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loop device in a Linux container? [Resolved]

I'm attempting to use a loop device inside a container, to mount some image file:

> sudo losetup /dev/loop0 test.img
losetup: /dev/loop0: failed to set up loop device: No such file or directory

/dev/loop0 indeed doesn't exist, and

> sudo mknod /dev/loop0 b 7 0
mknod: ‘/dev/loop0’: Operation not permitted

How can I make this work? Does the container need some cgroup permission that it might not have?

Question Credit: Johannes Ernst
Question Reference
Asked June 12, 2019
Posted Under: Network
2 Answers

If you're using systemd-nspawn, start up your container with the --capability=CAP_MKNOD command line switch. This will allow you to create device nodes inside your container. Then create a loop device like this:

# mknod /dev/loop0 b 7 0

Remember that this loop device is shared with the host and is called /dev/loop0 there as well. And that it is now possible to access host devices if you know the major and minor numbers. There could also be other consequences that I haven't thought about. Be warned.

credit: Troels Folke
Answered June 12, 2019
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