Date:   Tue, 22 May 2018 15:07:57 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     Neil Horman <nhorman@...driver.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 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().

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.

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.


>
> Thoughts?
> Neil
>

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