add kfuncs docs (#136)

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src/43-kfuncs/module/README.md

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+# write a basic kernel module
+
+## hello world
+
+Writing a Linux kernel module involves creating code that can be loaded into and unloaded from the kernel dynamically, without rebooting the system. Here’s a simple step-by-step guide to help you write a basic kernel module:
+
+### 1. Set Up Your Environment
+
+Make sure you have the Linux kernel headers installed and a suitable development environment ready. For Ubuntu or Debian, install them with:
+
+```bash
+sudo apt-get install linux-headers-$(uname -r) build-essential
+```
+
+### 2. Write the Kernel Module Code
+
+Here’s an example of a very basic Linux kernel module:
+
+```c
+// hello.c: A simple Linux kernel module
+#include <linux/init.h>    // Macros for module initialization
+#include <linux/module.h>  // Core header for loading modules
+#include <linux/kernel.h>  // Kernel logging macros
+
+// Function executed when the module is loaded
+static int __init hello_init(void)
+{
+    printk(KERN_INFO "Hello, world!\n");
+    return 0;  // Return 0 if successful
+}
+
+// Function executed when the module is removed
+static void __exit hello_exit(void)
+{
+    printk(KERN_INFO "Goodbye, world!\n");
+}
+
+// Macros to define the module’s init and exit points
+module_init(hello_init);
+module_exit(hello_exit);
+
+MODULE_LICENSE("GPL");               // License type (GPL)
+MODULE_AUTHOR("Your Name");          // Module author
+MODULE_DESCRIPTION("A simple module"); // Module description
+MODULE_VERSION("1.0");               // Module version
+```
+
+### 3. Create a Makefile
+
+To compile the kernel module, you’ll need a `Makefile`. Here's a simple one:
+
+```makefile
+obj-m += hello.o  # hello.o is the target
+
+all:
+    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
+
+clean:
+    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
+```
+
+### 4. Compile the Module
+
+Run the following command in the directory where your `hello.c` and `Makefile` are located:
+
+```bash
+make
+```
+
+This will generate a file called `hello.ko`, which is the compiled kernel module.
+
+### 5. Load the Module
+
+To insert the module into the kernel, use `insmod`:
+
+```bash
+sudo insmod hello.ko
+```
+
+### 6. Check the Logs
+
+To see the output from the `printk` statements, use the `dmesg` command:
+
+```bash
+dmesg | tail
+```
+
+You should see something like:
+
+```txt
+[ 1234.5678] Hello, world!
+```
+
+### 7. Remove the Module
+
+To unload the module, use `rmmod`:
+
+```bash
+sudo rmmod hello
+```
+
+Again, check the logs using `dmesg`:
+
+```bash
+sudo dmesg | tail
+```
+
+You should see:
+
+```txt
+[ 1234.9876] Goodbye, world!
+```
+
+### 8. Clean Up
+
+To clean up the build files, run:
+
+```bash
+make clean
+```
+
+### Notes
+
+- **License**: The `MODULE_LICENSE("GPL")` ensures the module is GPL-compliant, which allows it to use symbols (functions) exported by the kernel.
+- **Debugging**: Use `printk` for logging within the module. It behaves similarly to `printf` but is designed for kernel space.
+- **Module Parameters**: You can add parameters to modules using `module_param()` to pass arguments when the module is loaded.
+
+### Next Steps
+
+Once you are familiar with this basic example, you can explore:
+
+- Writing more advanced modules that interact with hardware or the filesystem.
+- Using kernel-specific APIs like work queues, kthreads, or handling interrupts.
+- Diving into eBPF or loadable kernel module techniques for debugging and tracing kernel events.