Docker Simplifications: Bugtowne City

I have a fairly complex docker container that I run on all of my machines. I would like to simplify it in a number of ways and for some reason I decided that it would be interesting to start on that project last night. The simplifications that I want to do are as follows:

  • Have all three daemons log to /proc/1/fd/1; this would remove one of my volumes and let me view logs with just docker logs offlineimap

  • Start docker with docker run ... -u 1000 instead of doing a sudo inside of the container. To some extent this should simplify things by removing a silly wrapper process, but also it should make things a little bit more secure.

  • Generate all config files at startup time so that parts of them can be based on env vars; this might allow me to remove the .netrc volume.

  • Finally, I based my image off of phusion, which is based off of ubuntu. This means that my image is 320M; non-trivial! It pulls in perl, python, and python3. If I rewrite my ~30 line perl program in python and don’t use python3, with a docker image based on alpine my container could easily shrink to 50M. The main thing missing there is the phusion init script that reaps zombie processes. Someone should rewrite that in Go or something.

So sadly, I had really weird problems with writing to init’s STDOUT. I know it can work:

Terminal 1:

$ docker run --rm --name tmp --privileged -it debian bash

Terminal 2:

$ docker exec -it tmp bash
root@38655f5d1154:/# echo "woo" > /proc/1/fd/1

Terminal 1: root@38655f5d1154:/# woo

But when I tried to use it from my container I got a lot of permission problems, likely because the processes were running as uid 1000. So then I figured that I’d switch to the second issue to consolidate the uid usage within the container and come back to the logging thing.

Note also that I used --privileged above. I suspect that this is due to some AppArmor stuff. I’ve already had to modify my AppArmor profile for Docker; I will track down the real modification so I don’t have to use --privileged, but this got me going faster for now.

Unfortunately I ran into other weird issues with that! I got this error running with -u 1000:

*** Killing all processes...
Traceback (most recent call last):
  File "/sbin/my_init", line 334, in <module>
  File "/sbin/my_init", line 252, in main
    import_envvars(False, False)
  File "/sbin/my_init", line 61, in import_envvars
    for envfile in listdir("/etc/container_environment"):
  File "/sbin/my_init", line 49, in listdir
    return sorted(os.listdir(path))
PermissionError: [Errno 13] Permission denied: '/etc/container_environment'

So I figured I needed to fix the perms of that directory, so I added this to the RUN statements inside the Dockerfile:

RUN chown 1000:1000 /etc/container_environment -R

I can then do a docker run -itu 1000 mycontainer bash and I see this:

user@1d0d69fe032e:/$ ls -dlh /etc/container_environment
drwx------ 2 user user 4.0K Apr  2 23:32 /etc/container_environment
user@1d0d69fe032e:/$ ls -lh /etc/container_environment
ls: cannot open directory /etc/container_environment: Permission denied

I don’t see why I wouldn’t be able to open that directory… Also, if I start without the -u 1000 everything looks fine:

root@54d1f1699037:/# ls -lh /etc/container_environment
total 12K
-rw-r--r-- 1 user user  2 Jan 20 10:08 INITRD
-rw-r--r-- 1 user user 11 Jan 20 10:09 LANG
-rw-r--r-- 1 user user 11 Jan 20 10:09 LC_CTYPE

The maintainers of phusion told me that this is likely a kernel bug, which is great. So my plan next is to see if I can boot up a newer kernel and still reproduce the problem. I might be able to ditch phusion, as I want to switch to alpine anyway, but I wanted to make a single change at a time.

Anyway, all of the above are things that should work. If I get them to work I’ll make other posts detailing how.

Posted Fri, Apr 3, 2015

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