Doubt in char device driver for linux 2.6

tomkaith13 · 11-21-2006, 09:21 AM

Hi everyone,

I am a newbie to the world of device drivers...
I tried my hand on character dev driver .. it doesn go smooth..

The problem is while tryin to read from the dev:

Say

cat /dev/scull ( i am includin the program later)

i get the followin:

================ error logged in /var/log/messages===============
Nov 21 20:06:46 datacom kernel: Unable to handle kernel NULL pointer dereference at virtual address 00000000
Nov 21 20:06:46 datacom kernel: printing eip:
Nov 21 20:06:46 datacom kernel: c01df982
Nov 21 20:06:46 datacom kernel: *pde = 00000000
Nov 21 20:06:46 datacom kernel: Oops: 0002 [#1]
Nov 21 20:06:46 datacom kernel: last sysfs file: /block/ram0/dev
Nov 21 20:06:46 datacom kernel: Modules linked in: chardev(U) i915 drm ntfs(U) parport_pc lp parport autofs4 sunrpc video button battery ac ipv6 uhci_hcd ehci_hcd hw_random i2c_i801 i2c_core snd_intel8x0 snd_ac97_codec snd_ac97_bus snd_seq_dummy snd_seq_oss snd_seq_midi_event snd_seq snd_seq_device snd_pcm_oss snd_mixer_oss snd_pcm snd_timer snd soundcore snd_page_alloc e100 mii floppy dm_snapshot dm_zero dm_mirror ext3 jbd dm_mod
Nov 21 20:06:46 datacom kernel: CPU: 0
Nov 21 20:06:46 datacom kernel: EIP: 0060:[<c01df982>] Tainted: P VLI
Nov 21 20:06:46 datacom kernel: EFLAGS: 00010213 (2.6.15-1.1831_FC4)
Nov 21 20:06:46 datacom kernel: EIP is at vsnprintf+0x54/0x62d
Nov 21 20:06:46 datacom kernel: eax: 00000049 ebx: 00000000 ecx: e08471f8 edx: 7fffffff
Nov 21 20:06:46 datacom kernel: esi: 00000000 edi: ffffffff ebp: ddd9f124 esp: d49cae94
Nov 21 20:06:46 datacom sshd(pam_unix)[2200]: session closed for user bibin
Nov 21 20:06:46 datacom kernel: ds: 007b es: 007b ss: 0068
Nov 21 20:06:46 datacom su(pam_unix)[2237]: session closed for user root
Nov 21 20:06:46 datacom kernel: Process cat (pid: 2543, threadinfo=d49ca000 task=d76c1ab0)
Nov 21 20:06:46 datacom kernel: Stack: badc0ded df08c000 df0c1eb0 00008001 00000000 d7267580 00000000 00000000
Nov 21 20:06:46 datacom kernel: 00000000 c01dcc97 00000000 c016350d da43bd00 00000000 e08471f8 00000000
Nov 21 20:06:46 datacom kernel: e0847d80 00000000 ddd9f124 c01e0006 d49caef4 d49caef4 e0847054 00000000
Nov 21 20:06:47 datacom kernel: Call Trace:
Nov 21 20:06:47 datacom kernel: [<c01dcc97>] kobject_get+0xf/0x13 [<c016350d>] cdev_get+0xe/0x4c
Nov 21 20:06:47 datacom kernel: [<c01e0006>] sprintf+0x20/0x24 [<e0847054>] device_open+0x54/0x79 [chardev]
Nov 21 20:06:47 datacom kernel: [<c01635f8>] chrdev_open+0x83/0x146 [<c0163575>] chrdev_open+0x0/0x146
Nov 21 20:06:47 datacom kernel: [<c015a3fe>] __dentry_open+0xa8/0x1d2 [<c015a5ff>] nameidata_to_filp+0x31/0x3a
Nov 21 20:06:47 datacom kernel: [<c015a557>] filp_open+0x2f/0x35 [<c013c3a4>] audit_syscall_entry+0x10b/0x15e
Nov 21 20:06:47 datacom kernel: [<c015a710>] get_unused_fd+0xac/0xce [<c013c7f4>] audit_getname+0x6c/0xd1
Nov 21 20:06:47 datacom kernel: [<c015a7f1>] do_sys_open+0x31/0xad [<c0102e75>] syscall_call+0x7/0xb

==============================================================

Here is the program:
Kindly request y'all to go thro and help me out:

==================Driver code=================================
#include <linux/module.h>
#include <linux/init.h>
#include <asm/current.h>
#include <asm/uaccess.h>
#include <linux/sched.h>
#include <linux/fs.h>
#include <linux/kernel.h>
#include <linux/kobject.h>
#include <linux/kobj_map.h>
#include <linux/cdev.h>
#ifdef CONFIG_KMOD
#include <linux/kmod.h>
#endif
#include <linux/cdev.h>

