Re: copy_to_user

[Hemanth Kumar <hemwire@xxxxxxxxxxx>]

--- On Thu, 23/12/10, mukti jain <muktijn@xxxxxxxxx> wrote:

From: mukti jain <muktijn@xxxxxxxxx>
Subject: Re: copy_to_user
To: "Srinivas G." <srinivasg@xxxxxxxxxxxxxxxxxxxxx>
Cc: "Dexter Haslem" <dexter.haslem@xxxxxxxxx>, "Hemanth Kumar" <hemwire@xxxxxxxxxxx>, Kernelnewbies@xxxxxxxxxxxxxxxxx
Date: Thursday, 23 December, 2010, 11:07 AM

On Thu, Dec 23, 2010 at 10:05 AM, Srinivas G. <srinivasg@xxxxxxxxxxxxxxxxxxxxx> wrote:

> 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,

I guess, you need to also implement open method in your driver. Because,
you are opening the device in your application. Could you try this?

Regards,
Srinivas G

> >
> >
> > #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,
> >
> >

The mutex initialization is missing.
Adding  mutex_init(&timer); in the driver init will make it work.

Thanks,
Mukti

Hi All,

    Can anybody please share some idea ,why I am getting segmentation fault & kernel oops,

Regards,

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