netdev - Re: [PATCH] tcp: do not forget FIN in tcp_shifted

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Message-ID: <alpine.DEB.2.02.1310042101040.32153@melkinpaasi.cs.helsinki.fi>
Date:	Fri, 4 Oct 2013 21:03:53 +0300 (EEST)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Neal Cardwell <ncardwell@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH] tcp: do not forget FIN in tcp_shifted_skb()

On Fri, 4 Oct 2013, Eric Dumazet wrote:

> From: Eric Dumazet <edumazet@...gle.com>
> 
> Yuchung found following problem :
> 
>  There are bugs in the SACK processing code, merging part in
>  tcp_shift_skb_data(), that incorrectly resets or ignores the sacked
>  skbs FIN flag. When a receiver first SACK the FIN sequence, and later
>  throw away ofo queue (e.g., sack-reneging), the sender will stop
>  retransmitting the FIN flag, and hangs forever.
> 
> Following packetdrill test can be used to reproduce the bug.
> 
> [...snip...]
> 
> First, a typo inverted left/right of one OR operation, then
> code forgot to advance end_seq if the merged skb carried FIN.
> 
> Bug was added in 2.6.29 by commit 832d11c5cd076ab
> ("tcp: Try to restore large SKBs while SACK processing")
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
> Acked-by: Neal Cardwell <ncardwell@...gle.com>
> Cc: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
> ---
>  net/ipv4/tcp_input.c |    5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 25a89ea..113dc5f 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -1284,7 +1284,10 @@ static bool tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
>  		tp->lost_cnt_hint -= tcp_skb_pcount(prev);
>  	}
>  
> -	TCP_SKB_CB(skb)->tcp_flags |= TCP_SKB_CB(prev)->tcp_flags;
> +	TCP_SKB_CB(prev)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags;
> +	if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
> +		TCP_SKB_CB(prev)->end_seq++;
> +
>  	if (skb == tcp_highest_sack(sk))
>  		tcp_advance_highest_sack(sk, skb);

Nice that it was finally found. For some reason my memory tries to say 
that it wouldn't have not even tried to merge skbs with FIN but either
I changed that at some point while deving or I just remember wrong.

Acked-by: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>

-- 
 i.