net/ipv4: Introduce IPSKB_FRAG_SEGS bit to inet_skb_parm.flags

This flag indicates whether fragmentation of segments is allowed.

Formerly this policy was hardcoded according to IPSKB_FORWARDED (set by
either ip_forward or ipmr_forward).

Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Florian Westphal <fw@strlen.de>
Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip.h

@@ -47,6 +47,7 @@ struct inet_skb_parm {
 #define IPSKB_REROUTED		BIT(4)
 #define IPSKB_DOREDIRECT	BIT(5)
 #define IPSKB_FRAG_PMTU		BIT(6)
+#define IPSKB_FRAG_SEGS		BIT(7)
 
 	u16			frag_max_size;
 };

net/ipv4/ip_forward.c

@@ -117,7 +117,7 @@ int ip_forward(struct sk_buff *skb)
 	if (opt->is_strictroute && rt->rt_uses_gateway)
 		goto sr_failed;
 
-	IPCB(skb)->flags |= IPSKB_FORWARDED;
+	IPCB(skb)->flags |= IPSKB_FORWARDED | IPSKB_FRAG_SEGS;
 	mtu = ip_dst_mtu_maybe_forward(&rt->dst, true);
 	if (ip_exceeds_mtu(skb, mtu)) {
 		IP_INC_STATS(net, IPSTATS_MIB_FRAGFAILS);

net/ipv4/ip_output.c

@@ -223,8 +223,10 @@ static int ip_finish_output_gso(struct net *net, struct sock *sk,
 	struct sk_buff *segs;
 	int ret = 0;
 
-	/* common case: locally created skb or seglen is <= mtu */
-	if (((IPCB(skb)->flags & IPSKB_FORWARDED) == 0) ||
+	/* common case: fragmentation of segments is not allowed,
+	 * or seglen is <= mtu
+	 */
+	if (((IPCB(skb)->flags & IPSKB_FRAG_SEGS) == 0) ||
 	      skb_gso_validate_mtu(skb, mtu))
 		return ip_finish_output2(net, sk, skb);
 

net/ipv4/ipmr.c

@@ -1749,7 +1749,7 @@ static void ipmr_queue_xmit(struct net *net, struct mr_table *mrt,
 		vif->dev->stats.tx_bytes += skb->len;
 	}
 
-	IPCB(skb)->flags |= IPSKB_FORWARDED;
+	IPCB(skb)->flags |= IPSKB_FORWARDED | IPSKB_FRAG_SEGS;
 
 	/* RFC1584 teaches, that DVMRP/PIM router must deliver packets locally
 	 * not only before forwarding, but after forwarding on all output