How to debug the Linux Kernel using VirtualBox

linux-debug.md

Assumptions: Host machine is linux.

1. Download virtualbox
2. Install a linux flavour. Preference for Debian since distro-dependent commands in the following steps will be debian-dependent
3. Run your VM.
4. Inside VM, download last kernel code from kernel.org
5. Extract linux src code
6. Copy your configuration file from /boot/config-$(uname -r) to .config in the extract linux src code folder.
7. Make sure .config has the following options: CONFIG_FRAME_POINTER=y , CONFIG_KGDB=y , CONFIG_KGDB_SERIAL_CONSOLE=y , CONFIG_KGDB_KDB=y , CONFIG_KDB_KEYBOARD=y. Change if necessary. Alternatively use make xconfig to configure in UI.
8. Install compilation dependencies (debian: sudo apt-get install build-essential libncurses-dev bison flex libssl-dev libelf-dev)
9. Compile the kernel: make -j8
10. Compile kernel modules: sudo make modules_install
11. Install kernel to /boot: sudo make install
12. Output will be in /boot directory. GRUB needs to be updated. On debian, run: sudo update-initramfs -c -k <KERNEL VERSION>. Example: sudo update-initramfs -c -k 5.6.9
13. After that, run sudo update-grub.
14. For more info about the compilation part, here: https://www.cyberciti.biz/tips/compiling-linux-kernel-26.html
15. Go back to linux src folder and find the compiled file vmlinux. Send this file to your host machine, either using scp or virtualbox shared folders.
16. Reboot the VM. Afterwards, run uname -rms. You should see the new just-compiled version.
17. Run sudo vim /etc/default/grub
18. Append to GRUB_CMDLINE_LINUX_DEFAULT the following parameters: kgdboc=ttyS0,115200 nokaslr. The first parameter tells the debugger to be active on serial port /dev/ttyS0. The second one tells to load the kernel without address space layout randomization, so the debugger can find the function addresses properly. Example: GRUB_CMDLINE_LINUX_DEFAULT=quiet splash kgdboc=ttyS0,115200 nokaslr.
19. Run sudo update-grub.
20. Turn off the VM.
21. On the host machine, open VM settings and go to Serial Ports. Go to tab Port 1. Mark Enable serial port. Choose Port Number=COM1, Port Mode=Host Pipe and Path/Address=/tmp/pipe. Don't mark Connect to existing pipe/socket.
22. Run the VM again.
23. Open a terminal in the VM and go to su mode by running sudo su
24. Run echo g > /proc/sysrq-trigger to make the kernel execution stop and wait for the debugger to be attached. Note that once you run this command, the VM will be frozen until you continue the execution using GDB later.
25. On the host machine, run socat -d /tmp/pipe TCP4-LISTEN:65335. This will create a TCP connection from the /tmp/pipe. Even though GDB supports connecting directly to serial ports, the /tmp/pipe is a UNIX socket, so GDB does not recognize it as a serial port. Using socat we can create a TCP connection and connect using GDB.
26. Now run GDB via gdb ./vmlinux where ./vmlinux is the path to the kernel binary file that you copied before.
27. In GDB run target remote localhost:65335.
28. At this point the debugger should be attached. Put breakpoints using break and type continue to resume the kernel execution in the VM.

Wonderful! Thanks for sharing.