It would be good to check whether other distros have the same or newer glibc as Bionic. I suspect Debian 9 has older glibc.

Older glibc is forward-compatible with newer, but the reverse isn't true -- Programs compiled with newer glibc won't run on older glibc.

I'll try to check the versions on the different distros some time today. Should work from just booting up a couple of LiveCD images in VirtualBox and running ldd --version on each of them...

We can also just check the Debian repositories: https://tracker.debian.org/pkg/glibc
Their old-stable (indeed Debian 9, Stretch) is on glibc 2.24

Ubuntu Bionic has glibc 2.27
so you are right about this possible implication. But also to think about: if targeting Debian as supported platform, and you want backwards-compatibility for their older stable releases, then building on Ubuntu might not be the best platform, because it reaches End-of-Life earlier than its Debian counterparts.

There is this PPA with many backports, including RPM, for Xenial: https://launchpad.net/~savoury1/+archive/ubuntu/backports

Maybe we could use that PPA, or make our own (to be more safe)

it is a bit problematic that official Xenial support is now dropped, so there are no more official Xenial repositories hosted by Ubuntu/Canonical themselves.

We could also use the Xenial image, and then just add the bionic repository in the script, and just install rpm from the official bionic repo.

I will close this PR in favor of #22433 because of the glibc complications mentioned by @DeeDeeG