filp_open

/**
 * filp_open - open file and return file pointer
 *
 * @filename:    path to open
 * @flags:    open flags as per the open(2) second argument
 * @mode:    mode for the new file if O_CREAT is set, else ignored
 *
 * This is the helper to open a file from kernelspace if you really
 * have to.  But in generally you should not do this, so please move
 * along, nothing to see here..
 */
struct file *filp_open(const char *filename, int flags, umode_t mode)
{
    struct filename *name = getname_kernel(filename);
    struct file *file = ERR_CAST(name);
    
    if (!IS_ERR(name)) {
        file = file_open_name(name, flags, mode);
        putname(name);
    }
    return file;
}
EXPORT_SYMBOL(filp_open);

old_fs = get_fs();
set_fs(KERNEL_DS);  //addr_limit  buf的地址与用户空间不同
 file->f_op->write(file, (char *)buf, sizeof(buf), &file->f_pos);
 set_fs(old_fs);