Subject: Re: [PATCH net-next 2/3] rds: Enable RDS IPv6 support

From: Sowmini Varadhan <[email-masked]>
Date: Mon, 25 Jun 2018 13:03:17 -0400

On (06/25/18 03:38), Ka-Cheong Poon wrote:
> @@ -1105,8 +1105,27 @@ int rds_sendmsg(struct socket *sock, struct msghdr *msg, size_t payload_len)
>  			break;
>  
>  		case sizeof(*sin6): {
> -			ret = -EPROTONOSUPPORT;
> -			goto out;
> +			int addr_type;
                         :
                         :
> +			daddr = sin6->sin6_addr;
> +			dport = sin6->sin6_port;
> +			scope_id = sin6->sin6_scope_id;
> +			break;
>  		}

In rds_sendmsg, the scopeid passed to rds_conn_create_outgoing
may come from the msg_name (if msg_name is a link-local) or
may come from the rs_bound_scope_id (for connected socket, change
made in Patch 1 of the series). 

This sounds inconsistent.

If I bind to scopeid if1 and then send to fe80::1%if2 (without connect()), 
we'd create an rds_connection with dev_if set to if2. 
(first off, its a bit unexpected to be sending to fe80::1%if2 when you
are bound to a link-local on if1!)

But then, if we got back a response from fe80::1%if2, I think we would
not find a matching conn in rds_recv_incoming? 

And this is even more confusing because the fastpath in rds_sendmsg
does not take the bound_scope_id into consideration at all:
1213         if (rs->rs_conn && ipv6_addr_equal(&rs->rs_conn->c_faddr, &daddr))
1214                 conn = rs->rs_conn;
1215         else {
1216                 conn = rds_conn_create_outgoing( /* .. */, scope_id)
so if I erroneously passed a msg_name on a connected rds socket, what
would happen? (see also question about rds_connect() itself, below)

Should we always use rs_bound_scope_id for creating the outgoing
rds_connection? (you may need something deterministic for this, 
like "if bound addr is linklocal, return error if daddr has a different
scopeid, else use the bound addr's scopeid", plus, "if bound addr is
not global, and daddr is link-local, we need a conn with the daddr's
scopeid")

Also, why is there no IPv6 support in rds_connect? 

(still looking through the rds-tcp changes, but wanted to get these
questions clarified first).

--Sowmini