tcp: Make simultaneous connect() RFC-compliant.

RFC 9293 states that in the case of simultaneous connect(), the connection gets established when SYN+ACK is received. [0]

TCP Peer A TCP Peer B

  1.  CLOSED                                           CLOSED
  2.  SYN-SENT     --> <SEQ=100><CTL=SYN>              ...
  3.  SYN-RECEIVED <-- <SEQ=300><CTL=SYN>              <-- SYN-SENT
  4.               ... <SEQ=100><CTL=SYN>              --> SYN-RECEIVED

5. SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...
6. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
7. ... <SEQ=100><ACK=301><CTL=SYN,ACK> --> ESTABLISHED

However, since commit 0c24604b68fc ("tcp: implement RFC 5961 4.2"), such a SYN+ACK is dropped in tcp_validate_incoming() and responded with Challenge ACK.

For example, the write() syscall in the following packetdrill script fails with -EAGAIN, and wrong SNMP stats get incremented.

0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_TCP) = 3
+0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)

+0 > S 0:0(0) <mss 1460,sackOK,TS val 1000 ecr 0,nop,wscale 8> +0 < S 0:0(0) win 1000 <mss 1000>
+0 > S. 0:0(0) ack 1 <mss 1460,sackOK,TS val 3308134035 ecr 0,nop,wscale 8> +0 < S. 0:0(0) ack 1 win 1000

+0 write(3, ..., 100) = 100
+0 > P. 1:101(100) ack 1

--

  # packetdrill cross-synack.pkt
  cross-synack.pkt:13: runtime error in write call: Expected result 100 but got -1 with errno 11 (Resource temporarily unavailable)
  # nstat
  ...
  TcpExtTCPChallengeACK           1                  0.0
  TcpExtTCPSYNChallenge           1                  0.0

The problem is that bpf_skops_established() is triggered by the Challenge ACK instead of SYN+ACK. This causes the bpf prog to miss the chance to check if the peer supports a TCP option that is expected to be exchanged in SYN and SYN+ACK.

Let's accept a bare SYN+ACK for active-open TCP_SYN_RECV sockets to avoid such a situation.

Note that tcp_ack_snd_check() in tcp_rcv_state_process() is skipped not to send an unnecessary ACK, but this could be a bit risky for net.git, so this targets for net-next.

Link https://www.rfc-editor.org/rfc/rfc9293.html#section-3.5-7 [0]

Signed-off-by Kuniyuki Iwashima <[email protected]>

---

RFC 9293 states that in the case of simultaneous connect(), the connection gets established when SYN+ACK is received. [0]

TCP Peer A TCP Peer B

  1.  CLOSED                                           CLOSED
  2.  SYN-SENT     --> <SEQ=100><CTL=SYN>              ...
  3.  SYN-RECEIVED <-- <SEQ=300><CTL=SYN>              <-- SYN-SENT
  4.               ... <SEQ=100><CTL=SYN>              --> SYN-RECEIVED

5. SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...
6. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
7. ... <SEQ=100><ACK=301><CTL=SYN,ACK> --> ESTABLISHED

However, since commit 0c24604b68fc ("tcp: implement RFC 5961 4.2"), such a SYN+ACK is dropped in tcp_validate_incoming() and responded with Challenge ACK.

For example, the write() syscall in the following packetdrill script fails with -EAGAIN, and wrong SNMP stats get incremented.

0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_TCP) = 3
+0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)

+0 > S 0:0(0) <mss 1460,sackOK,TS val 1000 ecr 0,nop,wscale 8> +0 < S 0:0(0) win 1000 <mss 1000>
+0 > S. 0:0(0) ack 1 <mss 1460,sackOK,TS val 3308134035 ecr 0,nop,wscale 8> +0 < S. 0:0(0) ack 1 win 1000

+0 write(3, ..., 100) = 100
+0 > P. 1:101(100) ack 1

--

  # packetdrill cross-synack.pkt
  cross-synack.pkt:13: runtime error in write call: Expected result 100 but got -1 with errno 11 (Resource temporarily unavailable)
  # nstat
  ...
  TcpExtTCPChallengeACK           1                  0.0
  TcpExtTCPSYNChallenge           1                  0.0

The problem is that bpf_skops_established() is triggered by the Challenge ACK instead of SYN+ACK. This causes the bpf prog to miss the chance to check if the peer supports a TCP option that is expected to be exchanged in SYN and SYN+ACK.

Let's accept a bare SYN+ACK for active-open TCP_SYN_RECV sockets to avoid such a situation.

