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, 2 insertions, 0 deletions

diff --git a/flow.h b/flow.h
index 7866477..078fd60 100644
--- a/flow.h
+++ b/flow.h
@@ -168,6 +168,8 @@ static inline bool flowside_eq(const struct flowside *left,
 
 int flowside_sock_l4(const struct ctx *c, enum epoll_type type, uint8_t pif,
 		     const struct flowside *tgt, uint32_t data);
+int flowside_connect(const struct ctx *c, int s,
+		     uint8_t pif, const struct flowside *tgt);
 
 /**
  * struct flow_common - Common fields for packet flows