Date:   Tue, 22 May 2018 07:51:51 -0400
From:   Neil Horman <nhorman@...driver.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     Michael Tuexen <tuexen@...muenster.de>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        network dev <netdev@...r.kernel.org>,
        linux-sctp@...r.kernel.org, davem <davem@...emloft.net>
Subject: Re: [PATCH net-next] sctp: add support for SCTP_REUSE_PORT sockopt

On Tue, May 22, 2018 at 03:07:57PM +0800, Xin Long wrote:
> On Mon, May 21, 2018 at 9:48 PM, Neil Horman <nhorman@...driver.com> wrote:
> > On Mon, May 21, 2018 at 02:16:56PM +0200, Michael Tuexen wrote:
> >> > On 21. May 2018, at 13:39, Neil Horman <nhorman@...driver.com> wrote:
> >> >
> >> > On Sun, May 20, 2018 at 10:54:04PM -0300, Marcelo Ricardo Leitner wrote:
> >> >> On Sun, May 20, 2018 at 08:50:59PM -0400, Neil Horman wrote:
> >> >>> On Sat, May 19, 2018 at 03:44:40PM +0800, Xin Long wrote:
> >> >>>> This feature is actually already supported by sk->sk_reuse which can be
> >> >>>> set by SO_REUSEADDR. But it's not working exactly as RFC6458 demands in
> >> >>>> section 8.1.27, like:
> >> >>>>
> >> >>>>  - This option only supports one-to-one style SCTP sockets
> >> >>>>  - This socket option must not be used after calling bind()
> >> >>>>    or sctp_bindx().
> >> >>>>
> >> >>>> Besides, SCTP_REUSE_PORT sockopt should be provided for user's programs.
> >> >>>> Otherwise, the programs with SCTP_REUSE_PORT from other systems will not
> >> >>>> work in linux.
> >> >>>>
> >> >>>> This patch reuses sk->sk_reuse and works pretty much as SO_REUSEADDR,
> >> >>>> just with some extra setup limitations that are neeeded when it is being
> >> >>>> enabled.
> >> >>>>
> >> >>>> "It should be noted that the behavior of the socket-level socket option
> >> >>>> to reuse ports and/or addresses for SCTP sockets is unspecified", so it
> >> >>>> leaves SO_REUSEADDR as is for the compatibility.
> >> >>>>
> >> >>>> Signed-off-by: Xin Long <lucien.xin@...il.com>
> >> >>>> ---
> >> >>>> include/uapi/linux/sctp.h |  1 +
> >> >>>> net/sctp/socket.c         | 48 +++++++++++++++++++++++++++++++++++++++++++++++
> >> >>>> 2 files changed, 49 insertions(+)
> >> >>>>
> >> >>> A few things:
> >> >>>
> >> >>> 1) I agree with Tom, this feature is a complete duplication of the SK_REUSEPORT
> >> >>> socket option.  I understand that this is an implementation of the option in the
> >> >>> RFC, but its definately a duplication of a feature, which makes several things
> >> >>> really messy.
> >> >>>
> >> >>> 2) The overloading of the sk_reuse opeion is a bad idea, for several reasons.
> >> >>> Chief among them is the behavioral interference between this patch and the
> >> >>> SO_REUSEADDR socket level option, that also sets this feature.  If you set
> >> >>> sk_reuse via SO_REUSEADDR, you will set the SCTP port reuse feature regardless
> >> >>> of the bind or 1:1/1:m state of the socket.  Vice versa, if you set this socket
> >> >>> option via the SCTP_PORT_REUSE option you will inadvertently turn on address
> >> >>> reuse for the socket.  We can't do that.
> >> >>
> >> >> Given your comments, going a bit further here, one other big
> >> >> implication is that a port would never be able to be considered to
> >> >> fully meet SCTP standards regarding reuse because a rogue application
> >> >> may always abuse of the socket level opt to gain access to the port.
> >> >>
> >> >> IOW, the patch allows the application to use such restrictions against
> >> >> itself and nothing else, which undermines the patch idea.
> >> >>
> >> > Agreed.
> >> >
> >> >> I lack the knowledge on why the SCTP option was proposed in the RFC. I
> >> >> guess they had a good reason to add the restriction on 1:1/1:m style.
> >> >> Does the usage of the current imply in any risk to SCTP sockets? If
> >> >> yes, that would give some grounds for going forward with the SCTP
> >> >> option.
> >> >>
> >> > I'm also not privy to why the sctp option was proposed, though I expect that the
> >> > lack of standardization of SO_REUSEPORT probably had something to do with it.
> >> > As for the reasoning behind restriction to only 1:1 sockets, if I had to guess,
> >> > I would say it likely because it creates ordering difficulty at the application
