ZFS?

We only tested mainline filesystems (and only disk-based ones), and even then only the ones that were straightforward to use (i.e. had packages for mkfs ready and had support in the distro we were using so that we could mount it on the host). Although adding support for new ones isn't that hard, especially when you've already done the work for other filesystems; I think we went from supporting ~5 filesystems to ~15 in about a day of work, so testing ZFS at some point in the future shouldn't be too difficult.

There are filesystems in the kernel which trust consistency if the checksums are correct. Ext4 and XFS are not among them, but we have talked to people after the talk who argued that this should be enough. I don't agree with that personally, but it's not my place to try to force filesystem maintainers to change, so the obvious next step seems to be for distros to restrict automouting to a set of whitelisted filesystems (and probably always prompt before mounting by default).