Debugging in Unikraft

The slides for this session can be found here.

Because unikernels aim to be a more efficient method of virtualization, this can sometimes cause problems. This session aims to familiarize you to solve any problem encountered during the development using GDB.


At this stage, you should be familiar with the steps of configuring, building and running any application within Unikraft and know the main parts of the architecture. Below you can see a list of the commands you have used so far.

Command Description
kraft list Get a list of all components that are available for use with kraft
kraft up -t <appname> <your_appname> Download, configure and build existing components into unikernel images
kraft run Run resulting unikernel image
kraft init -t <appname> Initialize the application
kraft configure Configure platform and architecture (interactive)
kraft configure -p <plat> -m <arch> Configure platform and architecture (non-interactive)
kraft build Build the application
kraft clean Clean the application
make menuconfig Configure application through the main menu

Support Files

Session support files are available in the repository. The repository is already cloned in the virtual machine.

If you want to clone the repository yourself, do

$ git clone

$ cd docs/content/en/community/hackathons/2022-05-lyon/debugging/

$ ls
demo/  images/  sol/  work/


Contrary to popular belief, debugging a unikernel is in fact simpler than debugging a standard operating system. Since the application and OS are linked into a single binary, debuggers can be used on the running unikernel to debug both application and OS code at the same time. A couple of hints that should help starting:

  1. In the configuration menu (presented with make menuconfig), under Build Options make sure that Drop unused functions and data is unselected. This prevents Unikraft from removing unused symbols from the final image and, if enabled, might hide missing dependencies during development.
  2. Use make V=1 to see verbose output for all of the commands being executed during the build. If the compilation for a particular file is breaking and you would like to understand why (e.g., perhaps the include paths are wrong), you can debug things by adding the -E flag to the command, removing the -o [objname], and redirecting the output to a file which you can then inspect.
  3. Check out the targets under Miscellaneous when typing make help, these may come in handy. For instance, make print-vars enables inspecting at the value of a particular variable in
  4. Use the individual make clean-[libname] targets to ensure that you’re cleaning only the part of Unikraft you’re working on and not all the libraries that it may depend on. This will speed up the build and thus the development process.
  5. Use the Linux user space platform target (linuxu) for quicker and easier development and debugging.

Debug Information

An old saying goes: “Two of the most useful tools for debuging are printf and your brain”. It’s very useful to just print information as part of your program to understand the runtime execution flow.

However, extensive printing may be too much and doesn’t make sens when you’re not debugging. Which is why, usually debug printing is configured by a logging level, i.e. how many messages are printed.

The debugging / logging facility in Unikraft is the ukdebug library. Function provided by the ukdebug interface (such as uk_printk, uk_pr_debug, uk_pr_info) print information conditionally is the logging level is past a pre-configured level.

When debugging Unikraft, it’s generally a good idea to enable logging messages. This is done by adding support for ukdebug at configuration stage.

Using GDB

The build system always creates two image files for each selected platform:

  • one that includes debugging information and symbols (.dbg file extension)
  • one that does not

Before using GDB, go to the configuration menu under Build Options and select a Debug information level that is bigger than 0. We recommend 3, the highest level.

Setting an optimization level
Figure 1: Setting an optimization level

Once set, save the configuration and build your images.


For the Linux user space target (linuxu) simply point GDB to the resulting debug image, for example:

$ gdb build/app-helloworld_linuxu-x86_64.dbg


For KVM, you can start the guest with the kernel image that includes debugging information, or the one that does not. We recommend creating the guest in a paused state (the -S option):

$ qemu-system-x86_64 -s -S -cpu host -enable-kvm -m 128 -nodefaults -no-acpi -display none -serial stdio -device isa-debug-exit -kernel build/app-helloworld_kvm-x86_64.dbg -append verbose

Note that the -s parameter is shorthand for -gdb tcp::1234. To avoid this long qemu-system-x86 command with a lot of arguments, we can use qemu-guest.

