I've been running the generic xfstests against it, with some
modifications to do things like mount/unmount the lower and upper fs
at once. Most of the failures I see there are related to missing
opcodes, like FUSE_SETLK, FUSE_GETLK, and FUSE_IOCTL. The main failure
I have been seeing is generic/126, which is happening due to some
additional checks we're doing in fuse_open_backing. I figured at some
point we'd add some tests into libfuse, and that sounds like a good
place to start.