The WORST bit of code I ever wrote….

Over the years, I have written quite a bit of code. Not mind boggling or anything, but a bit more than most other people I reckon. And looking back at it all, I’ve written some truly crappy stuff. I laugh at it, because I like to think I “know” stuff now. But today I was brought crashing down to earth.

Here is what I wrote:

/*….some code that initialised stuff…*/
Major=register_chrdev(253,”abc” , &fops);
if(Major < 0)
{
printk(“Registering failed !!\n”);
return Major;
}
printk(“Reading cruel world!!\n”);
return 0;
handle_list_head = (struct alloc_handle_list *)kmalloc (sizeof(struct alloc_handle_list), GFP_KERNEL);
if(handle_list_head==NULL)
{
printk(“Unable to allocate head”);
return 1;
}
INIT_LIST_HEAD(&handle_list_head->ptr);
if(handle_list_head->ptr.prev==NULL || handle_list_head->ptr.next==NULL)
/*….other init code… initialisation module ends…*/
/*…………………………………………….*/

/*……some other function….*/
list_for_each_safe(temp_list, temp_for_temp_list, &handle_list_head->ptr)
{
fun = list_entry(temp_list, struct alloc_handle_list, ptr);
if(fun->handle != handle) continue;
co_passage_page_assert_valid();
co_passage_page_acquire(&flags);
co_passage_page->operation=CO_OPERATION_UNREGISTER_HANDLE;
co_passage_page->params[0]=fun->handle;
co_switch_wrapper();
if(co_passage_page->params[0]==0)
{
co_passage_page_release(flags);
printk(“Did not unreg Handle (no such handle)!!\n”);
return -1;
}
co_passage_page_release(flags);
/*……….and some more…..*/

It isn’t important to know what this does, only that we were at a crucial point in the general proceedings. See if you can locate the flaw. It took poor Jitesh 4 hours to debug this little baby. Every time we executed the code, it would give us a Segmentation Fault. The kernel stack dump would tell us that the module was “Unable to deference pointer to kernel memory at virtual address…”.

We went through every detail of the pointer manipulation. We resorted to commenting the code line by line to figure out the offending snippet. Finally, unable to take it anymore, I called it a day and headed home. Jitesh, being the stubborn kind, struggled on. When he found the error, he called me up to dish out a few choice words and home truths. Here it is…

/*…in the initialisation function…*/
if(Major < 0)
{
printk(“Registering failed !!\n”);
return Major;
}
printk(“Reading cruel world!!\n”);
return 0;
handle_list_head = (struct alloc_handle_list *)kmalloc (sizeof(struct alloc_handle_list), GFP_KERNEL);
/*…continues…*/

As you can see, the head of our list structure was NEVER BEING INITIALIZED!!! This, my friends, is the absolute worst bit of code I have ever typed. And my punishment for doing this is to write truthfully about it in this post! Hopefully, this will remain the worst bit of code I ever write, and I won’t have to write another post like this ever!

Linux From Scratch

Here’s a great site for anyone interested in understanding the internal working of Linux. It’s called Linux From Scratch, and it gives step by step instructions on compiling your very own customized Linux OS from the source code, configuring absolutely everything. Why would you want to do this?

a) complete choice over what you want in your system
b) understanding of how the system actually works.

Warning though, this is not to be tried by beginners. You need some base in Linux before trying it. I plan to take it up soon, after I get confident enough to try n screw with the source code.

—

PS: I picked up the link from Abhishek’s Blog. So there. I’ve done my duty and let you know!

The Name of the Game is… Frustration.

Linux isn’t all fun. Try to touch the kernel, and you’ll see what I’m talking about. All I have learnt is, to do 2 minutes of useful work, one must first spend 5 days either banging one’s head on the keyboard or vacantly staring at the screen hoping for a miracle. It doesn’t work any other way. Mr. Murphy must indeed have been a visionary, for as he says, everything that can go wrong REALLY DOES go wrong!

And do you know what the real stinger is? It took my guide just 2 minutes to figure out what I was doing wrong, fix it, and run the damn module! 😥

P.S Much thanks to Sandeep, who pulled me out of the pit of depression I was wallowing in with a simple mail. Muchos gracias, and Namaste!