[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, 5 insertions, 6 deletions

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 66f8819f9568..f8c7eb79a27f 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -345,7 +345,7 @@ extern struct sk_buff *skb_realloc_headroom(struct sk_buff *skb,
 extern struct sk_buff *skb_copy_expand(const struct sk_buff *skb,
                                        int newheadroom, int newtailroom,
                                        gfp_t priority);
-extern struct sk_buff *         skb_pad(struct sk_buff *skb, int pad);
+extern int             skb_pad(struct sk_buff *skb, int pad);
 #define dev_kfree_skb(a)        kfree_skb(a)
 extern void           skb_over_panic(struct sk_buff *skb, int len,
                                      void *here);
@@ -1122,16 +1122,15 @@ static inline int skb_cow(struct sk_buff *skb, unsigned int headroom)
  *
  *      Pads up a buffer to ensure the trailing bytes exist and are
  *      blanked. If the buffer already contains sufficient data it
- *      is untouched. Returns the buffer, which may be a replacement
- *      for the original, or NULL for out of memory - in which case
- *      the original buffer is still freed.
+ *      is untouched. Otherwise it is extended. Returns zero on
+ *      success. The skb is freed on error.
  */
  
-static inline struct sk_buff *skb_padto(struct sk_buff *skb, unsigned int len)
+static inline int skb_padto(struct sk_buff *skb, unsigned int len)
 {
         unsigned int size = skb->len;
         if (likely(size >= len))
-                return skb;
+                return 0;
         return skb_pad(skb, len-size);
 }