Our secret driver – the write method

The end user can change the secret by writing a new secret into the driver, via a write(2) system call to the driver's device node. The kernel redirects the write (via the VFS layer) to our driver's write method (as you learned in the Understanding the connection between the process, the driver, and the kernel section):

static ssize_t
write_miscdrv_rdwr(struct file *filp, const char __user *ubuf, size_t count, loff_t *off)
{
    int ret = count;
    void *kbuf = NULL;
    struct device *dev = ctx->dev;
    char tasknm[TASK_COMM_LEN];

    PRINT_CTX();
    if (unlikely(count > MAXBYTES)) { /* paranoia */
        dev_warn(dev, "count %zu exceeds max # of bytes allowed, "
                "aborting write\n", count);
        goto out_nomem;
    }
    dev_info(dev, "%s wants to write %zd bytes\n", get_task_comm(tasknm, current), count);

    ret = -ENOMEM;
    kbuf = kvmalloc(count, GFP_KERNEL);
    if (unlikely(!kbuf))
        goto out_nomem;
    memset(kbuf, 0, count);

    /* Copy in the user supplied buffer 'ubuf' - the data content
     * to write ... */
    ret = -EFAULT;
    if (copy_from_user(kbuf, ubuf, count)) {
        dev_warn(dev, "copy_from_user() failed\n");
        goto out_cfu;
     }

    /* In a 'real' driver, we would now actually write (for 'count' bytes)
     * the content of the 'ubuf' buffer to the device hardware (or 
     * whatever), and then return.
     * Here, we do nothing, we just pretend we've done everything :-)
     */
    strscpy(ctx->oursecret, kbuf, (count > MAXBYTES ? MAXBYTES : count));
    [...]
    // Update stats
    ctx->rx += count; // our 'receive' is wrt this driver

    ret = count;
    dev_info(dev, " %zd bytes written, returning... (stats: tx=%d, rx=%d)\n",
            count, ctx->tx, ctx->rx);
out_cfu:
    kvfree(kbuf);
out_nomem:
    return ret;
}

We employ the kvmalloc() API to allocate memory for a buffer to hold the user data that we will copy in. The actual copying is done via the copy_from_user() routine, of course. Here, we use it to copy the data passed by the user space app to our kernel buffer, kbuf. We then (via the strscpy() routine) update our driver's context structure's oursecret member to this value, thus updating the secret! (A subsequent read on the driver will now reveal the new secret.) Also, do notice the following:

• How we now consistently use the dev_xxx() helpers in place of the usual printk routines. This is recommended for device drivers.
• The (now typical) usage of goto to perform optimal error handling.

This covers the meat of the driver.