Our secret driver – the init code

In the init code of our secret device driver (a kernel module, of course, thus invoked upon insmod(8)), we first register the driver as a misc character driver with the kernel (via the misc_register() API, as seen in the Writing the misc driver code – part 1 section earlier; we won't repeat this code here).

Next, we allocate kernel memory for our driver's "context" structure – via the useful managed allocation  devm_kzalloc() API (as you learned in the companion guide Linux Kernel Programming, Chapter 8, Kernel Memory Allocation for Module Authors – Part 1, in the Using the kernel's resource-managed memory allocation APIs section) – and initialize it. Notice that you must ensure you first get the device pointer dev before you can use this API; we retrieve it from our miscdevice structure's this_device member (as seen):

// ch1/miscdrv_rdwr/​miscdrv_rdwr.c
[ ... ]
static int __init miscdrv_rdwr_init(void)
{
    int ret;
    struct device *dev;

    ret = misc_register(&llkd_miscdev);
    [ ... ]
    dev = llkd_miscdev.this_device;
    [ ... ]
    ctx = devm_kzalloc(dev, sizeof(struct drv_ctx), GFP_KERNEL);
    if (unlikely(!ctx))
        return -ENOMEM;

    ctx->dev = dev;
    strscpy(ctx->oursecret, "initmsg", 8);
    [ ... ]
    return 0;         /* success */
}

Okay, clearly, we have initialized the dev member of our ctx private structure instance as well as the 'secret' string to the 'initmsg'  string (not a very convincing secret, but let's leave it at that). The idea here is that when a user space process (or thread) opens our device file and issues read(2) upon it, we pass back (copy) the secret to it; we do so by invoking the copy_to_user() helper function! Similarly, when the user-mode app writes data to us (yes, via the write(2) system call), we consider that data written to be the new secret. So, we fetch it from its user space buffer – via the copy_from_user() helper function – and update it in driver memory.

Why not simply use the strcpy() (or strncpy()) API to initialize the ctx->oursecret member? This is very important: they aren't safe enough security-wise. Also, the strlcpy() API has been marked as deprecated by the kernel community (https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy). In general, always avoid using deprecated stuff, as documented in the kernel documentation here: https://www.kernel.org/doc/html/latest/process/deprecated.html#deprecated-interfaces-language-features-attributes-and-conventions.

Quite clearly, the interesting parts of this new driver are the I/O functionality – the read and write methods; on with it!