disallow opening a file for write when a pin is bridged or when the file resides in the /ro/ part of the file system or the filename contains the string “ro”.
on the PC, I want to do
PC$ mount -t speak2mybbb /dev/usb1 /mnt/usb1 ## say my BBB sits on /dev/usb1
PC$ echo “hi” > /mnt/usb1/rw/file2 ## create a file
PC$ ls /mnt/usb1/rw/ ## note: my BBB has mangled the filename
PC$ cat /mnt/usb1/rw/file2-remangleme ## note: my BBB can remangle and figure this out
PC$ echo “hi” > /mnt/usb1/ro/file2 ## note: my BBB knows that /ro/ is read-only and does not allow writing here.
ERROR: no such file or directory
PC$ umount /dev/usb1
at first, I thought I should hook into the USB mass storage driver, because it already does EHCI and reading the USB spec, there is a lot of stuff that can go wrong. but the problem, I believe, is that this layer operates at too low a level. I deduct this because it supports many different higher-level file systems, like FAT or ext4. presumably, the USB-mass storage level is primarily “sector-read” and “sector-write,” which would make it very difficult to hook into a file-open.
the USB serial driver works and would allow me to filter requests, and I could write a fuse driver on the PC (not the BBB), but USB serial is slow.
has anyone created an EHCI fuse-like file-system communication example? any pointers by experts would be highly appreciated.