> >> > level.
> >> >
> >> > CC-ing Michael Tuxen, who I believe had some input on this RFC.  Hopefully he
> >> > can shed some light on this.
> >> Dear all,
> >>
> >> the reason this was added is to have a specified way to allow a system to
> >> behave like a client and server making use of the INIT collision.
> >>
> >> For 1-to-many style sockets you can do this by creating a socket, binding it,
> >> calling listen on it and trying to connect to the peer.
> >>
> >> For 1-to-1 style sockets you need two sockets for it. One listener and one
> >> you use to connect (and close it in case of failure, open a new one...).
> >>
> >> It was not clear if one can achieve this with SO_REUSEPORT and/or SO_REUSEADDR
> >> on all platforms. We left that unspecified.
> >>
> >> I hope this makes the intention clearer.
> >>
> > I think it makes the intention clearer yes, but it unfortunately does nothing in
> > my mind to clarify how the implementation should best handle the potential
> > overlap in functionality.  What I see here is that we have two functional paths
> > (the SO_REUSEPORT path and the SCTP_PORT_REUSE path), which may or may not
> > (depending on the OS implementation achieve the same functional goal (allowing
> > multiple sockets to share a port while allowing one socket to listen and the
> > other connect to a remote peer).  If both implementations do the same thing on a
> > given platform, we can either just alias one to another and be done, but if they
> > don't then we either have to implement both paths, and ensure that the
> > SO_REUSEPORT path is a no-op/error return for SCTP sockets, or that each path
> > implements a distinct feature set that is cleaarly documented.
> >
> > That said, I think we may be in luck.  Looking at the connect and listen paths,
> > it appears to me that:
> >
> > 1) Sockets ignore SO_REUSEPORT in the connect and listen paths (save for any
> > autobinding) so it would appear that the intent of the SCTP rfc can be honored
> > via SO_REUSEPORT on linux.
> >
> > 2) SO_REUSEPORT prevents changing state after a bind has occured, so we can honr
> > that part of the SCTP RFC.
> >
> > The only missing part is the restriction that SCTP_REUSE_PORT has which is
> > unaccounted for is that 1:M sockets aren't allowed to enable port reuse.
> > However, I think the implication from Michaels description above is that port
> > reuse on a 1:M socket is implicit because a single socket can connect and listen
> > in that use case, rather than there being a danger to doing so.
> >
> > As such, I would propose that we implement this socket option by simply setting
> > the sk->sk_reuseport field in the sock structure, and document the fact that
> > linux does not restrict port reuse from 1:M sockets.
> Note that, sk->sk_reuseport is not affecting linux SCTP socket at all now.
> linux SCTP socket doesn't really have SO_REUSEADDR (sk->sk_reuse)
> support, but use sk->sk_reuse as REUSE_PORT, (yes, it is confusing).
> Pls refer to sctp_get_port_local().
> 
No, its not used now, but if you do use it to do something specific to SCTP (via
the SCTP_REUSE_PORT socket option), you risk aliasing SO_REUSEPORT behavior to
it, and if it doesn't match what the RFC behavior mandates, thats a problem.

> So I'm not sure using sk->sk_reuseport here means we will drop sk->sk_reuse
> use in linux SCTP but use sk->sk_reuseport instead, or we will think that socket
> enables 'port reuse' when either of them is set.
> 
I don't think we would drop the behavior of sk_reuse here, why would we?  As far
as I can see, the behavior of SO_REUSEADDR (not SO_REUSEPORT), isn't in
question, is it?

> Note some users may be already using SO_REUSEADDR to enable the 'port
> reuse' in linux sctp socket. If we're changing to sk->sk_reuseport, we may face
> a compatibility problem.
> 
I don't see how the behavior of SO_REUSEADDR is in question here.  All I'm
suggesting is that you simplify this patch so that the SCTP_REUSE_PORT socket
option set sk_reuseport, as that option to my eyes conforms to the sctp rfc
requirements.  Or am I' missing something?

Neil

> 
> >
> > Thoughts?
> > Neil
> >
> 

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