netdev - Re: [PATCH] udp: Allow to defer reception until connect() happened

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Date:   Wed, 28 Nov 2018 18:19:07 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Christoph Paasch <cpaasch@...le.com>
Cc:     netdev <netdev@...r.kernel.org>, Ian Swett <ianswett@...gle.com>,
        lhedstrom@...le.com, jri.ietf@...il.com
Subject: Re: [PATCH] udp: Allow to defer reception until connect() happened

On Wed, Nov 28, 2018 at 5:57 PM Christoph Paasch <cpaasch@...le.com> wrote:
>
> There are use-cases where a host wants to use a UDP socket with a
> specific 4-tuple. The way to do this is to bind() and then connect() the
> socket. However, after the bind(), the socket starts receiving data even
> if it does not match the intended 4-tuple. That is because after the
> bind() UDP-socket will match in the lookup for all incoming UDP-traffic
> that has the specific IP/port.
>
> This patch prevents any incoming traffic until the connect() system-call
> is called whenever the app sets the UDP socket-option
> UDP_WAIT_FOR_CONNECT.

Please do not add something that could mislead applications writers to
think UDP stack can scale.

UDP stack does not have a full hash on 4-tuples, it means that
incoming traffic on a 'shared port' has
to scan a list of XXX sockets to find the best match ...

Also you add another cache line miss in UDP lookup to access
udp_sk()->wait_for_connect.

recvfrom() can be used to filter whatever frame that came before the connect()