Re: [PATCH v2] net:ipv4: send an ack when seg.ack > snd.nxt

[Zhouyi Zhou <zhouzhouyi@gmail.com>]

Thanks for reviewing my patch

On Sat, Mar 19, 2022 at 7:14 PM Eric Dumazet <edumazet@google.com> wrote:
>
> On Sat, Mar 19, 2022 at 4:04 AM <zhouzhouyi@gmail.com> wrote:
> >
> > From: Zhouyi Zhou <zhouzhouyi@gmail.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@ustc.edu.cn>
> > Signed-off-by: Wei Xu <xuweihf@ustc.edu.cn>
> > Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.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.
I am a beginner to TCP, although I have submitted once a patch to
netdev in 2013 (aaa0c23cb90141309f5076ba5e3bfbd39544b985), this is
first time I learned packetdrill test.
I think I should do the packetdrill test in the coming days, and
provide more details of how this (RFC793 related) can happen.

Apologize sincerely in advance if I have made noise.

Thank you for your time

Sincerely
Zhouyi
>
> 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
> >