  HPC centers face increasing demand for software flexibility, and there is
growing consensus that Linux containers are a promising solution. However,
existing container build solutions require root privileges and cannot be used
directly on HPC resources. This limitation is compounded as supercomputer
diversity expands and HPC architectures become more dissimilar from commodity
computing resources. Our analysis suggests this problem can best be solved with
low-privilege containers. We detail relevant Linux kernel features, propose a
new taxonomy of container privilege, and compare two open-source
implementations: mostly-unprivileged rootless Podman and fully-unprivileged
Charliecloud. We demonstrate that low-privilege container build on HPC
resources works now and will continue to improve, giving normal users a better
workflow to securely and correctly build containers. Minimizing privilege in
this way can improve HPC user and developer productivity as well as reduce
support workload for exascale applications.
