Linux VM Kernel Debugging

These are some handy steps for doing Linux live kernel debugging on the OpenXT platform. OpenXT makes this readily doable by using serial over IP within QEMU to connect the remote debugger GDB to the kernel debugger KGDB. Of course everyone has their favorite distro but for brevity's sake we will stick to just one – Debian. Most of the steps should be the same with other flavors aside from the package management bits and specifics of rebuilding the kernel sources.

Beforehand

Here are some things to setup up first and to keep in mind while going through this:

• This is based on using Debian Wheezy HVMs. That means apt package manager and .deb package files.
• Throughout this guide:
• target is the VM that is being kernel debugged
• host is the debugger, the VM where GDB is remotely connecting to the target.
• For simplicity, it is assumed both the host and target are the same OS or distro. The paths and users on both are the same. Also password-less SSH login and sudo setup are assumed.

So to get started, install OpenXT and create 2 Debian Wheezy HVMs. For debugging to work, SELinux and stubdoms need to be turned off. Stubdoms are disabled in the Advanced tabs for the VMs. To disable SELinux, run a terminal, use nr to log into the admin role then run rw to make the rootfs read-write. Next edit /etc/selinux/config and set SELINUX=permissive and save. Reboot.

Building a Kernel

The KGDB debugger components need to be enabled in the target kernel. This requires building a custom kernel. On the target, get the kernel source package - in this case for Wheezy:

$ sudo apt-get install linux-source-3.2