Research Spike on Docker Volume Permission

I love Docker. Who doesn’t! But it’s got one issue that continue to bugs me, and is always biting me in the arse.

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In many environments, such as on a dev box or a build slave, it can be useful to use docker to wrap a build tool, or some other one-shot app. In other words you have some folder on your host machine that you want to modify and update using a dockerised app.

The usually way to do this is use --volume to mount in the host’s directory into the container. However this causes problems.

Most dockerised apps are designed to run as root. (There is a --user flag for docker run which I’ll get to later but it doesn’t seem to be used all that often.) This means that any files or folders added by the app in your volume end up belonging to the root user, rather than the user running docker run.

While it’s possible to compensate for this, it’s awkward as hell. I’ve done a short research spike into some possible solutions and workarounds that I’ve explored in the past, or have discovered more recently.

I thought this is a good a place as any to store that research for myself and others.

Note: This is really only an initial research spike so don’t expect to find all the answers!

Using --user flag with docker run

This seems like the obvious answer at first. However it’s not quite so perfect as it seems.

The problem is that the docker image itself doesn’t know what user id you’re going to use when you docker run. So say you’re uid is 1111 there will be no entry in /etc/passwd, no home directory, nothing, unless you rebuild the docker image and add it there.

Without /etc/passwd and other bits, many common linux commands start acting squirely. For example python gets upset looking for site packages if there’s no home directory. SSH can misbehave too, even if it has a valid $HOME if there is no entry in /etc/passwd although I never got to the bottom of why. You really can’t blame the apps for this though, it’s not entirely mad to assume that the user owning the process has a minimal footprint on the system, but it does complicate matters.

This means you’ve got to potentially rebuild your docker image for every potential user you intend to use in --user.

It also means that if you want to safely do “root-like” things, one of the big advantages of using docker, you can’t do so.

Wrapping script inside the docker image

You could add a script in your docker image that wraps whatever command you intend to run with some clean up code to ensure any written files are chmodded back to the external users uid:gid.

This works well enough, but unless every image you want to use already does it, it will probably mean you have to create your own image for almost every image you use.

Said script also needs to know what the external UID is, so you’ve now got to pass that into the docker run command. Then you’ve got to standardise how you do this in multiple images, and chances are you’ll occasionally get something wrong and litter the volume with root files.

Wrapping script outside the docker image

Same principle as above, but instead of writing a script that’s part of the docker image you have a script that wraps the docker run command and fixes the files afterwards.

On the upside you no longer have to do anything special when you run docker, but on the downside it likely wont work without using sudo which might not be available.

A colleague got around this problem by calling docker run a second time to do whatever privileged cleanup steps he needed aka docker as a sudo replacement, which is a pretty novel solution, but obviously completely mad.

Also, as with the previous ‘solution’ things also get trickier if you want to have multiple non-root uids in play. I think it’s possible but a lot more gnarly.

Using bindfs

bindfs can be used to bind mount one directory in another location and map between users on one side and the other.

So for example if one had folder /foo one could mount it in /bar with the option --map=jblogs/root. If there are files in /foo owned by jblogs they will appear in /bar as owned by root. If new files are created by root in /bar they will appear on /foo as owned by jblogs.

So rather than just adding a working directory as a volume to docker, one would first create a bindfs mount of the working directory with a mapping and use that mount point as the volume with docker run.

IMO this is a nicer solution than any of the above, as it still allows you to be root inside the container, but without polluting the working directory.

It has a bunch of downsides though; It still requires you to do some scripting around the docker commands to make it work cleanly. You need bindfs (or something like it) on the host system and permission to use it. Bindfs in particular is apparently dog slow so I have read (Admittedly in a random github comment). I’m not sure how bindfs works if you try and use a user id not covered by the mappings. This could cause problems.

Using docker volume create

This command can be used to manually create a docker volume which can be later mounted into the container. When creating this volume one seems to be able to use any of the options normally present on the “mount” command. This presumably would allow one to use something like bindfs above but keeping the config within docker world.

That being said, it equally shares all of the problems with the previous approach.

It is however the closest thing we’ve got to an official solution to the problem. The bug Make uid & gid configurable for shared volumes #7198 was closed when the local volumes feature appeared;

"You can do this by creating local volumes, it just landed in master #20262
 ... Closing this as fixed."

Unfortunately how exactly local volumes could be used to achieve this was never explained and the rest of the bug is people trying to work out what this cryptic message meant. It’s possible there is some way to use local volumes to achieve this affect without bringing bindfs into the picture, but it’s not obvious from my brief investigation.

User Namespaces

I’m going to be slightly vague here because I’m no expert on lxc and this feature in particular.

User Namespaces allow a kernel level mapping between users inside and outside the container.

So if we mapped out container to a namespace starting at 10000, the root user (uid=0) inside the container maps to uid 10000 outside. User with uid=1 inside maps to 10001 outside and so on.

Immediately its obvious how this is almost exactly what we want. I haven’t yet had a chance to try it out, but it seems like you could setup a pretty nice solution.

To enable it’s use in dockerd see this help page. Or for a more complete guide there’s a good blog post here.

It’s not without it’s disadvantages though; Firstly it requires user namespaces to be compiled and enabled on the kernel. It’s not in the stock arch kernel yet, and afaik it’s not in ubuntu or redhat either.

It also requires host system configuration changes including careful planning of user id ranges.

I’m also unclear on the relationship between the owning uid, the subuid and if/how they overlap. It may still be possible that you end up with files owned by the wrong user, but at least it wont be root.

Conclusion

Unfortunately, I ran out of time about here so I couldn’t really play with user namespaces, but if I get the chance to do more it’s probably the approach I’ll try next.

If I do I’ll try and follow up with how well it went.

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