[PATCH] ipv4: add sysctl knob to control the discarding of skb from local in ip_forward

This change is meant to add a control for forwarding skb from local.
By default, ip forward will not receive the pakcet to/from the local.
But in some special cases, for example:
-
|  ovs-bridge  gw-port |  <---->   kube-proxy(iptables) |
-
Ovs sends the packet to the gateway, which requires iptables for nat,
such as kube-proxy (iptables), and then sends it back to the gateway
through routing for further processing in ovs.

Signed-off-by: Changliang Wu <redacted>
---
  include/net/netns/ipv4.h   | 1 +
  net/ipv4/af_inet.c         | 1 +
  net/ipv4/ip_forward.c      | 6 +++---
  net/ipv4/sysctl_net_ipv4.c | 7 +++++++
  4 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
index 2f65701..0dbe0d6 100644
--- a/include/net/netns/ipv4.h
+++ b/include/net/netns/ipv4.h

@@ -94,6 +94,7 @@ struct netns_ipv4 {
  	u8 sysctl_ip_no_pmtu_disc;
  	u8 sysctl_ip_fwd_use_pmtu;
  	u8 sysctl_ip_fwd_update_priority;
+	u8 sysctl_ip_fwd_accept_local;
  	u8 sysctl_ip_nonlocal_bind;
  	u8 sysctl_ip_autobind_reuse;
  	/* Shall we try to damage output packets if routing dev changes? */

diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index 0189e3c..b5dc205 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c

@@ -1844,6 +1844,7 @@ static __net_init int inet_init_net(struct net *net)
  	 */
  	net->ipv4.sysctl_ip_default_ttl = IPDEFTTL;
  	net->ipv4.sysctl_ip_fwd_update_priority = 1;
+	net->ipv4.sysctl_ip_fwd_accept_local = 0;
  	net->ipv4.sysctl_ip_dynaddr = 0;
  	net->ipv4.sysctl_ip_early_demux = 1;
  	net->ipv4.sysctl_udp_early_demux = 1;

diff --git a/net/ipv4/ip_forward.c b/net/ipv4/ip_forward.c
index 00ec819..06b7e00 100644
--- a/net/ipv4/ip_forward.c
+++ b/net/ipv4/ip_forward.c

@@ -95,9 +95,6 @@ int ip_forward(struct sk_buff *skb)
  	if (skb->pkt_type != PACKET_HOST)
  		goto drop;
  
-	if (unlikely(skb->sk))
-		goto drop;
-
  	if (skb_warn_if_lro(skb))
  		goto drop;
  

@@ -110,6 +107,9 @@ int ip_forward(struct sk_buff *skb)
  	skb_forward_csum(skb);
  	net = dev_net(skb->dev);
  
+	if (unlikely(!net->ipv4.sysctl_ip_fwd_accept_local && skb->sk))
+		goto drop;
+

  	/*
  	 *	According to the RFC, we must first decrease the TTL field. If
  	 *	that reaches zero, we must reply an ICMP control message telling

diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 97eb547..d95e2e3 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c

@@ -756,6 +756,13 @@ static int proc_fib_multipath_hash_fields(struct ctl_table *table, int write,
  		.extra2		= SYSCTL_ONE,
  	},
  	{
+		.procname	= "ip_forward_accept_local",
+		.data		= &init_net.ipv4.sysctl_ip_fwd_accept_local,
+		.maxlen		= sizeof(u8),
+		.mode		= 0644,
+		.proc_handler	= proc_dou8vec_minmax,
+	},
+	{
  		.procname	= "ip_nonlocal_bind",
  		.data		= &init_net.ipv4.sysctl_ip_nonlocal_bind,
  		.maxlen		= sizeof(u8),

diff --git a/net/ipv4/ip_forward.c b/net/ipv4/ip_forward.c
index 00ec819..06b7e00 100644
--- a/net/ipv4/ip_forward.c
+++ b/net/ipv4/ip_forward.c

@@ -95,9 +95,6 @@ int ip_forward(struct sk_buff *skb)
 	if (skb->pkt_type != PACKET_HOST)
 		goto drop;
 
-	if (unlikely(skb->sk))
-		goto drop;
-