netdev - Re: [Patch net] ipv4: clear all TCP_SKB_CB before passing to network layer

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Date:	Fri, 17 Oct 2014 11:18:48 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net,
	Krzysztof Kolasa <kkolasa@...soft.pl>,
	Eric Dumazet <edumazet@...gle.com>,
	Cong Wang <cwang@...pensource.com>
Subject: Re: [Patch net] ipv4: clear all TCP_SKB_CB before passing to
 network layer

On Fri, 2014-10-17 at 10:49 -0700, Cong Wang wrote:
> From: Cong Wang <cwang@...pensource.com>
> 
> Probably not a big deal, but IP is not the only network protocol,
> don't clear skb->cb just for IP.
> 
> Also, IPv6 header is not always defined in struct tcp_skb_cb.
> 
> Cc: Krzysztof Kolasa <kkolasa@...soft.pl>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Cong Wang <cwang@...pensource.com>
> Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
> ---
>  net/ipv4/tcp_output.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index e13d778..ee356e5 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -1005,9 +1005,7 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
>  	/* Our usage of tstamp should remain private */
>  	skb->tstamp.tv64 = 0;
>  
> -	/* Cleanup our debris for IP stacks */
> -	memset(skb->cb, 0, max(sizeof(struct inet_skb_parm),
> -			       sizeof(struct inet6_skb_parm)));
> +	memset(TCP_SKB_CB(skb), 0, sizeof(*TCP_SKB_CB(skb)));
>  
>  	err = icsk->icsk_af_ops->queue_xmit(sk, skb, &inet->cork.fl);
>  


Usually, each layer is responsible for clearing skb->cb[] at its entry
point. Or more exactly it does not care of previous garbage.

There is no evidence your patch is needed.

I was maybe too defensive when I added this, because I wanted to make
only TCP changes.

We should instead remove the memset() in TCP and fix IP/IPv6 if
necessary.

But this should wait net-next being open.


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