On Tue, May 25, 2021 at 01:54:21PM -0700, Andres Freund wrote: > Hi, > > On 2021-05-25 15:34:04 -0400, Bruce Momjian wrote: > > My point is that we have to full-page-write cases where we change the > > nonce --- we get a new LSN/nonce for free if we are using the LSN as the > > nonce. What has made this approach much easier is that you basically > > tie a change of the nonce to require a change of LSN, since you are WAL > > logging it and every nonce change has to be full-page-write WAL logged. > > This makes the LSN-as-nonce less fragile to breakage than a custom > > nonce, in my opinion, which may explain why my patch is so small. > > This disregards that we need to be able to increment nonces on standbys > / during crash recovery. > > It may look like that's not needed, with an (wrong!) argument like: The > only writes come from crash recovery, which always are associated with a > WAL record, guaranteeing nonce increases. Hint bits are not an issue > because they don't mark the buffer dirty. > > But unfortunately that analysis is wrong. Consider the following > sequence: > > 1) replay record LSN X affecting page Y (FPI replay) > 2) write out Y, encrypt Y using X as nonce > 3) crash > 4) replay record LSN X affecting page Y (FPI replay) > 5) hint bit update to Y, resulting in Y' > 6) write out Y', encrypt Y' using X as nonce > > While 5) did not mark the page as dirty, it still modified the page > contents. Which means that we'd encrypt different content with the same > nonce - which is not allowed. > > I'm pretty sure that there's several other ways to end up with page > contents that differ, despite the LSN not changing.
Yes, I can see that happening. I think occasional leakage of hint bit changes to be acceptable. We might decide they are all acceptable. -- Bruce Momjian <[email protected]> https://momjian.us EDB https://enterprisedb.com If only the physical world exists, free will is an illusion.