#define DEVICE_NAME "scull"
#define SUCCESS 0
#define FAIL -1
#define SIZE 100

static int Major, Minor;
static int device_opened = 0;

static char MSGBUF[SIZE];
static char *msg_ptr;
static char *msg;

static int device_open (struct inode *in, struct file *filp);
static int device_release (struct inode *in, struct file *filp);
static int device_read(struct file *filp,char *msg_ptr,int count,int *offp);
//static int device_write(struct file *filp,char *msg_ptr,int count,int *offp);

static int
device_open (struct inode *in, struct file *filp)
{
if (device_opened)
{
printk(KERN_ALERT "Driver is in use");
return -EBUSY;
}
device_opened++;

if(!try_module_get(THIS_MODULE))
return -EBUSY;

sprintf(msg,"Initial Driver Content !!");

printk("<1> %s",msg);

msg_ptr = msg;

return SUCCESS;
}

static int
device_release (struct inode *in, struct file *filp)
{

device_opened--;
module_put(THIS_MODULE);
return SUCCESS;
}

static struct file_operations scull_fops = {
.open = device_open,
.release = device_release,
.owner = THIS_MODULE,
.read = device_read,
// .write = device_write,
};

struct cdev scull_cdev ;
int dev_num;

static int scull_init (void)
{
/* dev_t dev=MKDEV(63,0); */
dev_t dev;
int retval;

retval=alloc_chrdev_region(&dev, 0, 1, DEVICE_NAME);
if(!retval)
{
Major=MAJOR(dev);
Minor=MINOR(dev);
}
else {
return FAIL;
}

printk("MAJOR(dev)=%d\n", MAJOR(dev));

printk("<1> Before cdev_init\n");

printk("<1> after cdev_init:\n");

cdev_init(&scull_cdev,&scull_fops);

// dev_num=MKDEV(Major,Minor);

retval=cdev_add(&scull_cdev,dev,1);
if(retval)
{
cdev_del(&scull_cdev);
unregister_chrdev_region(dev, 1);
printk("<1> Error in cdev_add:");
return -EBUSY;
}
printk ("<1> scull_init was a Success and the major device number is %d \n", Major);
return 0;

}

static int device_read(struct file *filp,char *msgusr,int count,int *offp)
{
int i;
unsigned long ret;

msgusr=NULL;
count=25;

if(*msg_ptr == 0)
return 0;

printk ("<1> \n\n Here we are Reading!!\n");

ret= copy_to_user(msgusr,msg_ptr,count);

if(ret <= 0)
{
printk("<1> Error in reading !!");
return -EFAULT;
}

// for(i=0;i<count;i++)
// printk("<1> The Msg read from the char driver is %c\n",*(msgusr++) );

return SUCCESS;
}

static void scull_exit (void)
{
printk ("<1> Removing scull drivers from /proc/devices\n");
cdev_del(&scull_cdev);
unregister_chrdev_region (MKDEV(Major,0), 1);
}

module_init (scull_init);
module_exit (scull_exit);
===============================================================

theNbomr · 11-21-2006, 09:34 AM

Code:

sprintf(msg,"Initial Driver Content !!");

The first error message is explained by the above code. You are asking sprintf to write to a string which has no storage allocated for it. The char pointer 'msg' is not initialized. Start by fixing that. I didn't look any further, so there may be other problems. This is not a problem specific to kernel/driver coding.

--- rod.

tomkaith13 · 11-21-2006, 09:43 AM

thanks a lot !!!

I never thought abt the sprintf causin the fuzz...

tomkaith13 · 11-22-2006, 03:59 AM

hi,

The sprintf issue is solved ..
The module is working fine except that it is not reading ..

If i try a cat /dev/scull,this is wat i get
cat: /dev/scull: Bad address

if i check the /var/log/messages ,this is the message:

Nov 22 15:05:12 datacom kernel: Here we are Reading!!
Nov 22 15:05:14 datacom kernel: no of bytes read :4096<1> Error in reading !!

Here is the read function alone:

====================Read function alone ==================
static int device_read(struct file *filp,char *msgusr,int count,int *offp)
{
int i;
unsigned long ret;

// msgusr=NULL;
//count=25;

if(*msg_ptr == 0)
return 0;

//while (data_not_ready) {
//interruptible_sleep_on(&scull_wait);
//}

printk ("<1> \n\n Here we are Reading!!\n");

ret= copy_to_user(msgusr,*(msg_ptr),count);

// ret is 4096 on execution, and not zero

printk("<1> no of bytes read :%d",ret);

if(ret != 0)
{
printk("<1> Error in reading !!");
return -EFAULT;
}

return SUCCESS;
}
=============================================================

Am i supposed to use offp ??
Please help !!

theNbomr · 11-22-2006, 09:18 AM

Code:

ret= copy_to_user(msgusr,*(msg_ptr),count);

I don't know what copy_to_user() does, but it appears to copy a buffer from kernel space to user space, of size 'count'. If so, then it looks like the second argument should just be 'msg_ptr' (Which you initialized to point at an information string in device_open() ). You seem to be having a bad char day.

