netdev - Re: [PATCH v3 net-next 0/3] rds: IPv6 support

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Date:   Tue, 17 Jul 2018 07:27:28 -0400
From:   Sowmini Varadhan <sowmini.varadhan@...cle.com>
To:     Ka-Cheong Poon <ka-cheong.poon@...cle.com>
Cc:     netdev@...r.kernel.org, santosh.shilimkar@...cle.com,
        davem@...emloft.net, rds-devel@....oracle.com
Subject: Re: [PATCH v3 net-next 0/3] rds: IPv6 support

On (07/17/18 13:32), Ka-Cheong Poon wrote:
> 
> The app can use either structures to make the call.  When the
> app fills in the structure, it knows what it is filling in,
> either sockaddr_in or sockaddr_in6.  So it knows the right size
> to use.  The app can also use IPv4 mapped address in a sockaddr_in6
> without a problem.

tupical applications that I have seen in routing applicaitons
will use a union like
  union {
	struct sockaddr_in sin;
	struct sockaddr_in sin5;
  }
Or they will use sockadd_storage. Passing down the sizeoof that structure
will do the worng thing thing in the existing code for ipv4 (even
though it will not generate EFAOIT)..

> Could you please explain the inconsistency?  An app can use IPv4
> mapped address in a sockaddr_in6 to operate on an IPv4 connection,
> in case you are thinking of this new addition in v3 of the patch.

bind() and connect() are using the sa_family/ss_family to have
the application signal to the kernel about whether ipv4 or ipv6 is 
desired. (and bind and connect are doing the right thing for
v4mapped, so that doesnt seem to be a problem there)

In this case you want the application to signal that info via
the optlen.  (And the reason for this inconsistency is that you dont 
want to deal with the user->kernel copy in the same way?)

--Sowmini