update element timeout support [was Re: [PATCH nf 1/2] netfilter: nft_set_rbtree: move sync GC from insert path to set->ops->commit]

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Sun, Oct 01, 2023 at 11:08:16PM +0200, Florian Westphal wrote:
> Not convinced. In particular, I don't understand what this has to do
> with async vs. sync gc.  To me this depends how we want to handle
> elements that have expired or expire during transaction.
> 
> Example:
> 
> Element E1, times out in 1 hour
> Element E2, times out in 1 second
> Element E3, timed out (1 second ago, 3 minutes ago, doesn't matter).
> 
> Userspace batch to kernel:
> Update Element E1 to time out in 2 hours.
> Update Element E2 to time out in 1 hour.
> Update Element E3 to time out in 1 hour.
> 
> What is the expected outcome of this request?
> 
> Ignore E3 being reaped already and refresh the timeout (resurrection?)

No resurrection, the element might have counters, it already expired.

> Ignore E3 being reaped already and ignore the request?
> Fail the transaction as E3 timed out already ("does not exist")?

Add a new E3. If NLM_F_EXCL is specified, then fail with "does not exist"

> Now, what about E2?  If transaction is large, it could become
> like E3 *during the transaction* unless we introduce some freezing
> mechanism.  Whats the expected outcome?
> 
> Whats the expected outcome if there is some other, unrelated
> failure? I assume we have to roll back all the timeout updates, right?

We annotate the new timeout in transaction object, then refresh the
timeout update in the commit phase.

> If so, why not temporarily make the timeouts effective right away
> and then roll back?

You mean, from the preparation phase? Then we need to undo what has
been done, in case of --check / abort path is exercised, this might
just create a bogus element listing.

> What if userspace asks to *shrink* timeouts? Same scenario:
> 
> Update Element E1 to time out in 1 second
> Update Element E2 to time out in 1 second
> Update Element E3 to time out in 1 second
> 
> We could say 'not supported' of course, would avoid the
> rollback headache, because in this case long transaction
> gc would definitely start to pick some of those elements
> up for reaping...

No need for rollback if new timeout is store in the transaction
object, we just set the new timeout from _commit() step in the
NEWSETELEM case, which has to deal with updates. Other objects follow
a similar approach.