Demi Marie Obenour <demiobenour@gmail.com> writes:
Bcachefs is not very stable right now,
Neither is Spectrum! Given that changing filesystem later if it doesn't work out will be a very easy change to make (up to a point), we can afford to wait. It's an approach that has served us well so far — sometimes focusing on other things means that by the time we have to look at something, the problem has been solved by somebody else. Filesystems are always going to have bugs, so in my opinion the most important thing is to make having good backups easy, so that recovery is possible when something goes wrong, regardless of choice of filesystem. I am very keen for Spectrum to have an integrated backup solution, ideally as easy to use as Time Machine.
and BTRFS is not a good choice from a verified boot perspective. f2fs is what is used in Android and ext4 is used in Chromebooks, so they at least have the backing of Google's security team when it comes to vulnerabilities involving maliciously crafted filesystem images. BTRFS doesn't.
The reason this matters for Spectrum is that verified boot aims to prevent system compromise from persisting across reboots, and an attacker who has compromised a Spectrum system can craft whatever image they want on the writable volume.
Would it make sense to use f2fs or ext4? That means no reflinks and no snapshots, which would be annoying at least. Another option might be to use FUSE for the writable volume, with kernel filesystems only used for the (signed and dm-verity protected) root volume. This is the only option supported by Linux's upstream maintainers, who (with the notable exception of Kent Overstreet) appear to have no interest in hardening filesystems against maliciously crafted images.
I think snapshots are going to be very important for us to do things like the aforementioned integrated backups, and it would be very unfortunate to have to limit ourselves to out of date filesystems that lack modern features like that.