Subject: Re: [PATCH net] tcp: fix wraparound issue in tcp_lp

From: Stephen Hemminger <[email-masked]>
Date: Mon, 1 May 2017 16:56:43 -0700

On Mon, 01 May 2017 15:29:48 -0700
Eric Dumazet <[email-masked]> wrote:

> From: Eric Dumazet <[email-masked]>
> 
> Be careful when comparing tcp_time_stamp to some u32 quantity,
> otherwise result can be surprising.
> 
> Fixes: 7c106d7e782b ("[TCP]: TCP Low Priority congestion control")
> Signed-off-by: Eric Dumazet <[email-masked]>
> ---
>  net/ipv4/tcp_lp.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/tcp_lp.c b/net/ipv4/tcp_lp.c
> index 046fd3910873306d74207615d6997e1c847ea361..d6fb6c067af4641f232b94e7c590c212648e8173 100644
> --- a/net/ipv4/tcp_lp.c
> +++ b/net/ipv4/tcp_lp.c
> @@ -264,13 +264,15 @@ static void tcp_lp_pkts_acked(struct sock *sk, const struct ack_sample *sample)
>  {
>  	struct tcp_sock *tp = tcp_sk(sk);
>  	struct lp *lp = inet_csk_ca(sk);
> +	u32 delta;
>  
>  	if (sample->rtt_us > 0)
>  		tcp_lp_rtt_sample(sk, sample->rtt_us);
>  
>  	/* calc inference */
> -	if (tcp_time_stamp > tp->rx_opt.rcv_tsecr)
> -		lp->inference = 3 * (tcp_time_stamp - tp->rx_opt.rcv_tsecr);
> +	delta = tcp_time_stamp - tp->rx_opt.rcv_tsecr;
> +	if ((s32)delta > 0)
> +		lp->inference = 3 * delta;

Agreed time wraparound would cause problems.
But why not use existing time_after() macro here?