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Kernel Debugging Ubuntu 16.04

24 Aug 2020

All the following steps are tested on Ubuntu 16.04

There are 3 steps here: Building qemu image, building and installing kernel, debugging

Building qemu image

Install qemu

sudo apt-get install qemu-kvm qemu virt-manager virt-viewer libvirt-bin

Create a qcow disk

Here, we will create a virtual disk on which ubuntu will be installed.

Why qcow2? not regular image? Because we can increase the size of qcow later, but increasing the size of regular image is tricky.

qemu-img create -f qcow2 ubuntu16.04.qcow 40G

Download ubuntu image

wget https://releases.ubuntu.com/16.04/ubuntu-16.04.7-desktop-amd64.iso

Installing Ubuntu 16.04 on the disk

qemu-system-x86_64 -hda ubuntu16.04.qcow -boot d -cdrom ./ubuntu-16.04.7-desktop-amd64.iso -device virtio-net,netdev=vmnic -netdev user,id=vmnic -m 4G

This will open a window on which you follow the instructions to complete the installation.

Build and install kernel

Follow these instructions on the host machine

Clone the kernel sources

git clone git://kernel.ubuntu.com/ubuntu/ubuntu-xenial.git
cd ubuntu-xenial
# check out the required kernel
git checkout tags/ubuntu-hwe-4.15.0-112.113_16.04.1

Configure

get the default config

Copy the config from the QEMU vm, you can find the config at the following path on the vm.

/boot/config-`uname -r`

Lets say you got the config out from the VM into the host as ubuntu16.04config

Now copy the config as .config in the ubuntu-xenial (i.e., folder where we checked out our kernel sources) i.e.,

cp ubuntu16.04config <path to ubuntu-xenial>/.config

Modify the config (Optional)

make menuconfig

This will open a window where you can enable or disable additional kernel configuration options.

Building kernel

	chmod a+x debian/scripts/*
	chmod a+x debian/scripts/misc/*
	cp debian/scripts/retpoline-extract-one scripts/ubuntu-retpoline-extract-one
	make deb-pkg

Installing kernel on to the guest

Copy all *.deb from host to guest and install the built kernel into the vm.

You should run the following command in guest VM

sudo dpkg -i linux-image-<..>.deb
sudo dpkg -i linux-headers-<..>.deb

Debugging Guest VM

First, run the QEMU vm and make qemu wait for the debugger using the following command:

qemu-system-x86_64 -s -S -hda ubuntu16.04.qcow -device virtio-net,netdev=vmnic -netdev user,id=vmnic -m 4G -enable-kvm -append "console=ttyS0"

This will cause the qemu wait untill the debugger gets attached.

Now in an other terminal window

# Go to the folder where we built the kernel
cd <path to ubuntu-xenial>
# gdb
> file vmlinux
> target remote:1234
# You are inside debugger and see that the break point is being hit.

Thats it!! You can use the regular gdb commands from now on.

References

[1] https://wiki.gentoo.org/wiki/QEMU/Options [2] https://help.ubuntu.com/community/Kernel/Compile#Alternate_Build_Method_.28B.29:_The_Old-Fashioned_Debian_Way