To prevent dst_entry leaks, add warning when the non-NULL dst_entry
is rewritten.

Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
---
 include/linux/skbuff.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 8240e0826204..2f9dac54d627 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1159,6 +1159,12 @@ static inline struct dst_entry *skb_dst(const struct sk_buff *skb)
 	return (struct dst_entry *)(skb->_skb_refdst & SKB_DST_PTRMASK);
 }
 
+static inline void skb_dst_check_unset(struct sk_buff *skb)
+{
+	DEBUG_NET_WARN_ON_ONCE((skb->_skb_refdst & SKB_DST_PTRMASK) &&
+			       !(skb->_skb_refdst & SKB_DST_NOREF));
+}
+
 /**
  * skb_dst_reset() - return current dst_entry value and clear it
  * @skb: buffer
@@ -1187,6 +1193,7 @@ static inline unsigned long skb_dst_reset(struct sk_buff *skb)
  */
 static inline void skb_dst_restore(struct sk_buff *skb, unsigned long refdst)
 {
+	skb_dst_check_unset(skb);
 	skb->_skb_refdst = refdst;
 }
 
@@ -1200,6 +1207,7 @@ static inline void skb_dst_restore(struct sk_buff *skb, unsigned long refdst)
  */
 static inline void skb_dst_set(struct sk_buff *skb, struct dst_entry *dst)
 {
+	skb_dst_check_unset(skb);
 	skb->slow_gro |= !!dst;
 	skb->_skb_refdst = (unsigned long)dst;
 }
@@ -1216,6 +1224,7 @@ static inline void skb_dst_set(struct sk_buff *skb, struct dst_entry *dst)
  */
 static inline void skb_dst_set_noref(struct sk_buff *skb, struct dst_entry *dst)
 {
+	skb_dst_check_unset(skb);
 	WARN_ON(!rcu_read_lock_held() && !rcu_read_lock_bh_held());
 	skb->slow_gro |= !!dst;
 	skb->_skb_refdst = (unsigned long)dst | SKB_DST_NOREF;
-- 
2.50.1