net: skb_condense() can also deal with empty skbs

It seems attackers can also send UDP packets with no payload at all.

skb_condense() can still be a win in this case.

It will be possible to replace the custom code in tcp_add_backlog()
to get full benefit from skb_condense()
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -4946,16 +4946,20 @@ EXPORT_SYMBOL(pskb_extract);
  */
 void skb_condense(struct sk_buff *skb)
 {
-	if (!skb->data_len ||
-	    skb->data_len > skb->end - skb->tail ||
+	if (skb->data_len) {
+		if (skb->data_len > skb->end - skb->tail ||
 		    skb_cloned(skb))
 			return;
 
 		/* Nice, we can free page frag(s) right now */
 		__pskb_pull_tail(skb, skb->data_len);
-
-	/* Now adjust skb->truesize, since __pskb_pull_tail() does
-	 * not do this.
+	}
+	/* At this point, skb->truesize might be over estimated,
+	 * because skb had a fragment, and fragments do not tell
+	 * their truesize.
+	 * When we pulled its content into skb->head, fragment
+	 * was freed, but __pskb_pull_tail() could not possibly
+	 * adjust skb->truesize, not knowing the frag truesize.
 	 */
 	skb->truesize = SKB_TRUESIZE(skb_end_offset(skb));
 }