Greetings,
* Bruce Momjian (bruce@momjian.us) wrote:
> On Tue, Jan 12, 2021 at 01:11:29PM -0500, Stephen Frost wrote:
> > > I think one big question is that, since we are using a streaming cipher,
> > > do we care about hint bit changes showing to users? I actually don't
> > > know. If we do, some kind of dummy LSN record might be required, as you
> > > suggested.
> >
> > I don't think there's any doubt that we need to make sure that the IV is
> > distinct and advancing the LSN to get a new one when needed for this
> > case seems like it's probably the way to do that. Hint bit change
> > visibility to users isn't really at issue here- we can't use the same IV
> > multiple times. The two options that we have are to either not actually
> > update the hint bit in such a case, or to make sure to change the
> > LSN/IV. Another option would be to, if we're able to make a hole to put
> > the GCM tag on to the page somewhere, further widen that hole to include
> > an additional space for a counter that would be mixed into the IV, to
> > avoid having to do an XLOG NOOP.
>
> Well, we have eight unused bits in the IV, so we could just increment
> that for every hint bit change that uses the same LSN, and then force a
> dummy WAL record when that 8-bit counter overflows --- that seems
> simpler than logging hint bits.
Sure, as long as we have a place to store that information.. We need to
have the full IV available when we go to decrypt the page.
Thanks,
Stephen
