* This function copies "len" bytes of data from a userspace pointer to a
* buffer backend, at the current context offset. This is more or less a buffer
* backend-specific memcpy() operation. Calls the slow path
* This function copies "len" bytes of data from a userspace pointer to a
* buffer backend, at the current context offset. This is more or less a buffer
* backend-specific memcpy() operation. Calls the slow path