Qt doesn't work in my ArchLinux docker

emberflare

@mviereck said in Qt doesn't work in my ArchLinux docker:

I gave an example calibre Dockerfile in the bugreport that I used for test runs. Can you check whether that one succeeds on your system?

Running that image fails for me with the usual error: Could not find the Qt platform plugin "xcb" in "".

Another possible check: run with --cap-add=ALL.

This fails with my minimal Dockerfile, and with your calibre Dockerfile.

Next check: Add to your seccomp profile whitelist syscalls not covered by --cap-add=ALL:

This fails with my minimal Dockerfile, and with your calibre Dockerfile.

Here's my minimal Dockerfile for reproducing the error.

It succeeds here with Docker version 18.06.1-ce, build e68fc7a and output QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-root'.

That is very strange. Perhaps my Docker installation is corrupted or my host is somehow incapable of applying seccomp configurations. In one of the PRs linked to this problem, somebody mentions a host dependency for seccomps, but it seems unlikely that they are missing on ubuntu 18.04.

I'll reinstall Docker and report back with the results.

EDIT: Same results with reinstalled docker. I'm looking into seccomp dependencies now.

mviereck

@emberflare Maybe different kernel versions make a difference. Here:

$ uname -a
Linux buster 4.17.0-3-amd64 #1 SMP Debian 4.17.17-1 (2018-08-18) x86_64 GNU/Linux

I'll reinstall Docker and report back with the results.

Make sure you don't have package docker.io installed. It interferes with docker-ce. Also there may be an old docker config file somewhere around that messes up your setup. Maybe purge-deinstall docker-ce, than install docker.io, purge it, too, than install docker-ce again. That hopefully removes all config files.

somebody mentions a host dependency for seccomps

Afaik the kernel can be compiled to support seccomp or not. If it does not support it, there should be no restriction.

emberflare

@mviereck

@emberflare Maybe different kernel versions make a difference. Here:
$ uname -a
Linux buster 4.17.0-3-amd64 #1 SMP Debian 4.17.17-1 (2018-08-18) x86_64 GNU/Linux

My host is a Google Cloud instance, with a seemingly custom compiled kernel (gcp suffix):

$ uname -a
Linux gpu-instance-1 4.15.0-1018-gcp #19~16.04.2-Ubuntu SMP Mon Aug 20 13:39:28 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

Perhaps the problem lies in that kernel, but as you mentioned, it should either be compiled to support seccomp or not, and theoretically work in both cases. Could a kernel update help?

I'll reinstall Docker and report back with the results.

purge-deinstall docker-ce, than install docker.io, purge it, too, than install docker-ce again

Good idea, I tried this, but still the same error.

mviereck

@emberflare

I found an interesting documentation about seccomp and docker: https://github.com/docker/labs/tree/master/security/seccomp

It may be worth a try to check which syscalls your qt app is using and whether one is missing in the seccomp whitelist.
Install strace in image, run it interactive -it with bash and try:

strace -c -f -S name /opt/qt511/bin/qmlplugindump 2>&1 1>/dev/null | tail -n +3 | head -n -2 | awk '{print $(NF)}'

strace probably needs --cap-add SYS_PTRACE. If in doubt, try --cap-add=ALL.

This blog post may help, too: https://blog.jessfraz.com/post/how-to-use-new-docker-seccomp-profiles/

I did not check all that above yet, I will look closer at this later.

emberflare

@mviereck Thanks for the command! The syscalls used by my application are the following:

access, arch_prctl, brk, close, connect, execve, fstat, futex, getcwd, getdents, geteuid, getpid, getrandom, getuid, lseek, lstat, mmap, mprotect, munmap, openat, prlimit64, read, readlink, rt_sigaction, rt_sigprocmask, set_robust_list, set_tid_address, socket, stat, statx, write

From what I can tell, all of these are allowed by the default seccomp profile of Docker 18.06, only the rule for arch_prctl is listed separately. I also verified that Docker has no problems accessing the seccomp file, which could've been a cause for my problem.

Perhaps I should do a kernel update, as it currently is my last idea to fix this issue (without using --security-opt seccomp=unconfined).