Debugging Linux Kernel using KGDB Part 1

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What is KGDB?

KGDB Stands for Kernel GDB.

User space programs can be debugged remotely using the combination of gdbserver on the target machine and gdb on the host machine/development machine. But you cannot use the same setup for debugging kernel, as gdbserver is still running as a user space application on the kernel which we want to debug. To solve this program, KGDB comes into picture. KGDB acts as a gdbserver inside the server.

Typical Setup

A typical KGDB setup requires two machines connected by serial cable.
  • Host/Development Machine: Runs gdb and performs debugging
  • Target Machine: Runs kgdb and is the machine to be debugged


Test Setup

Setup on Target VM:

1.    Build and install the latest Linux kernel on the target VM machine, make sure you add the following kernel configurations. Follow this link to build if you are beginner. https://embeddedguruji.blogspot.com/2018/12/steps-to-build-and-install-latest-kernel.html




CONFIG_KGDB
CONFIG_KGDB_SERIAL_CONSOLE


2.   Copy the vmlinux image from this target VM to the development VM
3. Establish Serial Communication between two Virtual Machines using the following documentation: https://embeddedguruji.blogspot.com/2018/12/connecting-serial-ports-of-two-ubuntu.html

4.  Append the following command -line to kernel command line in grub
kgdbwait kgdboc=ttyS1,115200 sysrq_always_enabled
kgdbdoc tells the kernel to use ttyS1 serial port and 115200 baud rate
kgdwait tells the kernel to wait until the debugger is attached
sysrq_always_enabled enabling the sysrq 


You can follow the following documentation, if you are beginner:


5. Reboot the machine and you will observe that the target VM is freezed after grub bootloader stage.



Setup on Host/Development Machine 

1. Run the following commands:
  • gdb vmlinux
  • target remote /dev/ttyS1
  • continue


Note: It is better to copy the whole linux build folder instead of just vmlinux to get source level debugging



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