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Old 09-09-2020, 12:05 PM   #1
Registered: Mar 2005
Posts: 147

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Modifying an image file inside a gitlab docker runner

Hi all,

I have a process that I run regularly on my host PC. I download the stock Debian beaglebone image, mount the image as loopback devices, chroot into it, and make some changes.

I would like to do this same thing in a docker container via a gitlab pipeline. That is, when I tag a new version of our software, the pipeline builds it and then makes a new image with the new version installed.

I can't figure out how to get this to work. It seems like the container needs elevated permissions and/or access to the host's loopback device nodes, and I can't really find good resources for how this can work in gitlab. Beyond that, though, all I'm doing is manipulating the contents of a file I own, so it seems strange that there's no way to do that without elevated privileges.

As it is, what I'm trying (and failing) to do, looks something like this:

mkdir -p /tmp/dev
mknod -m 0660 /tmp/dev/loop0 b 7 0
/sbin/losetup -P /tmp/dev/loop0 ./my-image.img
So, I make a loopback device (because one doesn't exist), then try to use losetup to hook it up to that image. Gitlab says the pipeline fails because "losetup: /tmp/dev/loop0: failed to set up loop device: Operation not permitted"

So, in googling that, I found a lot of people doing similar things, but no real solutions (and a lot of very old, obsolete posts). This got me thinking about why I need any elevated privileges at all, because really all I want to do is dump some files into the image.

I don't know much about fuse, but is that a potential answer? Or is there something else I'm missing that would work well here? And if I just need to force docker to let me do these thing, how would that work in gitlab?

Old 09-11-2020, 03:48 PM   #2
Registered: Mar 2015
Distribution: Linux Mint
Posts: 611

Rep: Reputation: 303Reputation: 303Reputation: 303Reputation: 303
Docker runs as root and typically docker containers will run as the root user as well.
I was able to run those commands in a docker container with a root user without that error. Interestingly, it showed up outside my docker container, which probably goes to show why you should be careful running software in a docker container under a root user (which is common).
However, a pipeline will probably not be running the container as a root user. It's probably using a unprivileged user, which would explain why you're getting a operation not permitted error.


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