[NET]: Avoid allocating skb in skb_pad

First of all it is unnecessary to allocate a new skb in skb_pad since
the existing one is not shared.  More importantly, our hard_start_xmit
interface does not allow a new skb to be allocated since that breaks
requeueing.

This patch uses pskb_expand_head to expand the existing skb and linearize
it if needed.  Actually, someone should sift through every instance of
skb_pad on a non-linear skb as they do not fit the reasons why this was
originally created.

Incidentally, this fixes a minor bug when the skb is cloned (tcpdump,
TCP, etc.).  As it is skb_pad will simply write over a cloned skb.  Because
of the position of the write it is unlikely to cause problems but still
it's best if we don't do it.

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

1 files changed, 26 insertions, 10 deletions

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index bb7210f..fe63d4e 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -781,24 +781,40 @@ struct sk_buff *skb_copy_expand(const struct sk_buff *skb,
  *	filled. Used by network drivers which may DMA or transfer data
  *	beyond the buffer end onto the wire.
  *
- *	May return NULL in out of memory cases.
+ *	May return error in out of memory cases. The skb is freed on error.
  */
  
-struct sk_buff *skb_pad(struct sk_buff *skb, int pad)
+int skb_pad(struct sk_buff *skb, int pad)
 {
-	struct sk_buff *nskb;
+	int err;
+	int ntail;
 	
 	/* If the skbuff is non linear tailroom is always zero.. */
-	if (skb_tailroom(skb) >= pad) {
+	if (!skb_cloned(skb) && skb_tailroom(skb) >= pad) {
 		memset(skb->data+skb->len, 0, pad);
-		return skb;
+		return 0;
 	}
-	
-	nskb = skb_copy_expand(skb, skb_headroom(skb), skb_tailroom(skb) + pad, GFP_ATOMIC);
+
+	ntail = skb->data_len + pad - (skb->end - skb->tail);
+	if (likely(skb_cloned(skb) || ntail > 0)) {
+		err = pskb_expand_head(skb, 0, ntail, GFP_ATOMIC);
+		if (unlikely(err))
+			goto free_skb;
+	}
+
+	/* FIXME: The use of this function with non-linear skb's really needs
+	 * to be audited.
+	 */
+	err = skb_linearize(skb);
+	if (unlikely(err))
+		goto free_skb;
+
+	memset(skb->data + skb->len, 0, pad);
+	return 0;
+
+free_skb:
 	kfree_skb(skb);
-	if (nskb)
-		memset(nskb->data+nskb->len, 0, pad);
-	return nskb;
+	return err;
 }	
  
 /* Trims skb to length len. It can change skb pointers.