[NET]: Avoid copying TCP packets unnecessarily

TCP packets all have writable heads, that is, even though it's cloned, it is
writable up to the end of the TCP header.  This patch makes skb_checksum_help
aware of this fact by using skb_clone_writable and avoiding a copy for TCP.

I've also modified the BUG_ON tests to be unsigned.  The only case where this
makes a difference is if csum_start points to a location before skb->data.
Since skb->data should always include the header where the checksum field
is (and all currently callers adhere to that), this change is safe and may
uncover bugs later.

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

diff --git a/net/core/dev.c b/net/core/dev.c
index 39aba4862f2122838476b5b0c2f0762a8b8c1c06..38b03da5c1ca93d061cfe250195bbfc8898b9da6 100644 (file)
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1362,22 +1362,21 @@ int skb_checksum_help(struct sk_buff *skb)
                goto out_set_summed;
        }
 
-       if (skb_cloned(skb)) {
+       offset = skb->csum_start - skb_headroom(skb);
+       BUG_ON(offset >= skb_headlen(skb));
+       csum = skb_checksum(skb, offset, skb->len - offset, 0);
+
+       offset += skb->csum_offset;
+       BUG_ON(offset + sizeof(__sum16) > skb_headlen(skb));
+
+       if (skb_cloned(skb) &&
+           !skb_clone_writable(skb, offset + sizeof(__sum16))) {
                ret = pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
                if (ret)
                        goto out;
        }
 
-       offset = skb->csum_start - skb_headroom(skb);
-       BUG_ON(offset > (int)skb->len);
-       csum = skb_checksum(skb, offset, skb->len-offset, 0);
-
-       offset = skb_headlen(skb) - offset;
-       BUG_ON(offset <= 0);
-       BUG_ON(skb->csum_offset + 2 > offset);
-
-       *(__sum16 *)(skb->head + skb->csum_start + skb->csum_offset) =
-               csum_fold(csum);
+       *(__sum16 *)(skb->data + offset) = csum_fold(csum);
 out_set_summed:
        skb->ip_summed = CHECKSUM_NONE;
 out: