[PATCH 3.16 009/233] netem: fix skb_orphan_partial()

[Ben Hutchings]

3.16.48-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <edumazet@xxxxxxxxxx>

commit f6ba8d33cfbb46df569972e64dbb5bb7e929bfd9 upstream.

I should have known that lowering skb->truesize was dangerous :/

In case packets are not leaving the host via a standard Ethernet device,
but looped back to local sockets, bad things can happen, as reported
by Michael Madsen ( https://bugzilla.kernel.org/show_bug.cgi?id=195713 )

So instead of tweaking skb->truesize, lets change skb->destructor
and keep a reference on the owner socket via its sk_refcnt.

Fixes: f2f872f9272a ("netem: Introduce skb_orphan_partial() helper")
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Reported-by: Michael Madsen <mkm@xxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
[bwh: Backported to 3.16: drop changes to the preceding comment and the
 fast path, which we don't have]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1654,17 +1654,17 @@ EXPORT_SYMBOL(sock_wfree);
 
 void skb_orphan_partial(struct sk_buff *skb)
 {
-	/* TCP stack sets skb->ooo_okay based on sk_wmem_alloc,
-	 * so we do not completely orphan skb, but transfert all
-	 * accounted bytes but one, to avoid unexpected reorders.
-	 */
 	if (skb->destructor == sock_wfree
 #ifdef CONFIG_INET
 	    || skb->destructor == tcp_wfree
 #endif
 		) {
-		atomic_sub(skb->truesize - 1, &skb->sk->sk_wmem_alloc);
-		skb->truesize = 1;
+		struct sock *sk = skb->sk;
+
+		if (atomic_inc_not_zero(&sk->sk_refcnt)) {
+			atomic_sub(skb->truesize, &sk->sk_wmem_alloc);
+			skb->destructor = sock_efree;
+		}
 	} else {
 		skb_orphan(skb);
 	}