If you have installed the hardened Linux Kernel on Fedora, you may have encountered the following error when trying to launch Flatpak applications.
bwrap: No permissions to creating new namespace, likely because the kernel does not allow non-privileged user namespaces. On e.g. debian this can be enabled with 'sysctl kernel.unprivileged_userns_clone=1'. error: Failed to sync with dbus proxy
The issue looks to arise from the fact that the hardened Linux Kernel disables unprivileged name space and Fedora does not have setuid on by default on the bubblewrap executable.
Enabling setuid on bubblewrap
You can set the setuid permission on the bubblewrap executable with
sudo chmod u+s /usr/bin/bwrap
Allow Unprivileged Name Space (Alternative work around)
You could also allow unprivileged name space by running
Note that setting the setuid seems the safer/recommended option.
It looks like using the setuid binary for bubblewrap would be better to use then enabling unprivileged user space.
Remove setuid on bubblewrap
If you would like to remove the setuid permission for any reason, you can with the following command.
sudo chmod u-s /usr/bin/bwrap