MC504/MC514 - Sistemas Operacionais

  $ insmod drivers/ofcd.ko
  $ lsmod
   ...
  $ dmesg | tail
  $ cat /proc/devices
   ...
   252 ofcd

  $ mknod /dev/ofcd0 c 252 0
  $ mknod /dev/ofcd1 c 252 1
  $ mknod /dev/ofcd2 c 252 2
  $ ls /dev

  $ cat /dev/ofcd0

ofcd-null

  $ insmod drivers/ofcd-null.ko
  $ ls /dev/
  $ cat /dev/ofcd-null
  $ echo "teste" > /dev/ofcd-null

Fazendo alterações

Primeira tentativa com crash

static char c;

static ssize_t my_read(struct file *f, char __user *buf, size_t len, loff_t *off)
{
    printk(KERN_INFO "Driver: read()\n");
    buf[0] = c;
    return 1;
}
static ssize_t my_write(struct file *f, const char __user *buf, size_t len, loff_t *off)
{
    printk(KERN_INFO "Driver: write()\n");
    c = buf[len - 1];
    return len;
}

ofcd-lastchar: copy_to_user e copy_from_user

static char c;

static ssize_t my_read(struct file *f, char __user *buf, size_t len, loff_t *off)
{
    printk(KERN_INFO "Driver: read()\n");
    if (copy_to_user(buf, &c, 1) != 0)
        return -EFAULT;
    else
        return 1;
}
static ssize_t my_write(struct file *f, const char __user *buf, size_t len, loff_t *off)
{
    printk(KERN_INFO "Driver: write()\n");
    if (copy_from_user(&c, buf + len - 1, 1) != 0)
        return -EFAULT;
    else
        return len;
}

Se você procurar a documentação sobre copy_to_user e copy_from_user verá que estas funções podem dormir. Por quê?

Sugestão: tente implementar uma versão deste driver que armazene a última escrita com kmalloc e kfree.

ioctl

./query_app      to display the driver variables
./query_app -c   to clear the driver variables
./query_app -g   to display the driver variables
./query_app -s   to set the driver variables

No capítulo 4 do Kernel Hacking: ioctls: Not writing a new system call podemos encontrar a seguinte informação:

A system call generally looks like this

asmlinkage long sys_mycall(int arg)
{
        return 0;
}

First, in most cases you don't want to create a new system call. You
create a character device and implement an appropriate ioctl for
it. This is much more flexible than system calls, doesn't have to be
entered in every architecture's include/asm/unistd.h and
arch/kernel/entry.S file, and is much more likely to be accepted by
Linus.