linux-kernel - Re: [PATCH v2] net:ipv4: send an ack when seg.ack

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Date:   Sat, 19 Mar 2022 04:14:43 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     zhouzhouyi@...il.com
Cc:     fw@...len.de, davem@...emloft.net, yoshfuji@...ux-ipv6.org,
        dsahern@...nel.org, kuba@...nel.org, pabeni@...hat.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Wei Xu <xuweihf@...c.edu.cn>
Subject: Re: [PATCH v2] net:ipv4: send an ack when seg.ack > snd.nxt

On Sat, Mar 19, 2022 at 4:04 AM <zhouzhouyi@...il.com> wrote:
>
> From: Zhouyi Zhou <zhouzhouyi@...il.com>
>
> In RFC 793, page 72: "If the ACK acks something not yet sent
> (SEG.ACK > SND.NXT) then send an ACK, drop the segment,
> and return."
>
> Fix Linux's behavior according to RFC 793.
>
> Reported-by: Wei Xu <xuweihf@...c.edu.cn>
> Signed-off-by: Wei Xu <xuweihf@...c.edu.cn>
> Signed-off-by: Zhouyi Zhou <zhouzhouyi@...il.com>
> ---
> Thank Florian Westphal for pointing out
> the potential duplicated ack bug in patch version 1.

I am travelling this week, but I think your patch is not necessary and
might actually be bad.

Please provide more details of why nobody complained of this until today.

Also I doubt you actually fully tested this patch, sending a V2 30
minutes after V1.

If yes, please provide a packetdrill test.

Thank you.

> --
>  net/ipv4/tcp_input.c | 21 +++++++++++++--------
>  1 file changed, 13 insertions(+), 8 deletions(-)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index bfe4112e000c..4bbf85d7ea8c 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -3771,11 +3771,13 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
>                 goto old_ack;
>         }
>
> -       /* If the ack includes data we haven't sent yet, discard
> -        * this segment (RFC793 Section 3.9).
> +       /* If the ack includes data we haven't sent yet, then send
> +        * an ack, drop this segment, and return (RFC793 Section 3.9 page 72).
>          */
> -       if (after(ack, tp->snd_nxt))
> -               return -1;
> +       if (after(ack, tp->snd_nxt)) {
> +               tcp_send_ack(sk);
> +               return -2;
> +       }
>
>         if (after(ack, prior_snd_una)) {
>                 flag |= FLAG_SND_UNA_ADVANCED;
> @@ -6385,6 +6387,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
>         struct request_sock *req;
>         int queued = 0;
>         bool acceptable;
> +       int ret;
>
>         switch (sk->sk_state) {
>         case TCP_CLOSE:
> @@ -6451,14 +6454,16 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
>                 return 0;
>
>         /* step 5: check the ACK field */
> -       acceptable = tcp_ack(sk, skb, FLAG_SLOWPATH |
> -                                     FLAG_UPDATE_TS_RECENT |
> -                                     FLAG_NO_CHALLENGE_ACK) > 0;
> +       ret = tcp_ack(sk, skb, FLAG_SLOWPATH |
> +                               FLAG_UPDATE_TS_RECENT |
> +                               FLAG_NO_CHALLENGE_ACK);
> +       acceptable = ret > 0;
>
>         if (!acceptable) {
>                 if (sk->sk_state == TCP_SYN_RECV)
>                         return 1;       /* send one RST */
> -               tcp_send_challenge_ack(sk);
> +               if (ret > -2)
> +                       tcp_send_challenge_ack(sk);
>                 goto discard;
>         }
>         switch (sk->sk_state) {
> --
> 2.25.1
>