[NET]: Avoid copying TCP packets unnecessarily (a030847e) · Commits · 戴 / test

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>

net/core/dev.c

+10 −11

Original line number	Diff line number	Diff line
		@@ -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:

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