Note that tcp_ack_snd_check() in tcp_rcv_state_process() is skipped not to send an unnecessary ACK, but this could be a bit risky for net.git, so this targets for net-next.

Link https://www.rfc-editor.org/rfc/rfc9293.html#section-3.5-7 [0]

Signed-off-by Kuniyuki Iwashima <[email protected]>

---
 net/ipv4/tcp_input.c | 9 +++++++++
 1 file changed, 9 insertions(+)

net/ipv4/tcp_input.c +6 -0

index 47dacb575f74..1eddb6b9fb2a 100644
@@ -5989,6 +5989,11 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 	 * RFC 5961 4.2 : Send a challenge ack
 	 */
 	if (th->syn) {
+		if (sk->sk_state == TCP_SYN_RECV && sk->sk_socket && th->ack &&
+		    TCP_SKB_CB(skb)->seq + 1 == TCP_SKB_CB(skb)->end_seq &&
+		    TCP_SKB_CB(skb)->seq + 1 == tp->rcv_nxt &&
+		    TCP_SKB_CB(skb)->ack_seq == tp->snd_nxt)
+			goto pass;
 syn_challenge:
 		if (syn_inerr)
 			TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);

@@ -5998,6 +6003,7 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 		goto discard;
 	}
 
+pass:
 	bpf_skops_parse_hdr(sk, skb);
 
 	return true;

Hi Kuniyuki,

Quoted 48 lines · Kuniyuki Iwashima

RFC 9293 states that in the case of simultaneous connect(), the connection gets established when SYN+ACK is received. [0]

TCP Peer A TCP Peer B

  1.  CLOSED                                           CLOSED
  2.  SYN-SENT     --> <SEQ=100><CTL=SYN>              ...
  3.  SYN-RECEIVED <-- <SEQ=300><CTL=SYN>              <-- SYN-SENT
  4.               ... <SEQ=100><CTL=SYN>              --> SYN-RECEIVED

5. SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...
6. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
7. ... <SEQ=100><ACK=301><CTL=SYN,ACK> --> ESTABLISHED

However, since commit 0c24604b68fc ("tcp: implement RFC 5961 4.2"), such a SYN+ACK is dropped in tcp_validate_incoming() and responded with Challenge ACK.

For example, the write() syscall in the following packetdrill script fails with -EAGAIN, and wrong SNMP stats get incremented.

0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_TCP) = 3
+0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)

+0 > S 0:0(0) <mss 1460,sackOK,TS val 1000 ecr 0,nop,wscale 8> +0 < S 0:0(0) win 1000 <mss 1000>
+0 > S. 0:0(0) ack 1 <mss 1460,sackOK,TS val 3308134035 ecr 0,nop,wscale 8> +0 < S. 0:0(0) ack 1 win 1000

+0 write(3, ..., 100) = 100
+0 > P. 1:101(100) ack 1

--

  # packetdrill cross-synack.pkt
  cross-synack.pkt:13: runtime error in write call: Expected result 100 but got -1 with errno 11 (Resource temporarily unavailable)
  # nstat
  ...
  TcpExtTCPChallengeACK           1                  0.0
  TcpExtTCPSYNChallenge           1                  0.0

The problem is that bpf_skops_established() is triggered by the Challenge ACK instead of SYN+ACK. This causes the bpf prog to miss the chance to check if the peer supports a TCP option that is expected to be exchanged in SYN and SYN+ACK.

Let's accept a bare SYN+ACK for active-open TCP_SYN_RECV sockets to avoid such a situation.

Quoted 6 lines

Note that tcp_ack_snd_check() in tcp_rcv_state_process() is skipped not to send an unnecessary ACK, but this could be a bit risky for net.git, so this targets for net-next.

Link https://www.rfc-editor.org/rfc/rfc9293.html#section-3.5-7 [0]

Signed-off-by Kuniyuki Iwashima <[email protected]>

Thank you for having worked on this patch!

