커널 패닉 메시지가 표시되지 않는 이유는 무엇입니까?

copy_to_user및 기능을 사용하지 않기 때문에 copy_from _user다음 커널 모듈은 커널에 긴급 메시지를 발행해야 합니다. 그러나 나는 올바른 결과를 얻습니다. 아키텍처는 ubuntu 14.04 LTE의 x86입니다.

$ sudo mknod /dev/mem_char c 61 0
$ sudo chmod 777 /dev/char
$ echo 123asa >/dev/mem_char

dmesg 출력:

[  494.821648] mem_driver: module license 'unspecified' taints kernel.
[  494.821653] Disabling lock debugging due to kernel taint
[  494.821678] mem_driver: module verification failed: signature and/or  required key missing - tainting kernel
[  573.581104] data from user : 123asa
[  573.581104]  tushar!!
[  573.581104] eload
[  573.581104] t
[  573.581104] /bin/grub-script-check
[  573.581104] bkdf2
[  573.581104] e

모듈 코드:

#include<linux/module.h>
#include<linux/fs.h>
#include<linux/init.h>

ssize_t mem_read (struct file *, char __user *,size_t, loff_t *);
ssize_t mem_write (struct file *, const char __user *, size_t, loff_t *);
int mem_open (struct inode *, struct file *);
int mem_release (struct inode *, struct file *);

int major_no=61;    //if major no=0 then dynamic mean kernel alocate it 

struct file_operations fops={
    .open=mem_open,
    .read=mem_read,
    .write=mem_write,
    .release=mem_release,
};

ssize_t mem_read (struct file *fp, char __user *buff,size_t len,  loff_t *off)
{
    printk("buffer :%s \n",buff);
    return 0;
}
ssize_t mem_write (struct file *fp, const char __user *buff, size_t len, loff_t *off)
{
    //  char kbuff;
    //  printk("Function name : %s \n",__func__);
    printk("data from user : %s\n",buff);
    //    kmalloc();      
    return len;
}
int mem_open (struct inode *ip, struct file *fp)
{
    // printk("Function name : %s \n",__func__);
    return 0;
}
int mem_release (struct inode *ip, struct file *fp)
{
    // printk("Function name : %s \n",__func__);
    return 0;
}
static int mem_init(void)
{
    int ret;
    //        printk("mem_init\n");
    ret =register_chrdev(major_no,"mem_driver",&fops);
    if(ret<0)
    {
        //       printk("registetation fails with major no : %d",major_no);
        return ret; 
    }
    // printk("mem_driver register with major no %d\n",major_no);      
    return 0;
}
static void mem_exit(void)
{
    //  printk("exit\n");
    unregister_chrdev(major_no,"mem_driver");   
}
module_init(mem_init);
module_exit(mem_exit);