[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 bb7210f4005..fe63d4efbd4 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.