Re: [PATCH] TCP: fix a bug that triggers large number of TCP RST by mistake

From: Jerry Chu <hkchu@google.com>
To: David Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Subject: Re: [PATCH] TCP: fix a bug that triggers large number of TCP RST by mistake
Date: Tue, 25 Jan 2011 15:48:10 -0800	[thread overview]
Message-ID: <AANLkTinVrXDUUgpq2=CXqkNdTiidUPYcPVrY3Z3dgtEY@mail.gmail.com> (raw)
In-Reply-To: <20110125.134816.48501871.davem@davemloft.net>

On Tue, Jan 25, 2011 at 1:48 PM, David Miller <davem@davemloft.net> wrote:
> From: "H.K. Jerry Chu" <hkchu@google.com>
> Date: Sat, 22 Jan 2011 11:06:17 -0800
>
>> From: Jerry Chu <hkchu@google.com>
>>
>> This patch fixes a bug that causes TCP RST packets to be generated
>> on otherwise correctly behaved applications, e.g., no unread data
>> on close,..., etc. To trigger the bug, at least two conditions must
>> be met:
>>
>> 1. The FIN flag is set on the last data packet, i.e., it's not on a
>> separate, FIN only packet.
>> 2. The size of the last data chunk on the receive side matches
>> exactly with the size of buffer posted by the receiver, and the
>> receiver closes the socket without any further read attempt.
>>
>> This bug was first noticed on our netperf based testbed for our IW10
>> proposal to IETF where a large number of RST packets were observed.
>> netperf's read side code meets the condition 2 above 100%.
>>
>> Before the fix, tcp_data_queue() will queue the last skb that meets
>> condition 1 to sk_receive_queue even though it has fully copied out
>> (skb_copy_datagram_iovec()) the data. Then if condition 2 is also met,
>> tcp_recvmsg() often returns all the copied out data successfully
>> without actually consuming the skb, due to a check
>> "if ((chunk = len - tp->ucopy.len) != 0) {"
>> and
>> "len -= chunk;"
>> after tcp_prequeue_process() that causes "len" to become 0 and an
>> early exit from the big while loop.
>>
>> I don't see any reason not to free the skb whose data have been fully
>> consumed in tcp_data_queue(), regardless of the FIN flag.  We won't
>> get there if MSG_PEEK is on. Am I missing some arcane cases related
>> to urgent data?
>>
>> Signed-off-by: H.K. Jerry Chu <hkchu@google.com>
>
> This bug goes as far back as January, 2000 right after the softnet
> mega-merge happened via the netdev CVS tree (netdev-vger-cvs GIT
> commit 214d457e)

Yes I also tried to trace how long the bug has been there and it seems
to go back to the prehistoric era :). Guess only TCP types got alarmed
by spurious RSTs.

Jerry

>
> Good work, applied, thanks!
>