tcp: Use defaults when no route options are available

Trying to parse the option of a SYN packet that we have
no route entry for should just use global wide defaults
for route entry options.
Signed-off-by: NGilad Ben-Yossef <gilad@codefidence.com>
Tested-by: Valdis.Kletnieks@vt.edu
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/dst.h

@@ -113,7 +113,7 @@ dst_metric(const struct dst_entry *dst, int metric)
 static inline u32
 dst_feature(const struct dst_entry *dst, u32 feature)
 {
-	return dst_metric(dst, RTAX_FEATURES) & feature;
+	return (dst ? dst_metric(dst, RTAX_FEATURES) & feature : 0);
 }
 
 static inline u32 dst_mtu(const struct dst_entry *dst)

net/ipv4/tcp_input.c

@@ -3704,8 +3704,6 @@ void tcp_parse_options(struct sk_buff *skb, struct tcp_options_received *opt_rx,
 	struct tcphdr *th = tcp_hdr(skb);
 	int length = (th->doff * 4) - sizeof(struct tcphdr);
 
-	BUG_ON(!estab && !dst);
-
 	ptr = (unsigned char *)(th + 1);
 	opt_rx->saw_tstamp = 0;