Friday, April 29, 2022

Debugging Flatpak applications

Flatpak is a way to distribute applications on Linux. Its container-style approach allows applications to run across Linux distributions. This means native packages (rpm, deb, etc) are not needed and it's relatively easy to get your app to Linux users with fewer worries about distro compatibility. This makes life a lot easier for developers and is also convenient for users.

I've run popular applications like OBS Studio as flatpaks and even publish my own on Flathub, a popular hosting site for applications. Today I figured out how to debug flatpaks, which requires some extra steps that I'll share below so I don't forget them myself!

Bonus Tip: Testing local flatpaks

If you're building a flatpak of your own application it's handy to use the dir sources type in the manifest to compile your application's source code from a local directory instead of a git tag or tarball URL. This way you can make changes to the source code and test them quickly inside Flatpak.

Put something along these lines in the manifest's modules object where /home/user/my-app is you local directory with your app's source code:

     "name": "my-app",
     "sources": [
             "type": "dir",
             "path": "/home/user/my-app"

Building and installing apps with debuginfo

flatpak-builder(1) automatically creates a separate .Debug extension for your flatpak that contains your application's debuginfo. You'll need the .Debug extension if you want proper backtraces and source level debugging. At the time of writing the Flatpak documentation did not mention how to install the locally-built .Debug extension. Here is how:

$ flatpak-builder --user --force-clean --install build
$ flatpak install --user --reinstall --assumeyes "$(pwd)/.flatpak-builder/cache"

It might be a good idea to install debuginfo for the system libraries in your SDK too in case it's not already installed:

$ flatpak install org.kde.Sdk.Debug # or your runtime's SDK

Running applications for debugging

There is a flatpak(1) option that launches the application with the SDK instead of the Runtime:

$ flatpak run --user --devel

The SDK contains development tools whereas the Runtime just has the files needed to run applications.

It can also be handy to launch a shell so you can control the launch of your app and maybe use gdb or strace:

$ flatpak run --user --devel --command=sh
[📦 ~]$ gdb /app/bin/my-app

Working with core dumps

If your application crashes it will dump core like any other process. However, existing ways of inspecting core dumps like coredumpctl(1) are not fully functional because the process ran inside namespaces and debuginfo is located inside flatpaks instead of the usual system-wide /usr/lib/debug location. coredumpctl(1), gdb, etc aren't Flatpak-aware and need extra help.

Use the flatpak-coredumpctl wrapper to launch gdb:

$ flatpak-coredumpctl -m <PID>

You can get PID from the list printed by coredumpctl(1).


This article showed how to install locally-built .Debug extensions and inspect core dumps when using Flatpak. I hope that over time these manual steps will become unnecessary as flatpak-builder(1) and coredumpctl(1) are extended to automatically install .Debug extensions and handle Flatpak core dumps. For now it just takes a few extra commands compared to debugging regular applications.

Monday, March 7, 2022

vhost-user is coming to non-Linux hosts!

Sergio Lopez sent a QEMU patch series and vhost-user protocol specification update that maps vhost-user to non-Linux POSIX host operating systems. This is great news because vhost-user has become a popular way to develop emulated devices in any programming language that execute as separate processes with their own security sandboxing. Until now they have only been available on Linux hosts.

At the moment the BSD and macOS implementation is slower than the Linux implementation because the KVM ioeventfd and irqfd primitives are unavailable on those operating systems. Instead POSIX pipes is used and the VMM (QEMU) needs to acts as a forwarder for MMIO/PIO accesses and interrupt injections. On Linux the kvm.ko kernel module has direct support for this, bypassing the VMM process and achieving higher efficiency. However, similar mechanisms could be added to non-KVM virtualization drivers in the future.

This means that vhost-user devices can now start to support multiple host operating systems and I'm sure they will be used in new ways that no one thought about before.

Friday, February 4, 2022

Speaking at FOSDEM '22 about "What's coming in VIRTIO 1.2"

I will give a talk titled What's coming in VIRTIO 1.2: New virtual I/O devices and features on Saturday, February 5th 2022 at 10:00 CET at the FOSDEM virtual conference (it's free and there is no registration!). The 9 new device types will be covered, as well as some of the other features that have been added to the upcoming 1.2 release of the VIRTIO specification. I hope to see you there and if you miss it there will be slides and video available afterwards.