It would be helpful to wrap your code listings in code tags to preserve formatting.

--- rod.

tomkaith13 · 11-23-2006, 04:15 AM

Ah !! finally the code worked ..
Thanks for the help...you are right abt the copy_to_user()..

No one seem to use copy_to_user func ...
Everyone seems to be content with put_user() itself..

I thought i wil try the latter func .... thats wat landed me in this mess ..

tomkaith13 · 11-27-2006, 04:46 AM

hi all,
Here is the final working code of a simple char driver implemented using copy_to_user and copy_from_user :

Quote:

#include <linux/module.h>
#include <linux/init.h>
#include <asm/current.h>
#include <asm/uaccess.h>
#include <linux/sched.h>
#include <linux/fs.h>
#include <linux/kernel.h>
#include <linux/kobject.h>
#include <linux/kobj_map.h>
#include <linux/cdev.h>
#ifdef CONFIG_KMOD
#include <linux/kmod.h>
#endif
#include <linux/cdev.h>

#define DEVICE_NAME "scull"
#define SUCCESS 0
#define FAIL -1
#define SIZE 100

static int Major, Minor;
static int device_opened = 0;
int bytes=25,bytes_read=0;
static char *msg_ptr;
static char msg[SIZE];

static int device_open (struct inode *in, struct file *filp);
static int device_release (struct inode *in, struct file *filp);
static int device_read(struct file *,char *,int ,int *);
static int device_write(struct file *,char *,int ,int *);

static int
device_open (struct inode *in, struct file *filp)
{
if (device_opened)
{
printk(KERN_ALERT "Driver is in use");
return -EBUSY;
}
device_opened++;

if(!try_module_get(THIS_MODULE))
return -EBUSY;

msg_ptr = msg;

return SUCCESS;
}

static int
device_release (struct inode *in, struct file *filp)
{

device_opened--;
module_put(THIS_MODULE);
return SUCCESS;
}

static struct file_operations scull_fops = {
.open = device_open,
.release = device_release,
.owner = THIS_MODULE,
.read = device_read,
.write = device_write,
};

struct cdev scull_cdev ;
int dev_num;

static int scull_init (void)
{

dev_t dev;
int retval;

retval=alloc_chrdev_region(&dev, 0, 1, DEVICE_NAME);
if(!retval)
{
Major=MAJOR(dev);
Minor=MINOR(dev);
}
else {
return FAIL;
}

printk("MAJOR(dev)=%d\n", MAJOR(dev));

printk("<1> Before cdev_init\n");

printk("<1> after cdev_init:\n");

cdev_init(&scull_cdev,&scull_fops);

retval=cdev_add(&scull_cdev,dev,1);
if(retval)
{
cdev_del(&scull_cdev);
unregister_chrdev_region(dev, 1);
printk("<1> Error in cdev_add:");
return -EBUSY;
}
printk ("<1> scull_init was a Success and the major device number is %d \n", Major);

memset(msg,'0',100);
sprintf(msg,"Initial Driver Content !!");

printk("<1> %s",msg);

return 0;

}

static int device_read(struct file *filp,char *msgusr,int count,int *offp)
{
int i;
unsigned long ret;

if(*msg_ptr == 0)
return 0;

printk ("<1> \n\n Here we are Reading!!\n");

if(bytes==0)
return 0;

ret= copy_to_user(msgusr,msg_ptr,bytes);

if(ret != 0)
{
printk("<1> Error in reading !!");
return -EFAULT;
}

bytes_read=bytes;
bytes=0;

return bytes_read;
}

static ssize_t device_write (struct file *filp,char *msgusr,int count,int *offp)
{
int err,i=1;
i++;
printk("<1> i is :%d",i);
printk("<1> We are writing !!");

err = copy_from_user(msg_ptr,msgusr,count);
if (err != 0)
{
printk("<1>\n Write error\n");
return -EFAULT;
}

offp-=count;
bytes += count;

printk("<1>\n\n no of chars wriiten :%d\n",count);
return count;
}
static void scull_exit (void)
{
printk ("<1> Removing scull drivers from /proc/devices\n");
cdev_del(&scull_cdev);
unregister_chrdev_region (MKDEV(Major,0), 1);
}

module_init (scull_init);
module_exit (scull_exit);