Re: [PATCH] ipv4: avoid divide 0 error in tcp_incr_quickack

[Eric Dumazet]

On Mon, 2014-11-03 at 09:29 +0800, Chen Weilong wrote:
> From: Weilong Chen <chenweilong@xxxxxxxxxx>
> 
> We got a problem like this:

> There was a wrong ack packet coming during TCP handshake. The socket's state
> was TCP_SYN_RECV, its rcv_mss was not initialize yet. So
> tcp_send_dupack -> tcp_enter_quickack_mode got a divide 0 error.
> This patch add a state check before tcp_enter_quickack_mode.
> 
> Signed-off-by: Weilong Chen <chenweilong@xxxxxxxxxx>
> ---
>  net/ipv4/tcp_input.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 4e4617e..9eb56dc 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -3986,7 +3986,8 @@ static void tcp_send_dupack(struct sock *sk, const struct sk_buff *skb)
>  	if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
>  	    before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
>  		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
> -		tcp_enter_quickack_mode(sk);
> +		if (sk->sk_state != TCP_SYN_RECV)
> +			tcp_enter_quickack_mode(sk);
>  
>  		if (tcp_is_sack(tp) && sysctl_tcp_dsack) {
>  			u32 end_seq = TCP_SKB_CB(skb)->end_seq;


Sorry I do not think this is the right fix.

We have to not simply avoid the divide, but fix this issue by
understanding the missing steps.



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