$ qemu-guest -P -g 1234 -k build/app-helloworld_kvm-x86_64.dbg

Now connect GDB by using the debug image with:

$ gdb --eval-command="target remote :1234" build/app-helloworld_kvm-x86_64.dbg

Unless you’re debugging early boot code (until _libkvmplat_start32), you’ll need to set a hardware break point: Hardware breakpoints have the same effect as the common software breakpoints you are used to, but they are different in the implementation. As the name suggests, hardware breakpoints are based on direct hardware support. This may limit the number of breakpoints you can set, but makes them especially useful when debugging kernel code.

hbreak [location]

We’ll now need to set the right CPU architecture:

set arch i386:x86-64:intel

And reconnect:

tar remote localhost:1234

You can now run continue and debug as you would normally.

Practical Work

Support Files

Session support files are available in the repository. The repository is already cloned in the virtual machine.

If you want to clone the repository yourself, do

$ git clone

$ cd docs/content/en/community/hackathons/2022-05-lyon/debugging/

$ ls
demo/  images/  sol/  work/

01. Debugging information

Setup, configure, build and run the app-helloworld application. Build it both for Linuxu and for KVM.

Then configure ukdebug for app-helloworld. Enable all messages (all debug levels) then build and run the application, both for Linuxu and KVM.

Add a uk_pr_* call (such as uk_pr_info or uk_pr_warn) in the main.c file to print a message. Configure the application such that it would print and then it would not print the message.

02. Tutorial. Use GDB in Unikraft

For this tutorial, we will just start the app-helloworld application and inspect it with the help of GDB.

First make sure you have the following file structure in your working directory:

|-- apps
|   `-- helloworld
|-- libs
`-- unikraft


For the image for the linuxu platform we can use GDB directly with the binary already created.

$ gdb build/app-helloworld_linuxu-x86_64.dbg


To avoid using a command with a lot of parameters that you noticed above in the KVM section, we can use qemu-guest.

$ qemu-guest -P -g 1234 -k build/app-helloworld_kvm-x86_64.dbg

Open another terminal to connect to GDB by using the debug image with:

$ gdb --eval-command="target remote :1234" build/app-helloworld_kvm-x86_64.dbg

First you can set the right CPU architecture and then reconnect:

set arch i386:x86-64:intel
tar remote localhost:1234

Then you can put a hardware break point at main function and run continue:

hbreak main

All steps described above can be done using the script kvm_gdb_debug located in the work/02-tutorial-gdb/ folder. All you need to do is to provide the path to kernel image.

kvm_gdb_debug build/app-helloworld_kvm-x86_64.dbg

03. Mystery: Find the secret using GDB

Before starting the task let’s get familiar with some GDB commands.

ni - go to the next instruction, but skip function calls

si - go to the next instruction, but enters function calls

c - continue execution to the next breakpoint

p expr - display the value of an expression

x addr - get the value at the indicated address (similar to p *addr)

whatis arg - print the data type of arg

GDB provides convenience variables that you can use within GDB to hold on to a value and refer to it later. For example:

set $foo = *object_ptr

Note that you can also cast variables in GDB similar to C:

set $var = (int *) ptr

If you want to dereference a pointer and actually see the value, you can use the following command:

p *addr

You can find more GDB commands here Also, if you are unfamiliar with X86_64 calling convention you can read more about it here.

Now, let’s get back to the task. Download the mystery_kvm-x86_64 file from here. Copy the mystery_kvm-x86_64 file to the work/03-mystery/ directory. Navigate to work/03-mystery/ directory. Use the 2 scripts in the directory ( and to start the mystery_kvm-x86_64.dbg executable using GDB. Do you think you can find out the secret?

HINT Use the nm utility on the binary as a starting point.

04. Bug or feature?

There are two kernel images located in the work/04-app-bug/ folder. One of them is build for Linuxu, the other for KVM.

First try to inspect what it’s wrong with Linuxu image. You will notice that if you run the program you will get a segmentation fault. Why does this happen?

After you figure out what it’s happening with Linuxu image have a look also at the KVM one. It was built from the code source, but when you will try to run it, you will not get a segmentation fault. Is this a bug or a feature?

Further Reading