I wonder if we can just require URING registered FDs (using IORING_REGISTER_FILES). I think io_uring does checks on the file permissions when the FD is registered.It is possible, for example set FOPEN_PASSTHROUGH_FD to interpret backing_id as backing_fd, but note that in the current implementation of passthrough_hp, not every open does fuse_passthrough_open(). The non-first open of an inode uses a backing_id stashed in inode, from the first open so we'd need different server logic depending on the commands channel, which is not nice.
