ipv4: tcp: fix TOS value in ACK messages sent from TIME_WAIT

There is a long standing bug in linux tcp stack, about ACK messages sent
on behalf of TIME_WAIT sockets.

In the IP header of the ACK message, we choose to reflect TOS field of
incoming message, and this might break some setups.

Example of things that were broken :
- Routing using TOS as a selector
- Firewalls
- Trafic classification / shaping

We now remember in timewait structure the inet tos field and use it in
ACK generation, and route lookup.

Notes :
- We still reflect incoming TOS in RST messages.
- We could extend MuraliRaja Muniraju patch to report TOS value in
netlink messages for TIME_WAIT sockets.
- A patch is needed for IPv6

Signed-off-by: Eric Dumazet <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/inet_timewait_sock.h

···
 	/* And these are ours. */
 	unsigned int		tw_ipv6only     : 1,
 				tw_transparent  : 1,
-				tw_pad		: 14,	/* 14 bits hole */
+				tw_pad		: 6,	/* 6 bits hole */
+				tw_tos		: 8,
 				tw_ipv6_offset  : 16;
 	kmemcheck_bitfield_end(flags);
 	unsigned long		tw_ttd;

include/net/ip.h

···
 	int	    csumoffset; /* u16 offset of csum in iov[0].iov_base */
 				/* -1 if not needed */ 
 	int	    bound_dev_if;
+	u8  	    tos;
 }; 
 
 #define IP_REPLY_ARG_NOSRCCHECK 1
···
 }
 
 void ip_send_reply(struct sock *sk, struct sk_buff *skb, __be32 daddr,
-		   struct ip_reply_arg *arg, unsigned int len);
+		   const struct ip_reply_arg *arg, unsigned int len);
 
 struct ipv4_config {
 	int	log_martians;

net/ipv4/inet_timewait_sock.c

···
 		tw->tw_daddr	    = inet->inet_daddr;
 		tw->tw_rcv_saddr    = inet->inet_rcv_saddr;
 		tw->tw_bound_dev_if = sk->sk_bound_dev_if;
+		tw->tw_tos	    = inet->tos;
 		tw->tw_num	    = inet->inet_num;
 		tw->tw_state	    = TCP_TIME_WAIT;
 		tw->tw_substate	    = state;

net/ipv4/ip_output.c

···
  *     	structure to pass arguments.
  */
 void ip_send_reply(struct sock *sk, struct sk_buff *skb, __be32 daddr,
-		   struct ip_reply_arg *arg, unsigned int len)
+		   const struct ip_reply_arg *arg, unsigned int len)
 {
 	struct inet_sock *inet = inet_sk(sk);
 	struct ip_options_data replyopts;
···
 	}
 
 	flowi4_init_output(&fl4, arg->bound_dev_if, 0,
-			   RT_TOS(ip_hdr(skb)->tos),
+			   RT_TOS(arg->tos),
 			   RT_SCOPE_UNIVERSE, sk->sk_protocol,
 			   ip_reply_arg_flowi_flags(arg),
 			   daddr, rt->rt_spec_dst,
···
 	   with locally disabled BH and that sk cannot be already spinlocked.
 	 */
 	bh_lock_sock(sk);
-	inet->tos = ip_hdr(skb)->tos;
+	inet->tos = arg->tos;
 	sk->sk_priority = skb->priority;
 	sk->sk_protocol = ip_hdr(skb)->protocol;
 	sk->sk_bound_dev_if = arg->bound_dev_if;

net/ipv4/tcp_ipv4.c

···
 	arg.flags = (sk && inet_sk(sk)->transparent) ? IP_REPLY_ARG_NOSRCCHECK : 0;
 
 	net = dev_net(skb_dst(skb)->dev);
+	arg.tos = ip_hdr(skb)->tos;
 	ip_send_reply(net->ipv4.tcp_sock, skb, ip_hdr(skb)->saddr,
 		      &arg, arg.iov[0].iov_len);
 
···
 static void tcp_v4_send_ack(struct sk_buff *skb, u32 seq, u32 ack,
 			    u32 win, u32 ts, int oif,
 			    struct tcp_md5sig_key *key,
-			    int reply_flags)
+			    int reply_flags, u8 tos)
 {
 	const struct tcphdr *th = tcp_hdr(skb);
 	struct {
···
 	arg.csumoffset = offsetof(struct tcphdr, check) / 2;
 	if (oif)
 		arg.bound_dev_if = oif;
-
+	arg.tos = tos;
 	ip_send_reply(net->ipv4.tcp_sock, skb, ip_hdr(skb)->saddr,
 		      &arg, arg.iov[0].iov_len);
 
···
 			tcptw->tw_ts_recent,
 			tw->tw_bound_dev_if,
 			tcp_twsk_md5_key(tcptw),
-			tw->tw_transparent ? IP_REPLY_ARG_NOSRCCHECK : 0
+			tw->tw_transparent ? IP_REPLY_ARG_NOSRCCHECK : 0,
+			tw->tw_tos
 			);
 
 	inet_twsk_put(tw);
···
 			req->ts_recent,
 			0,
 			tcp_v4_md5_do_lookup(sk, ip_hdr(skb)->daddr),
-			inet_rsk(req)->no_srccheck ? IP_REPLY_ARG_NOSRCCHECK : 0);
+			inet_rsk(req)->no_srccheck ? IP_REPLY_ARG_NOSRCCHECK : 0,
+			ip_hdr(skb)->tos);
 }
 
 /*