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Message-ID: <CAJPywTJzpZAXGdgZLJ+y7G2JoQMyd_JG+G8kgG+xruVVmZD-OA@mail.gmail.com>
Date: Wed, 27 Nov 2019 15:07:55 +0100
From: Marek Majkowski <marek@...udflare.com>
To: Eric Dumazet <edumazet@...gle.com>, ncardwell@...gle.com,
maze@...gle.com, network dev <netdev@...r.kernel.org>
Cc: kernel-team <kernel-team@...udflare.com>
Subject: Delayed source port allocation for connected UDP sockets
Morning,
In my applications I need something like a connectx()[1] syscall. On
Linux I can get quite far with using bind-before-connect and
IP_BIND_ADDRESS_NO_PORT. One corner case is missing though.
For various UDP applications I'm establishing connected sockets from
specific 2-tuple. This is working fine with bind-before-connect, but
in UDP it creates a slight race condition. It's possible the socket
will receive packet from arbitrary source after bind():
s = socket(SOCK_DGRAM)
s.bind((192.0.2.1, 1703))
# here be dragons
s.connect((198.18.0.1, 58910))
For the short amount of time after bind() and before connect(), the
socket may receive packets from any peer. For situations when I don't
need to specify source port, IP_BIND_ADDRESS_NO_PORT flag solves the
issue. This code is fine:
s = socket(SOCK_DGRAM)
s.setsockopt(IP_BIND_ADDRESS_NO_PORT)
s.bind((192.0.2.1, 0))
s.connect((198.18.0.1, 58910))
But the IP_BIND_ADDRESS_NO_PORT doesn't work when the source port is
selected. It seems natural to expand the scope of
IP_BIND_ADDRESS_NO_PORT flag. Perhaps this could be made to work:
s = socket(SOCK_DGRAM)
s.setsockopt(IP_BIND_ADDRESS_NO_PORT)
s.bind((192.0.2.1, 1703))
s.connect((198.18.0.1, 58910))
I would like such code to delay the binding to port 1703 up until the
connect(). IP_BIND_ADDRESS_NO_PORT only makes sense for connected
sockets anyway. This raises a couple of questions though:
- IP_BIND_ADDRESS_NO_PORT name is confusing - we specify the port
number in the bind!
- Where to store the source port in __inet_bind. Neither
inet->inet_sport nor inet->inet_num seem like correct places to store
the user-passed source port hint. The alternative is to introduce
yet-another field onto inet_sock struct, but that is wasteful.
Suggestions?
Marek
[1] https://www.unix.com/man-page/mojave/2/connectx/
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