Re: [PATCH nf] netfilter: nf_tables: do not refresh timeout when resetting element

[Phil Sutter <phil@nwl.cc>]

On Tue, Oct 03, 2023 at 08:03:10PM +0200, Pablo Neira Ayuso wrote:
> On Tue, Oct 03, 2023 at 07:52:31PM +0200, Phil Sutter wrote:
> > On Tue, Oct 03, 2023 at 07:21:33PM +0200, Pablo Neira Ayuso wrote:
> > > On Tue, Oct 03, 2023 at 05:57:59PM +0200, Phil Sutter wrote:
> > > > On Tue, Oct 03, 2023 at 09:46:46AM +0200, Pablo Neira Ayuso wrote:
> > > > > On Tue, Oct 03, 2023 at 12:55:41AM +0200, Phil Sutter wrote:
> > > > > > On Tue, Oct 03, 2023 at 12:17:53AM +0200, Pablo Neira Ayuso wrote:
> > > > > > > On Mon, Oct 02, 2023 at 11:50:25PM +0200, Pablo Neira Ayuso wrote:
> > > > > > > > On Mon, Oct 02, 2023 at 08:06:42PM +0200, Phil Sutter wrote:
> > > > > > > > > On Mon, Oct 02, 2023 at 11:05:16AM +0200, Pablo Neira Ayuso wrote:
> > > > > > > > > > The dump and reset command should not refresh the timeout, this command
> > > > > > > > > > is intended to allow users to list existing stateful objects and reset
> > > > > > > > > > them, element expiration should be refresh via transaction instead with
> > > > > > > > > > a specific command to achieve this, otherwise this is entering combo
> > > > > > > > > > semantics that will be hard to be undone later (eg. a user asking to
> > > > > > > > > > retrieve counters but _not_ requiring to refresh expiration).
> > > > > > > > > 
> > > > > > > > > From a users' perspective, what is special about the element expires
> > > > > > > > > value disqualifying it from being reset along with any counter/quota
> > > > > > > > > values?
> > > > > > > > >
> > > > > > > > > Do you have a PoC for set element reset via transaction yet? Can we
> > > > > > > > > integrate non-timeout resets with it, too? Because IIUC, that's an
> > > > > > > > > alternative to the pending reset locking.
> > > > > > > > 
> > > > > > > > Problem is listing is not supported from transaction path, this is
> > > > > > > > using existing netlink dump infrastructure which runs lockless via
> > > > > > > > rcu. So we could support reset, but we could not use netlink dump
> > > > > > > > semantics to fetch the values, and user likely wants this to
> > > > > > > > fetch-and-reset as in ctnetlink.
> > > > > > > 
> > > > > > > Well, with NLM_F_ECHO, it should be possible to explore reset under
> > > > > > > commit_mutex, but is it really worth the effort?
> > > > > > 
> > > > > > I don't understand. Wasn't it your proposal to move things into the
> > > > > > transaction? Above you write: "element expiration should be refresh via
> > > > > > transaction instead". I asked what is special about timeout, why not
> > > > > > handle all element state reset the same way?
> > > > > > 
> > > > > > > With two concurrent threads, we just want to ensure that no invalid
> > > > > > > state shows in the listing (you mentioned it is possible to list
> > > > > > > negative values with two threads listing-and-resetting at the same
> > > > > > > time).
> > > > > > 
> > > > > > It's not just in the listing, the actual values underrun. A quota e.g.
> > > > > > will immediately deny.
> > > > > > 
> > > > > > > I think we should just make sure something valid is included in the
> > > > > > > listing, but as for the two threads performing list-and-reset, why
> > > > > > > ensure strict serialization?
> > > > > > 
> > > > > > I seem to lack context here. Is there an alternative to "strict"
> > > > > > serializing? Expressions' dump+reset callbacks must not run multiple
> > > > > > times at the same time for the same expression. At least not how they
> > > > > > are currently implemented.
> > > > > > 
> > > > > > > This is a rare operation to fetch statistics, most likely having a
> > > > > > > single process performing this in place? So we are currently
> > > > > > > discussing how to fix the (negative) non-sense in the listing.
> > > > > > > 
> > > > > > > > > What we have now is a broad 'reset element', not specifying what to
> > > > > > > > > reset. If the above is a feature being asked for, I'd rather implement
> > > > > > > > > 'reset element counter', 'reset element timeout', 'reset element quota',
> > > > > > > > > etc. commands.
> > > > > > > > 
> > > > > > > > We are currently discussing how to implement refresh timeout into the
> > > > > > > > transaction model.
> > > > > > > > 
> > > > > > > > I would suggest we keep this chunk away by now for the _RESET command,
> > > > > > > > until we agree on next steps.
> > > > > > 
> > > > > > I would suggest to leave things as-is until there's hard evidence why it
> > > > > > has to change now or there is a viable alternative implementation.
> > > > > 
> > > > > Leave things as is means we will have this implicit refresh in the
> > > > > element refresh. We have no such semantics in conntrack, for example,
> > > > > and conntrack can be seen as a hardcoded set with a fixed number of
> > > > > tuples.
> > > > 
> > > > It's just as implicit as counter or quota reset. I define "reset
> > > > element" command as "reset any state in given element", so from my
> > > > perspective it makes perfectly sense to reset the timeout as well.
> > > 
> > > The timer is usually updated from the packet path. We are now planning
> > > to support for refreshing the timer from control plane which is not
> > > supported.
> > 
> > This implies that people create dynamic sets and then use reset command
> > on the temporary elements. I would assume one either lets packet path
> > "maintain" elements or control path but not both.
> > 
> > > Note it is possible to set custom timeouts in elements too, for
> > > example:
> > > 
> > >         # nft add element x y { 1.2.3.4 expires 50s }
> > 
> > I assumed this is for dump'n'restore purposes. If you set a custom
> > *timeout* when adding a set, the current reset code respects that.
> 
> This could be an exception, ie. a specific timeout for a given
> element, I am afraid there are more usecases that just a
> dump'n'restore. User is really free to add specific timeouts for the
> sets it adds. The default set timeout only applies if user does not
> specify a timeout for new elements.

Yes, and if a user chooses a custom timeout like so:

| nft add element x y { 1.2.3.4 timeout 50s }

the element will timeout in 50s and reset command will reset expires
value to 50s not the set's default. Adding an element with a defined
expires value makes it expire within that time but the set's timeout
value applies.

Or am I missing a use-case requiring to use expires instead of timeout?

> > > while default set policy might be different 3m. In this case, you
> > > assume resetting to the default set timeout policy (3m) is fine.
>