Kernel Code : where can I find and how to debug the kernel

Question

Recently I have installed Ubuntu 12.04 LTS ISO image in my desktop. Below is the output of the kernel version I have installed:

# uname -r
3.5.0-41-generic

I am trying to develop a VFS and want the kernel source code version '3.5.0-41-generic' for reference purpose - where can I find the same?

What are the excellent kernel debugging options looking at logs and mapping them to kernel code?

How and which debugger I can use to debug a live kernel flow execution?

Are there ways I can add more printk methods and re-modify the modules? Say I want to know how a FS mount method works - I can modify the required FS code (adding more printk functions) re-compile and reload the modules. Now with aid of my new printk functions I can understand the flow

Answer 1

Find and download kernel source code:

• For Ubuntu-specific kernel sources, use apt cmd:

sudo apt-get install linux-source

This fetches the generic kernel source.

To fetch specific version use:

sudo apt-get source linux-image-6.11.8-generic

• Also you can download vanilla/original kernel source from kernel.org.

Kernel debugging options:

• Read logs: Use dmesg cmd to view kernel logs.
• Search in source: Match log messages with code by searching for keywords in source code files (like grep -r "log text" /path/to/source).
• Tracing tools:
  • Use ftrace, perf, or systemtap cmds for tracing execution paths.
  • For filesystem-specific debugging, use tracepoints cmd.

Live kernel debugging options:

• Use KGDB:
  • Configure KGDB in your kernel config (CONFIG_KGDB=y and CONFIG_GDBSTUB=y).
  • Attach it via gdb using:

    target remote /dev/ttyS0
    

• Explore it by advanced tracing tools like bpftrace.

Adding printk and reloading modules:

• Modify kernel modules:
  1. Locate the kernel source file for FS subsystem, like fs/super.c.
  2. Add printk() to inside functions of your targets (like mount function).
  3. Rebuild and install module:

     make modules
     sudo insmod your_module.ko
     

  4. Use dmesg to watch new logs.

Note: Live kernel debugging means inspecting and modifying the Linux kernel while it is actively running.