copy_to_user

[Hemanth Kumar]

--- On Thu, 23/12/10, Dexter Haslem <dexter.haslem at gmail.com> wrote:

> From: Dexter Haslem <dexter.haslem at gmail.com>
> Subject: Re: copy_to_user
> To: "Hemanth Kumar" <hemwire at yahoo.co.in>
> Cc: Kernelnewbies at kernelnewbies.org
> Date: Thursday, 23 December, 2010, 6:48 AM
> On 12/22/2010 5:59 PM, Hemanth Kumar
> wrote:
> > Hi All,
> >
> >             I
> have small problem with copy_to_user in read function,below
> is my code,when I try to read from userspace I get
> segmentation fault,
> > Can any please point me where I went wrong,
> >
> >
> > #include<linux/kernel.h>
> > #include<linux/module.h>
> > #include<linux/init.h>
> > #include<linux/types.h>
> > #include<linux/proc_fs.h>
> > #include<linux/fs.h>
> > #include<linux/kdev_t.h>
> > #include<linux/jiffies.h>
> > #include<linux/cdev.h>
> > #include<asm/uaccess.h>
> > #include<linux/mutex.h>
> >
> > struct mutex timer;
> > static struct cdev my_cdev;
> > dev_t devn;
> > int maj = 300;
> > int min = 0;
> > int count = 1;
> > char modname[] = "mytimer";
> > short x[10] = {1,2,3,4,5,6,7,8,9,10};
> >
> >
> >
> >
> > ssize_t my_read(struct file *file,char *buf,size_t
> count,loff_t *pos){
> >       unsigned long res;
> >       void *k = (void
> *)&x;
> >            void *l =
> (void *)&x+1;
> >            void *j =
> (void *)&x+2;
> >
> >               
>        mutex_lock(&timer);
> >               
>            
>    res =   
> copy_to_user(buf,k,sizeof(short));
> >               
>            
>    res =   
> copy_to_user(buf,l,sizeof(short));
> >               
>            
>    res =   
> copy_to_user(buf,j,sizeof(short));
> >
> >               
>          /*    res
> =  copy_to_user(buf,&x+4,sizeof(short));
> >               
>            
>    res =   
> copy_to_user(buf,&x+5,sizeof(short));
> >               
>            
>    res =   
> copy_to_user(buf,&x+6,sizeof(short));
> >               
>            
>    res =   
> copy_to_user(buf,&x+7,sizeof(short));
> >               
>            
>    res =   
> copy_to_user(buf,&x+8,sizeof(short));
> >               
>            
>    res =   
> copy_to_user(buf,&x+9,sizeof(short));
> >               
>           */
> >               
>    mutex_unlock(&timer);
> >
> >     return 20;
> >
> > }
> >
> >
> > static struct file_operations my_fops = {
> >               
>    .owner = THIS_MODULE,
> >               
>    .read = my_read,
> >
> > };
> >
> >
> > static int __init my_init(void){
> >          int ret;
> >     devn = MKDEV(maj,min);
> >
> >       ret =
> register_chrdev_region(devn,count,modname);
> >
> >       
> cdev_init(&my_cdev,&my_fops);
> >       
> cdev_add(&my_cdev,devn,count);
> >
> >        printk("<1> 
> Register timer maj = %d\n",maj);
> >
> >
> >
> >
> >   return 0;
> > }
> >
> >
> >
> > static void __exit my_exit(void){
> >
> >        cdev_del(&my_cdev);
> >     
>    unregister_chrdev_region(devn,count);
> >         
> printk("<1>  Bye Bye \n");
> >
> > }
> >
> >
> > module_init(my_init);
> > module_exit(my_exit);
> > MODULE_LICENSE("Dual BSD/GPL");
> >
> >
> >
> >
> > my userspace App:
> >
> > #include<stdio.h>
> > #include<fcntl.h>
> > #include<stdlib.h>
> > #include<unistd.h>
> >
> > int main()
> > {
> >           int
> nbytes ;
> >           char
> n[20];
> >           short a =
> *((short *)&n[0]);
> >           short b =
> *((short *)&n[2]);
> >           short c =
> *((short *)&n[4]);
> >
> >          int 
>    fd = open( "/dev/mytimer", O_RDONLY );
> >          if ( fd<  0
> ) { perror( "/dev/mytimer" ); exit(1); }
> >
> > while ( 1 )
> >               
>   {
> >
> >               
>    nbytes = read( fd, n, 40 );
> >               
>   if ( nbytes<  0 ) break;
> >
> >               
>   printf( "\r a = %d \n ", a);
> >               
>   printf("\r b = %d \n",b);
> >               
>   printf("\r c = %d \n",c);
> >
> >               
>   sleep(1);
> >               
>   fflush( stdout );
> >               
>   }
> > return 0;
> > }
> >
> >
> >
> >
> > Best regards,
> >
> >
> >
> >
> >
> > _______________________________________________
> > Kernelnewbies mailing list
> > Kernelnewbies at kernelnewbies.org
> > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
> Hi,
> 
> At first glance, you have char n[20], but read 40 bytes in
> the read 
> call. char is only 1 byte on x86 I believe so that might be
> your problem.
 
  Hi Dexter, 

           I have commented remaining copy_to_user read function , it should be 6bytes,


> -- 
> -Dexter Haslem
>