Date:   Mon, 21 May 2018 16:09:31 +0200
From:   Michael Tuexen <tuexen@...muenster.de>
To:     Neil Horman <nhorman@...driver.com>
Cc:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Xin Long <lucien.xin@...il.com>,
        network dev <netdev@...r.kernel.org>,
        linux-sctp@...r.kernel.org, davem@...emloft.net
Subject: Re: [PATCH net-next] sctp: add support for SCTP_REUSE_PORT sockopt

> On 21. May 2018, at 15:48, 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.
> 
> Thoughts?
Sounds acceptable to me...

Best regards
Michael
> Neil
> 


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