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📝 Netlink Protocol and Linux Kernel Module Revision Notes

🌐 Concept of Netlink Protocol

  • Netlink allows communication between userspace applications and the Linux kernel.
  • To specify which kernel subsystem the application wants to communicate with, a unique ID, called the Netlink protocol number, is used.
  • Pre-existing Linux subsystems have reserved netlink protocol numbers.
  • The routing infrastructure and TCP IP stack in the kernel have a Netlink protocol number of zero, known as Netlink route.
  • To see what netlink protocol numbers are being used, check the file: Linux/netlink.h.
  • In the project described, the unused netlink protocol number 31 is used for a new kernel subsystem.

📁 Greet Netlink LCMC File Creation

  • Inside the directory named "Greetings", a file named greet Netlink lcmc is created to represent the Linux kernel module.

🚀 Steps for Linux Kernel Module Creation

  1. Initialization and Cleanup Functions

    • Every Linux kernel module starts with an initialization function and ends with a cleanup function.
    • A message is printed during initialization to indicate successful insertion of the module. A return value of 0 means success.
    • During cleanup, a message is printed to indicate the module's successful removal.

  2. Receiving Data from User Space

    • A variable of type netlink_kernel_cfg is defined. This data structure is provided by the Linux kernel.
    • Within this structure, there's an 'input' member, which is a pointer to a function meant to receive data from user space.
    • When data is received from user space, this function is invoked in kernel space.
    • The prototype of this function matches the datatype of the 'input' pointer in the netlink_kernel_cfg structure.

  3. Creating a Netlink Socket

    • Upon inserting the Linux kernel module, it should create a netlink socket that defines the type of data it will receive from user space.
    • The socket is created using the netlink_kernel_create API.
    • The first argument is a global variable init_net, representing the complete networking subsystem in the kernel.
    • The second argument is the netlink protocol number, which for this project is defined as 31.
    • The third argument is the configuration parameter (netlink_kernel_cfg).

  4. Releasing Kernel Resources

    • To prevent kernel resource leaks, any resources created by the kernel module must be released upon its removal.
    • For this module, the only resource is the netlink socket, which is released using the netlink_kernel_release API.

  5. Header Files Inclusion

    • Always include Linux/module.h when writing a Linux kernel module.
    • For netlink related functionalities, include Linux/netlink.h and net/socket.h.
    • For string handling APIs in kernel space, include linux/string.h.

📖 Next Steps

  • The subsequent steps involve having the Linux kernel module receive messages from user space, process them, and reply back.

🎤 Interview Questions

  1. What is the main purpose of the Netlink protocol in the Linux kernel?

    • Answer: 📌 Netlink allows communication between userspace applications and the Linux kernel, and each kernel subsystem is identified using a unique Netlink protocol number.

  2. What is the significance of the netlink_kernel_cfg structure in relation to the Netlink protocol?

    • Answer: 📌 The netlink_kernel_cfg data structure allows us to define various parameters of our netlink communication in kernel space. The 'input' member within this structure points to a function meant to receive data from user space.

  3. Why is it crucial to release resources when a kernel module is removed, and how is it done for a netlink socket?

    • Answer: 📌 Releasing resources prevents kernel resource leaks. If not released, resources continue to exist in kernel space without any code using them. For a netlink socket, the netlink_kernel_release API is used.

  4. Which header file must always be included when writing any Linux kernel module?

    • Answer: 📌 The Linux/module.h header file must always be included when writing any Linux kernel module.

  5. Describe the process of creating a netlink socket in the Linux kernel module.

    • Answer: 📌 To create a netlink socket, the netlink_kernel_create API is used. It takes three arguments: the global variable init_net (representing the complete networking subsystem in the kernel), the netlink protocol number, and the configuration parameter (netlink_kernel_cfg).

I hope these detailed notes and interview questions help you in your revision for interviews! 🚀