The new thing isn't the holes after they've been punched, it's that holes can be pre-marked, such that they will be punched by the kernel some undetermined amount of time in the future. Perhaps 2 months later, after rebooting 20 times, and plugging the disk into another computer.
> Tie this to the ability to modify/truncate the file and you are not adding any new possibilities, just new ways to trigger the possibilities (someone who can modify the file can truncate it, write a new file missing some data, etc)
But without taking extra preventative measures, the ability to ever once *have* *had* permission to modify a file might then result in the ability to modify the file (by zeroing out some blocks) any arbitrary time in the future.
> the same way you would find out how much space is used by other sparse files today.
But these new files aren't sparse immediately; volatile data does use up actual space, until it gets dropped on the floor. That's a brand new type of thing.