Kernel Module Programming

Question

I'm trying to read and write to a proc file through kernel module
But when I run this command :

echo "hello" >> /proc/hello && cat /proc/hello

It doesn't print anything and when i open the file through text editor. I found mysterious symbols like this

 ^@^@^@^@^@^@^@^@^@^@

Any help will be appreciated thanks in advance

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/proc_fs.h>
#include<linux/sched.h>
#include <asm/uaccess.h>
#include <linux/slab.h>

int len,temp;
char *msg;

int read_proc(struct file *filp,char *buf,size_t count,loff_t *offp ){
    if(count>temp){count=temp;}
    temp=temp-count;
    copy_to_user(buf,msg, count);
    if(count==0)temp=len;
    return count;
}

int write_proc(struct file *filp,const char *buf,size_t count,loff_t *offp){
    copy_from_user(msg,buf,count);
    len=count;
    temp=len;
    return count;
}

struct file_operations proc_fops = {
    read: read_proc,
    write: write_proc
};

void create_new_proc_entry(void){
    proc_create("hello",0,NULL,&proc_fops);
    msg=kmalloc(GFP_KERNEL,10*sizeof(char));
}

int proc_init (void){
    create_new_proc_entry();
    return 0;
}

void proc_cleanup(void){
    remove_proc_entry("hello",NULL);
}

MODULE_LICENSE("GPL"); 
module_init(proc_init);
module_exit(proc_cleanup);

Just a remark: you copy something from user in msg, but msg has a size of 10 characters => in write_proc be sure to not allowed count more than 10, otherwise you will have a buffer overflow! — Garf365
– Garf365, Commented Apr 13, 2016 at 14:57

LPs · Accepted Answer · 2016-04-13 15:12:41Z

4

Apart from other problems of your kernel module (like boundaries check)

This

msg=kmalloc(GFP_KERNEL,10*sizeof(char));

have to be

 msg=kmalloc(10*sizeof(char), GFP_KERNEL);

With your call to kmalloc you are trying, probably, to allocate too many or not enough bytes and it refuses your kmalloc request.

You should always check the kmalloc return value to be consistent: != NULL

edited Apr 13, 2016 at 15:12

answered Apr 13, 2016 at 14:58

LPs

16.2k8 gold badges34 silver badges64 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Master AN HA Over a year ago

ya you are completely right , thanks a lot , and i'm sorry for this bad implementation , i'm just a beginner in kernel programming and trying to understand how things work

wack · Accepted Answer · 2016-04-21 08:16:52Z

0

You can find kmalloc in slab.h:

static __always_inline void *kmalloc(size_t size, gfp_t flags)
{
    if (__builtin_constant_p(size)) {
        if (size > KMALLOC_MAX_CACHE_SIZE)
            return kmalloc_large(size, flags);
    #ifndef CONFIG_SLOB
        if (!(flags & GFP_DMA)) {
            int index = kmalloc_index(size);

            if (!index)
                return ZERO_SIZE_PTR;

            return kmem_cache_alloc_trace(kmalloc_caches[index],
                    flags, size);
        }
    #endif
    }
    return __kmalloc(size, flags);
}

answered Apr 21, 2016 at 8:16

wack

811 silver badge4 bronze badges

Collectives™ on Stack Overflow

Kernel Module Programming

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related