udp: Handle "spliced" datagrams with per-flow sockets

When forwarding a datagram to a socket, we need to find a socket with a
suitable local address to send it.  Currently we keep track of such sockets
in an array indexed by local port, but this can't properly handle cases
where we have multiple local addresses in active use.

For "spliced" (socket to socket) cases, improve this by instead opening
a socket specifically for the target side of the flow.  We connect() as
well as bind()ing that socket, so that it will only receive the flow's
reply packets, not anything else.  We direct datagrams sent via that socket
using the addresses from the flow table, effectively replacing bespoke
addressing logic with the unified logic in fwd.c

When we create the flow, we also take a duplicate of the originating
socket, and use that to deliver reply datagrams back to the origin, again
using addresses from the flow table entry.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/flow_table.h b/flow_table.h
index df253be..a499e7b 100644
--- a/flow_table.h
+++ b/flow_table.h
@@ -100,6 +100,21 @@ static inline uint8_t pif_at_sidx(flow_sidx_t sidx)
 	return flow->f.pif[sidx.sidei];
 }
 
+/** flowside_at_sidx() - Retrieve a specific flowside
+ * @sidx:    Flow & side index
+ *
+ * Return: Flowside for the flow & side given by @sidx
+ */
+static inline const struct flowside *flowside_at_sidx(flow_sidx_t sidx)
+{
+	const union flow *flow = flow_at_sidx(sidx);
+
+	if (!flow)
+		return PIF_NONE;
+
+	return &flow->f.side[sidx.sidei];
+}
+
 /** flow_sidx_opposite() - Get the other side of the same flow
  * @sidx:	Flow & side index
  *