[NETLINK]: Unshare SKB, as necessary, in netlink_trim()

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/af_netlink.c

@@ -660,7 +660,7 @@ void netlink_detachskb(struct sock *sk, struct sk_buff *skb)
 	sock_put(sk);
 }
 
-static inline void netlink_trim(struct sk_buff *skb, int allocation)
+static inline struct sk_buff *netlink_trim(struct sk_buff *skb, int allocation)
 {
 	int delta;
 
@@ -668,10 +668,20 @@ static inline void netlink_trim(struct sk_buff *skb, int allocation)
 
 	delta = skb->end - skb->tail;
 	if (delta * 2 < skb->truesize)
-		return;
-	if (pskb_expand_head(skb, 0, -delta, allocation))
-		return;
-	skb->truesize -= delta;
+		return skb;
+
+	if (skb_shared(skb)) {
+		struct sk_buff *nskb = skb_clone(skb, allocation);
+		if (!nskb)
+			return skb;
+		kfree_skb(skb);
+		skb = nskb;
+	}
+
+	if (!pskb_expand_head(skb, 0, -delta, allocation))
+		skb->truesize -= delta;
+
+	return skb;
 }
 
 int netlink_unicast(struct sock *ssk, struct sk_buff *skb, u32 pid, int nonblock)
@@ -680,7 +690,7 @@ int netlink_unicast(struct sock *ssk, struct sk_buff *skb, u32 pid, int nonblock
 	int err;
 	long timeo;
 
-	netlink_trim(skb, gfp_any());
+	skb = netlink_trim(skb, gfp_any());
 
 	timeo = sock_sndtimeo(ssk, nonblock);
 retry:
@@ -778,6 +788,8 @@ int netlink_broadcast(struct sock *ssk, struct sk_buff *skb, u32 pid,
 	struct hlist_node *node;
 	struct sock *sk;
 
+	skb = netlink_trim(skb, allocation);
+
 	info.exclude_sk = ssk;
 	info.pid = pid;
 	info.group = group;
@@ -788,8 +800,6 @@ int netlink_broadcast(struct sock *ssk, struct sk_buff *skb, u32 pid,
 	info.skb = skb;
 	info.skb2 = NULL;
 
-	netlink_trim(skb, allocation);
-
 	/* While we sleep in clone, do not allow to change socket list */
 
 	netlink_lock_table();