netfilter: have ip*t REJECT set the sock err when an icmp is to be sent

Allow the REJECT --reject-with icmp*blabla to also set the matching error
locally on the socket affected by the reject.
This allows the process to see an error almost as if it received it
via ICMP.
It avoids the local process who's ingress packet is rejected to have to
wait for a pseudo-eternity until some timeout kicks in.

Ideally, this should be enabled with a new iptables flag similar to
   --reject-with-sock-err
For now it is enabled with CONFIG_IP*_NF_TARGET_REJECT_SKERR option.

Change-Id: I649a4fd5940029ec0b3233e5abb205da6984891e
Signed-off-by: JP Abgrall <jpa@google.com>

net/ipv4/netfilter/Kconfig

@@ -110,6 +110,18 @@ config IP_NF_TARGET_REJECT
 
 	  To compile it as a module, choose M here.  If unsure, say N.
 
+config IP_NF_TARGET_REJECT_SKERR
+	bool "Force socket error when rejecting with icmp*"
+	depends on IP_NF_TARGET_REJECT
+	default n
+	help
+          This option enables turning a "--reject-with icmp*" into a matching
+          socket error also.
+	  The REJECT target normally allows sending an ICMP message. But it
+          leaves the local socket unaware of any ingress rejects.
+
+	  If unsure, say N.
+
 config IP_NF_TARGET_ULOG
 	tristate "ULOG target support"
 	default m if NETFILTER_ADVANCED=n

net/ipv4/netfilter/ipt_REJECT.c

@@ -129,6 +129,14 @@ static void send_reset(struct sk_buff *oldskb, int hook)
 static inline void send_unreach(struct sk_buff *skb_in, int code)
 {
 	icmp_send(skb_in, ICMP_DEST_UNREACH, code, 0);
+#ifdef CONFIG_IP_NF_TARGET_REJECT_SKERR
+	if (skb_in->sk) {
+		skb_in->sk->sk_err = icmp_err_convert[code].errno;
+		skb_in->sk->sk_error_report(skb_in->sk);
+		pr_debug("ipt_REJECT: sk_err=%d for skb=%p sk=%p\n",
+			skb_in->sk->sk_err, skb_in, skb_in->sk);
+	}
+#endif
 }
 
 static unsigned int

net/ipv6/netfilter/Kconfig

@@ -153,6 +153,18 @@ config IP6_NF_TARGET_REJECT
 
 	  To compile it as a module, choose M here.  If unsure, say N.
 
+config IP6_NF_TARGET_REJECT_SKERR
+	bool "Force socket error when rejecting with icmp*"
+	depends on IP6_NF_TARGET_REJECT
+	default n
+	help
+          This option enables turning a "--reject-with icmp*" into a matching
+          socket error also.
+	  The REJECT target normally allows sending an ICMP message. But it
+          leaves the local socket unaware of any ingress rejects.
+
+	  If unsure, say N.
+
 config IP6_NF_MANGLE
 	tristate "Packet mangling"
 	default m if NETFILTER_ADVANCED=n

net/ipv6/netfilter/ip6t_REJECT.c

@@ -180,6 +180,15 @@ send_unreach(struct net *net, struct sk_buff *skb_in, unsigned char code,
 		skb_in->dev = net->loopback_dev;
 
 	icmpv6_send(skb_in, ICMPV6_DEST_UNREACH, code, 0);
+#ifdef CONFIG_IP6_NF_TARGET_REJECT_SKERR
+	if (skb_in->sk) {
+		icmpv6_err_convert(ICMPV6_DEST_UNREACH, code,
+				   &skb_in->sk->sk_err);
+		skb_in->sk->sk_error_report(skb_in->sk);
+		pr_debug("ip6t_REJECT: sk_err=%d for skb=%p sk=%p\n",
+			skb_in->sk->sk_err, skb_in, skb_in->sk);
+	}
+#endif
 }
 
 static unsigned int