netdev - Re: [PATCH net-next] tcp: reduce skb overhead in selected places

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Date:   Wed, 25 Jan 2017 13:17:17 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net-next] tcp: reduce skb overhead in selected places

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Tue, 24 Jan 2017 14:57:36 -0800

> From: Eric Dumazet <edumazet@...gle.com>
> 
> 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@...gle.com>

Applied, thanks Eric.