From: Wei Wang <weiwan@google.com>
Date: Sat, 17 Jun 2017 10:42:25 -0700
Subject: udp: call dst_hold_safe() in udp_sk_rx_set_dst()
Patch-mainline: v4.13-rc1
Git-commit: d24406c85d123df773bc4df88ad5da2233896919
References: bsc#1061739

In udp_v4/6_early_demux() code, we try to hold dst->__refcnt for
dst with DST_NOCACHE flag. This is because later in udp_sk_rx_dst_set()
function, we will try to cache this dst in sk for connected case.
However, a better way to achieve this is to not try to hold dst in
early_demux(), but in udp_sk_rx_dst_set(), call dst_hold_safe(). This
approach is also more consistant with how tcp is handling it. And it
will make later changes simpler.

Signed-off-by: Wei Wang <weiwan@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Michal Kubecek <mkubecek@suse.cz>

---
 net/ipv4/udp.c | 22 +++++++++++-----------
 net/ipv6/udp.c | 11 +++++------
 2 files changed, 16 insertions(+), 17 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 5693928641b1..afd211b29a0a 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1881,10 +1881,12 @@ bool udp_sk_rx_dst_set(struct sock *sk, struct dst_entry *dst)
 {
 	struct dst_entry *old;
 
-	dst_hold(dst);
-	old = xchg(&sk->sk_rx_dst, dst);
-	dst_release(old);
-	return old != dst;
+	if (dst_hold_safe(dst)) {
+		old = xchg(&sk->sk_rx_dst, dst);
+		dst_release(old);
+		return old != dst;
+	}
+	return false;
 }
 
 /*
@@ -2208,13 +2210,11 @@ void udp_v4_early_demux(struct sk_buff *skb)
 	if (dst)
 		dst = dst_check(dst, 0);
 	if (dst) {
-		/* DST_NOCACHE can not be used without taking a reference */
-		if (dst->flags & DST_NOCACHE) {
-			if (likely(atomic_inc_not_zero(&dst->__refcnt)))
-				skb_dst_set(skb, dst);
-		} else {
-			skb_dst_set_noref(skb, dst);
-		}
+		/* set noref for now.
+		 * any place which wants to hold dst has to call
+		 * dst_hold_safe()
+		 */
+		skb_dst_set_noref(skb, dst);
 	}
 }
 
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 723d44184047..cf5f5bd1742c 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -939,12 +939,11 @@ static void udp_v6_early_demux(struct sk_buff *skb)
 	if (dst)
 		dst = dst_check(dst, inet6_sk(sk)->rx_dst_cookie);
 	if (dst) {
-		if (dst->flags & DST_NOCACHE) {
-			if (likely(atomic_inc_not_zero(&dst->__refcnt)))
-				skb_dst_set(skb, dst);
-		} else {
-			skb_dst_set_noref(skb, dst);
-		}
+		/* set noref for now.
+		 * any place which wants to hold dst has to call
+		 * dst_hold_safe()
+		 */
+		skb_dst_set_noref(skb, dst);
 	}
 }
 
-- 
2.14.2