gro: prevent ACE field corruption & better AccECN handling

There are important differences in how the CWR field behaves
in RFC3168 and AccECN. With AccECN, CWR flag is part of the
ACE counter and its changes are important so adjust the flags
changed mask accordingly.

Also, if CWR is there, set the Accurate ECN GSO flag to avoid
corrupting CWR flag somewhere.

Signed-off-by: Ilpo Järvinen <[email protected]>
Signed-off-by: Chia-Yu Chang <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: Ilpo Järvinen

net/ipv4/tcp_offload.c

@@ -332,7 +332,7 @@ struct sk_buff *tcp_gro_receive(struct list_head *head, struct sk_buff *skb,
 	th2 = tcp_hdr(p);
 	flush = (__force int)(flags & TCP_FLAG_CWR);
 	flush |= (__force int)((flags ^ tcp_flag_word(th2)) &
-		  ~(TCP_FLAG_CWR | TCP_FLAG_FIN | TCP_FLAG_PSH));
+		  ~(TCP_FLAG_FIN | TCP_FLAG_PSH));
 	flush |= (__force int)(th->ack_seq ^ th2->ack_seq);
 	for (i = sizeof(*th); i < thlen; i += 4)
 		flush |= *(u32 *)((u8 *)th + i) ^
@@ -408,7 +408,7 @@ void tcp_gro_complete(struct sk_buff *skb)
 	shinfo->gso_segs = NAPI_GRO_CB(skb)->count;
 
 	if (th->cwr)
-		shinfo->gso_type |= SKB_GSO_TCP_ECN;
+		shinfo->gso_type |= SKB_GSO_TCP_ACCECN;
 }
 EXPORT_SYMBOL(tcp_gro_complete);