netdev - Re: [PATCHv4 net-next 0/2] net: enable udp v6 sockets receiving v4 packets with UDP GRO

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Message-ID: <655776.1612343656@warthog.procyon.org.uk>
Date:   Wed, 03 Feb 2021 09:14:16 +0000
From:   David Howells <dhowells@...hat.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     dhowells@...hat.com, network dev <netdev@...r.kernel.org>,
        davem <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Martin Varghese <martin.varghese@...ia.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        vfedorenko@...ek.ru
Subject: Re: [PATCHv4 net-next 0/2] net: enable udp v6 sockets receiving v4 packets with UDP GRO

Xin Long <lucien.xin@...il.com> wrote:

> BTW, I'm also thinking to use udp_sock_create(), the only problem I can
> see is it may not do bind() in rxrpc_open_socket(), is that true? or we
> can actually bind to some address when a local address is not supplied?

If a local address isn't explicitly bound to the AF_RXRPC socket, binding the
UDP socket to a random local port is fine.  In fact, sometimes I want to
explicitly bind an rxrpc server socket to a random port.  See fs/afs/rxrpc.c
function afs_open_socket():

	/* bind the callback manager's address to make this a server socket */
	memset(&srx, 0, sizeof(srx));
	srx.srx_family			= AF_RXRPC;
	srx.srx_service			= CM_SERVICE;
	srx.transport_type		= SOCK_DGRAM;
	srx.transport_len		= sizeof(srx.transport.sin6);
	srx.transport.sin6.sin6_family	= AF_INET6;
	srx.transport.sin6.sin6_port	= htons(AFS_CM_PORT);
	...
	ret = kernel_bind(socket, (struct sockaddr *) &srx, sizeof(srx));
	if (ret == -EADDRINUSE) {
		srx.transport.sin6.sin6_port = 0;

		^^^ That's hoping to get a random port bound.

		ret = kernel_bind(socket, (struct sockaddr *) &srx, sizeof(srx));
	}
	if (ret < 0)
		goto error_2;

The client cache manager server socket here is used to receive notifications
back from the fileserver.  There's a standard port (7001) for the service, but
if that's in use, we can use any other port.  The fileserver grabs the source
port from incoming RPC requests - and then uses that when sending 3rd-party
change notifications back.

If you could arrange for a random port to be assigned in such a case (and
indicated back to the caller), that would be awesome.  Possibly I just don't
need to actually use bind in this case.

David