RE: [PATCHv2 net-next 3/5] sctp: add SCTP_EXPOSE_POTENTIALLY_FAILED_STATE sockopt

[David Laight <David.Laight@ACULAB.COM>]

From: Neil Horman <nhorman@tuxdriver.com>
> Sent: 14 October 2019 13:42
> To: Xin Long <lucien.xin@gmail.com>
> Cc: David Laight <David.Laight@ACULAB.COM>; network dev <netdev@vger.kernel.org>; linux-sctp@vger.kernel.org; Marcelo
> Ricardo Leitner <marcelo.leitner@gmail.com>; davem@davemloft.net
> Subject: Re: [PATCHv2 net-next 3/5] sctp: add SCTP_EXPOSE_POTENTIALLY_FAILED_STATE sockopt
> 
> On Mon, Oct 14, 2019 at 04:36:34PM +0800, Xin Long wrote:
> > On Thu, Oct 10, 2019 at 12:18 AM Neil Horman <nhorman@tuxdriver.com> wrote:
> > >
> > > On Tue, Oct 08, 2019 at 11:28:32PM +0800, Xin Long wrote:
> > > > On Tue, Oct 8, 2019 at 9:02 PM David Laight <David.Laight@aculab.com> wrote:
> > > > >
> > > > > From: Xin Long
> > > > > > Sent: 08 October 2019 12:25
> > > > > >
> > > > > > This is a sockopt defined in section 7.3 of rfc7829: "Exposing
> > > > > > the Potentially Failed Path State", by which users can change
> > > > > > pf_expose per sock and asoc.
> > > > >
> > > > > If I read these patches correctly the default for this sockopt in 'enabled'.
> > > > > Doesn't this mean that old application binaries will receive notifications
> > > > > that they aren't expecting?
> > > > >
> > > > > I'd have thought that applications would be required to enable it.
> > > > If we do that, sctp_getsockopt_peer_addr_info() in patch 2/5 breaks.
> > > >
> > > I don't think we can safely do either of these things.  Older
> > > applications still need to behave as they did prior to the introduction
> > > of this notification, and we shouldn't allow unexpected notifications to
> > > be sent.
> > Hi, Neil
> >
> > I think about again, and also talked with QE, we think to get unexpected
> > notifications shouldn't be a problem for user's applications.
> >
> On principle, I disagree.  Regardless of what the RFC does, we shouldn't
> send notifications that an application aren't subscribed to.  Just
> because QE doesn't think it should be a problem (and for their uses it
> may well not be an issue), we can't make that general assumption.
> 
> > RFC actually keeps adding new notifications, and a user shouldn't expect
> > the specific notifications coming in some exact orders. They should just
> > ignore it and wait until the ones they expect. I don't think some users
> > would abort its application when getting an unexpected notification.
> >
> To make that assertion is to discount the purpose of the SCTP_EVENTS
> sockopt entirely.  the SCTP_EVENTS option is a whitelist operation, so
> they expect to get what they subscribe to, and no more.
> 
> > We should NACK patchset v3 and go with v2. What do you think?
> >
> No, we need to go with an option that maintains backwards compatibility
> without relying on the assumption that applications will just ignore
> events they didn't subscribe to.  Davids example is a case in point.

Although I don't enable the SCTP_PEER_ADDR_CHANGE indications.
But rfc 6458 doesn't say that the list might be extended.

Aren't there 3 separate items here:
1) The SCTP protocol changes (to better handle primary path failure).
2) The SCTP_GET_PEER_ADDR_INFO sockopt.
3) The MSG_NOTIFICATION indication for SCTP_ADDR_POTENTIALLY_FAILED.

Looking at RFC 7829 section 7.3.
7.3 defines SCTP_EXPOSE_POTENTIALLY_FAILED_STATE.
For compatibility this must default to 'disabled'.
This is even true if the application has set the SCTP_PEER_ADDR_THLDS.

	David

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