netdev - Re: [PATCH net-next] sctp: add support for SCTP_REUSE

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Message-ID: <20180521005059.GA15233@neilslaptop.think-freely.org>
Date:   Sun, 20 May 2018 20:50:59 -0400
From:   Neil Horman <nhorman@...driver.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     network dev <netdev@...r.kernel.org>, linux-sctp@...r.kernel.org,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        davem@...emloft.net
Subject: Re: [PATCH net-next] sctp: add support for SCTP_REUSE_PORT sockopt

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.

Its a bit frustrating, since SO_REUSEPORT is widely available on multiple
operating systems, but isn't standard (AFAIK).  I would say however, given the
prevalence of the socket level option, we should likely advocate for the removal
of the sctp specific option, or at the least implement and document it as being
an alias for SO_REUSEPORT

As this stands however, its a NACK from me.

Neil