QT5 issue: could not find or load the Qt platform plugin "xcb"

[mviereck]

Some QT5 applications running with recent QT5 versions fail in docker containers with error message:

This application failed to start because it could not find or load the Qt platform plugin "xcb"
in "".

Running with QT5 debug option --env QT_DEBUG_PLUGINS=1 shows some more error messages. The important part seems to be:

Cannot allocate memory

Solutions found so far:

• Use an older QT5 version. For example, QT5 images based on debian stretch work well, while images based on debian buster fail.
• A quick and dirty fix is to run the container with flag --security-opt seccomp=unconfined. That degrades container security and must not be a general solution.

Related thread: https://forum.qt.io/topic/92783/qt-doesn-t-work-in-my-archlinux-docker

[mviereck]

It turns out to be an issue with statx syscall. It is already whitelisted in recent docker versions, but did not reach the distributions yet. It is fixed in docker-ce 18.06, but not in 18.03.

This PR added statx to docker seccomp whitelist: moby/moby#36417
Related SO thread: https://stackoverflow.com/questions/48995826/which-capabilities-are-needed-for-statx-to-stop-giving-eperm

Compare QT5 bug report: https://bugreports.qt.io/browse/QTBUG-70447

Possible solutions for docker versions before 18.06:

• Discouraged, degrades container isolation: add option --security-opt seccomp=unconfined
• Encouraged: download https://raw.githubusercontent.com/moby/moby/master/profiles/seccomp/default.json and run with --security-opt seccomp=/path/to/downloaded-seccomp.json

[mviereck]

Probably there is no Docker version around anymore that is affected by this bug. Closing now.