tcp: merge 'init_req' and 'route_req' functions

The Multipath-TCP standard (RFC 8684) says that an MPTCP host should send
a TCP reset if the token in a MP_JOIN request is unknown.

At this time we don't do this, the 3whs completes and the 'new subflow'
is reset afterwards.  There are two ways to allow MPTCP to send the
reset.

1. override 'send_synack' callback and emit the rst from there.
   The drawback is that the request socket gets inserted into the
   listeners queue just to get removed again right away.

2. Send the reset from the 'route_req' function instead.
   This avoids the 'add&remove request socket', but route_req lacks the
   skb that is required to send the TCP reset.

Instead of just adding the skb to that function for MPTCP sake alone,
Paolo suggested to merge init_req and route_req functions.

This saves one indirection from syn processing path and provides the skb
to the merged function at the same time.

'send reset on unknown mptcp join token' is added in next patch.

Suggested-by: Paolo Abeni <pabeni@redhat.com>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 7 insertions, 2 deletions

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index e4b31e70bd30..af2338294598 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1444,9 +1444,15 @@ static void tcp_v4_init_req(struct request_sock *req,
 }
 
 static struct dst_entry *tcp_v4_route_req(const struct sock *sk,
+					  struct sk_buff *skb,
 					  struct flowi *fl,
-					  const struct request_sock *req)
+					  struct request_sock *req)
 {
+	tcp_v4_init_req(req, sk, skb);
+
+	if (security_inet_conn_request(sk, skb, req))
+		return NULL;
+
 	return inet_csk_route_req(sk, &fl->u.ip4, req);
 }
 
@@ -1466,7 +1472,6 @@ const struct tcp_request_sock_ops tcp_request_sock_ipv4_ops = {
 	.req_md5_lookup	=	tcp_v4_md5_lookup,
 	.calc_md5_hash	=	tcp_v4_md5_hash_skb,
 #endif
-	.init_req	=	tcp_v4_init_req,
 #ifdef CONFIG_SYN_COOKIES
 	.cookie_init_seq =	cookie_v4_init_sequence,
 #endif