Delayed source port allocation for connected UDP sockets

[Marek Majkowski <marek@cloudflare.com>]

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/