Re: storing an explicit nonce

On Wed, May 26, 2021 at 04:40:48PM -0400, Bruce Momjian wrote:
> On Wed, May 26, 2021 at 01:56:38PM -0400, Robert Haas wrote:
> > However, I believe that if we store the nonce in the page explicitly,
> > as proposed here, rather trying to derive it from the LSN, then we
> > don't need to worry about this kind of masking, which I think is
> > better from both a security perspective and a performance perspective.
>
> You are saying that by using a non-LSN nonce, you can write out the page
> with a new nonce, but the same LSN, and also discard the page during
> crash recovery and use the WAL copy?
>
> I am confused why checksums, which are widely used, acceptably require
> wal_log_hints, but there is concern that file encryption, which is
> heavier, cannot acceptably require wal_log_hints. I must be missing
> something.
>
> Why can't checksums also throw away hint bit changes like you want to do
> for file encryption and not require wal_log_hints?

One detail might be this extra hint bit FPW case:

https://github.com/postgres/postgres/compare/bmomjian:cfe-01-doc..bmomjian:_cfe-02-internaldoc.patch

However, if a hint-bit-modified page is written to the file system
during a checkpoint, and there is a later hint bit change switching
the same page from clean to dirty during the same checkpoint, we
need a new LSN, and wal_log_hints doesn't give us a new LSN here.
The fix for this is to update the page LSN by writing a dummy
WAL record via xloginsert.c::LSNForEncryption() in such cases.

Is this how file encryption is different from checksum wal_log_hints,
and the big concern?

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EDB https://enterprisedb.com

If only the physical world exists, free will is an illusion.