XFS parent pointers

> Unless they can modify the filesystem on disk, that is.

And that, folks, is why we don't let anyone other than root access block devices directly or mount filesystem images.

Because if anyone can modify the filesystem on disk then we're completely and utterly screwed, parent pointers or not. And that goes for any filesystem that doesn't have a cryptographically secure on-disk format (e.g. XFS, ext4, btrfs, f2fs, etc). i.e. no filesystem except maybe bcachefs is robust against such tampering.

-Dave.