2022-03-21 16:09:14

by Zhouyi Zhou

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

From: Zhouyi Zhou <[email protected]>

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 <[email protected]>
Signed-off-by: Wei Xu <[email protected]>
Signed-off-by: Zhouyi Zhou <[email protected]>
---
Thank Florian Westphal for pointing out
the potential duplicated ack bug in patch version 1.
--
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

2022-03-21 23:07:41

by Eric Dumazet

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

On Sat, Mar 19, 2022 at 4:04 AM <[email protected]> wrote:
>
> From: Zhouyi Zhou <[email protected]>
>
> 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 <[email protected]>
> Signed-off-by: Wei Xu <[email protected]>
> Signed-off-by: Zhouyi Zhou <[email protected]>
> ---
> 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
>