tcp: reduce skb overhead in selected places
tcp_add_backlog() can use skb_condense() helper to get better gains and less SKB_TRUESIZE() magic. This only happens when socket backlog has to be used. Some attacks involve specially crafted out of order tiny TCP packets, clogging the ofo queue of (many) sockets. Then later, expensive collapse happens, trying to copy all these skbs into single ones. This unfortunately does not work if each skb has no neighbor in TCP sequence order. By using skb_condense() if the skb could not be coalesced to a prior one, we defeat these kind of threats, potentially saving 4K per skb (or more, since this is one page fragment). A typical NAPI driver allocates gro packets with GRO_MAX_HEAD bytes in skb->head, meaning the copy done by skb_condense() is limited to about 200 bytes. Signed-off-by: Eric Dumazet <edumazet@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
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@ -4507,6 +4507,7 @@ add_sack:
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end:
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if (skb) {
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tcp_grow_window(sk, skb);
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skb_condense(skb);
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skb_set_owner_r(skb, sk);
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}
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}
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@ -1556,8 +1556,7 @@ bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb)
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* It has been noticed pure SACK packets were sometimes dropped
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* (if cooked by drivers without copybreak feature).
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*/
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if (!skb->data_len)
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skb->truesize = SKB_TRUESIZE(skb_end_offset(skb));
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skb_condense(skb);
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if (unlikely(sk_add_backlog(sk, skb, limit))) {
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bh_unlock_sock(sk);
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