a container image is a tarball of a filesystem

(or several tarballs: 1 per layer)

pensive stick figure with short curly hair: if someone sends me a tarball of their filesystem, how do I use that though?

chroot: change a process’s root directory

If you chroot to /fake/root when it opens the file /usr/bin/redis it’ll get /fake/root/usr/bin/redis instead.

You can “run” a container just by using chroot, like this:

$ mkdir redis; cd redis
$ tar -xzf redis. tar
$ chroot $PWD /usr/bin/redis
# done ! redis is running!

programs can break out of a chroot

chroot:

Illustration of a box labelled “whole filesystem”. Inside it is another box labelled “redis container directory”.

All these files are still there! A root process can access them if it wants.

pivot_root

Illustration of a box labelled “redis container directory”.

You can unmount the old filesystem so it’s impossible to access it.

Containers use pivot_root instead of chroot.

to have a “container” you need more than pivot_root

pivot_root alone won’t let you:

• set CPU/memory limits
• hide other running processes
• use the same port as another process
• restrict dangerous system calls