Hi all
Again I have problems with some kernel programming stuff. I decided to
make my first device driver more robust. So I wanted to add
access_ok() both in function read() and write() from fops. But when I
type: echo "Message" >> /dev/mydevice I get this error:
bash: echo: write error: Bad address
Here my source code. First, implementation of read() function:
static ssize_t FirstModule_Write(struct file *flip, const char
*buffer, size_t length, loff_t *off)
{
int i;
if(!access_ok(VERIFY_READ, buffer, length));
return -EFAULT;
for(i=0; i < length && i < BUF_LEN; i++)
get_user(msg2[i],buffer+i);
msg2_Ptr = msg2;
printk(KERN_INFO "Your message from Userland is: %s",msg2);
I think that I have to verify reading mode because I`ll get some data
from userspace buffer. Second argument is of course address of this
buffer and last one ... that`s right, I don`t know how to interpret
this argument. It is some length of memory block. There is the snippet
from #include <asm/uaccess.h>:
* access_ok: - Checks if a user space pointer is valid
* @type: Type of access: %VERIFY_READ or %VERIFY_WRITE. Note that
* %VERIFY_WRITE is a superset of %VERIFY_READ - if it is safe
* to write to a block, it is always safe to read from it.
* @addr: User space pointer to start of block to check
* @size: Size of block to check
Size of what block ? Size of address of this buffer or WHOLE buffer ?
My invocation of access_ok() is based on this code
(drivers/char/mem.c) :
static ssize_t read_port(struct file * file, char __user * buf,
size_t count, loff_t *ppos)
{
unsigned long i = *ppos;
char __user *tmp = buf;
if (!access_ok(VERIFY_WRITE, buf, count))
return -EFAULT;
while (count-- > 0 && i < 65536) {
if (__put_user(inb(i),tmp) < 0)
return -EFAULT;
i++;
tmp++;
}
*ppos = i;
return tmp-buf;
}
I thought my code is good but not. Of course not. If someone have some
ideas just go ahead :) And thanks for any explanation :)
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