netdev - Re: TCP output handling bug ?

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Date:	Fri, 22 Aug 2014 11:18:22 -0700
From:	Yuchung Cheng <ycheng@...gle.com>
To:	David Miller <davem@...emloft.net>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Dave Jones <davej@...hat.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	alan@...rguk.ukuu.org.uk, netdev <netdev@...r.kernel.org>
Subject: Re: TCP output handling bug ?

On Fri, Aug 22, 2014 at 10:41 AM, David Miller <davem@...emloft.net> wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Fri, 22 Aug 2014 06:56:30 -0700
>
> CC:'ing Yuchung Cheng
>
>> On Thu, 2014-08-21 at 22:13 -0400, Dave Jones wrote:
>>> On Thu, Aug 21, 2014 at 05:24:45PM -0700, Stephen Hemminger wrote:
>>>  > On Thu, 21 Aug 2014 19:02:08 +0100
>>>  > Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
>>>  >
>>>  > >
>>>  > > tcp_send_syn_data:
>>>  > >
>>>  > >         TCP_SKB_CB(data)->tcp_flags &= ~TCPHDR_SYN;
>>>  > >         TCP_SKB_CB(data)->tcp_flags = (TCPHDR_ACK|TCPHDR_PSH);
>>>  > >
>>>  > > the reporter has a point 8)
>>>  > >
>>>  > > https://bugzilla.kernel.org/show_bug.cgi?id=82101
>>>  >
>>>  > I wonder if covertity or smatch could be smart enough to catch this kind of bug?
>>>
>>> For coverity: it should be, but isn't :)
>>> I've pointed them at the bugzilla, maybe they can add a check in a
>>> future update.
>>>
>>> I bet this isn't the only instance of a bug like this left in the tree.
>>> We've definitely had similar cases before.
>>>
>>>      Dave
>>
>> There is no bug here as a matter of fact.
>>
>> You can simply remove the first line, as it is useless.
>>
>> -     TCP_SKB_CB(data)->tcp_flags &= ~TCPHDR_SYN;
>>
>> At the end of the day, data segment has ACK and PSH flags only.
>
> What about ECE/CWR?

sorry for the late response. my original intention was
TCP_SKB_CB(data)->tcp_flags &= ~TCPHDR_SYN;
TCP_SKB_CB(data)->tcp_flags |= (TCPHDR_ACK|TCPHDR_PSH);

so that we can keep other flags intact b/c in the future we may have
xmas tree packet :-)
I will send a patch soon.
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