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

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Message-ID: <CAHA+R7O0tUinJV4mMOWKF0A6pR3HQL1HA2Zurs=EPDu6suUZGA@mail.gmail.com>
Date:	Fri, 17 Oct 2014 11:26:15 -0700
From:	Cong Wang <cwang@...pensource.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Cong Wang <xiyou.wangcong@...il.com>,
	netdev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Krzysztof Kolasa <kkolasa@...soft.pl>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [Patch net] ipv4: clear all TCP_SKB_CB before passing to network layer

On Fri, Oct 17, 2014 at 11:18 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> 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.

This is why I said "probably not a big deal" in changelog, I never
say it fixes anything, just a cleanup.

>
> 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.

That works too and sounds better.

>
> But this should wait net-next being open.
>
>

Yeah, agreed, I will update and resend when net-next is reopen.

Thanks.
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