net: implement IP_RECVTOS for IP_PKTOPTIONS

Currently, it is not easily possible to get TOS/DSCP value of packets from
an incoming TCP stream. The mechanism is there, IP_PKTOPTIONS getsockopt
with IP_RECVTOS set, the same way as incoming TTL can be queried. This is
not actually implemented for TOS, though.

This patch adds this functionality, both for IPv4 (IP_PKTOPTIONS) and IPv6
(IPV6_2292PKTOPTIONS). For IPv4, like in the IP_RECVTTL case, the value of
the TOS field is stored from the other party's ACK.

This is needed for proxies which require DSCP transparency. One such example
is at http://zph.bratcheda.org/.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/af_inet.c

diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index f7b5670..e588a34 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -381,6 +381,7 @@
 	inet->mc_all	= 1;
 	inet->mc_index	= 0;
 	inet->mc_list	= NULL;
+	inet->rcv_tos	= 0;
 
 	sk_refcnt_debug_inc(sk);
 

net/ipv4/ip_sockglue.c

diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index 9125529..ca50d9f 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -1289,6 +1289,10 @@
 			int hlim = inet->mc_ttl;
 			put_cmsg(&msg, SOL_IP, IP_TTL, sizeof(hlim), &hlim);
 		}
+		if (inet->cmsg_flags & IP_CMSG_TOS) {
+			int tos = inet->rcv_tos;
+			put_cmsg(&msg, SOL_IP, IP_TOS, sizeof(tos), &tos);
+		}
 		len -= msg.msg_controllen;
 		return put_user(len, optlen);
 	}

net/ipv4/tcp_ipv4.c

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 4d6f81c..94abee8 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1463,6 +1463,7 @@
 	ireq->opt	      = NULL;
 	newinet->mc_index     = inet_iif(skb);
 	newinet->mc_ttl	      = ip_hdr(skb)->ttl;
+	newinet->rcv_tos      = ip_hdr(skb)->tos;
 	inet_csk(newsk)->icsk_ext_hdr_len = 0;
 	if (inet_opt)
 		inet_csk(newsk)->icsk_ext_hdr_len = inet_opt->opt.optlen;