rem: return EINVAL;
add: return FS_EINVAL;
context: read_pos(int fd, fs_off_t _pos, void *data,  size_t nbytes){    off_t  pos = (off_t)_pos;    size_t ret;        if (lseek(fd, pos, SEEK_SET) < 0) {        perror("read lseek");        return EINVAL;    }        ret = read(fd, data, nbytes);    if (ret != nbytes) {        printf("read_pos: wanted %d, got %d\n", nbytes, ret);        return -1;    }    return ret;}