On Tue, Jan 12, 2021 at 10:47 AM Stephen Frost <sfrost@snowman.net> wrote:
This is an interesting question but ultimately I don't think we should be looking at this from the perspective of allowing arbitrary changes to the page format. The challenge is that much of the page format, today, is defined by a C struct and changing the way that works would require a great deal of code to be modified and turn this into a massive effort, assuming we wish to have the same compiled binary able to work with both unencrypted and encrypted clusters, which I do believe is a requirement.
The thought that I had was to, instead, try to figure out if we could fudge some space by, say, putting a 128-bit 'hole' at the end of the page and just move pd_special back, effectively making the page seem 'smaller' to all of the code that uses it, except for the code that knows how to do the decryption. I ran into some trouble with that but haven't quite sorted out what happened yet. Other ideas would be to put it before pd_special, or maybe somewhere else, but a lot depends on the code's expectations.
I agree that we should not make too many changes to affect the use of unencrypted clusters. But as a personal opinion only, I don't think it's a good idea to add some "implicit" tricks. To provide an inspiration, can we add a flag to mark whether the page format has been changed:
In this way, I think it has little effect on the unencrypted cluster, and we can also modify the page format as we wish. Of course, it's also possible that I didn't understand your design correctly, or there's something wrong with my idea. :D