Quoted 27 lines

---
 net/ipv4/tcp_input.c | 9 +++++++++
 1 file changed, 9 insertions(+)

net/ipv4/tcp_input.c +6 -0

index 47dacb575f74..1eddb6b9fb2a 100644
@@ -5989,6 +5989,11 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 	 * RFC 5961 4.2 : Send a challenge ack
 	 */
 	if (th->syn) {
+		if (sk->sk_state == TCP_SYN_RECV && sk->sk_socket && th->ack &&
+		    TCP_SKB_CB(skb)->seq + 1 == TCP_SKB_CB(skb)->end_seq &&
+		    TCP_SKB_CB(skb)->seq + 1 == tp->rcv_nxt &&
+		    TCP_SKB_CB(skb)->ack_seq == tp->snd_nxt)
+			goto pass;
 syn_challenge:
 		if (syn_inerr)
 			TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);

@@ -5998,6 +6003,7 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 		goto discard;
 	}
 
+pass:
 	bpf_skops_parse_hdr(sk, skb);
 

net/ipv4/tcp_input.c tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb) · line 6815

Quoted 8 lines

net/ipv4/tcp_input.c +3 -0

 	return true;

@@ -6804,6 +6810,9 @@ tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 		tcp_fast_path_on(tp);
 		if (sk->sk_shutdown & SEND_SHUTDOWN)
 			tcp_shutdown(sk, SEND_SHUTDOWN);
+
+		if (sk->sk_socket)
+			goto consume;

It looks like this modification changes the behaviour for MPTCP Join requests for listening sockets: when receiving the 3rd ACK of a request adding a new path (MP_JOIN), sk->sk_socket will be set, and point to the MPTCP sock that has been created when the MPTCP connection got created before with the first path.

This new 'goto' here will then skip the
process of the segment text (step 7) and not go through tcp_data_queue() where the MPTCP options are validated, and some actions are triggered, e.g. sending the MPJ 4th ACK [1].

This doesn't fully break MPTCP, mainly the 4th MPJ ACK that will be delayed,

but it looks like it affects the MPTFO feature as well -- probably in case of retransmissions I suppose -- and being the reason why the selftests started to be unstable the last few days [2].

[1] https://datatracker.ietf.org/doc/html/rfc8684#fig_tokens
[2]
https://netdev.bots.linux.dev/contest.html?executor=vmksft-mptcp-dbg&test=mptcp-connect-sh

Looking at what this patch here is trying to fix, I wonder if it would not be enough to apply this patch:

Quoted 8 lines

net/ipv4/tcp_input.c +0 -0

index ff9ab3d01ced..ff981d7776c3 100644
@@ -6820,7 +6820,7 @@ tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
                if (sk->sk_shutdown & SEND_SHUTDOWN)
                        tcp_shutdown(sk, SEND_SHUTDOWN);
 

net/ipv4/tcp_input.c tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb) · line 6826

Quoted 5 lines

net/ipv4/tcp_input.c +1 -1

-               if (sk->sk_socket)
+               if (sk->sk_socket && !sk_is_mptcp(sk))
                        goto consume;
                break;
 

But I still need to investigate how the issue that is being addressed by your patch can be translated to the MPTCP case. I guess we could add additional checks for MPTCP: new connection or additional path? etc. Or maybe that's not needed.

Hi Matthieu,

From: Matthieu Baerts <[email protected]>
Date: Mon, 15 Jul 2024 17:58:49 +0200

Quoted 87 lines

Hi Kuniyuki,

Quoted 48 lines · Kuniyuki Iwashima

RFC 9293 states that in the case of simultaneous connect(), the connection gets established when SYN+ACK is received. [0]

TCP Peer A TCP Peer B

  1.  CLOSED                                           CLOSED
  2.  SYN-SENT     --> <SEQ=100><CTL=SYN>              ...
  3.  SYN-RECEIVED <-- <SEQ=300><CTL=SYN>              <-- SYN-SENT
  4.               ... <SEQ=100><CTL=SYN>              --> SYN-RECEIVED

5. SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...
6. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
7. ... <SEQ=100><ACK=301><CTL=SYN,ACK> --> ESTABLISHED

However, since commit 0c24604b68fc ("tcp: implement RFC 5961 4.2"), such a SYN+ACK is dropped in tcp_validate_incoming() and responded with Challenge ACK.

For example, the write() syscall in the following packetdrill script fails with -EAGAIN, and wrong SNMP stats get incremented.

0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_TCP) = 3
+0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)

+0 > S 0:0(0) <mss 1460,sackOK,TS val 1000 ecr 0,nop,wscale 8> +0 < S 0:0(0) win 1000 <mss 1000>
+0 > S. 0:0(0) ack 1 <mss 1460,sackOK,TS val 3308134035 ecr 0,nop,wscale 8> +0 < S. 0:0(0) ack 1 win 1000

+0 write(3, ..., 100) = 100
+0 > P. 1:101(100) ack 1

--

  # packetdrill cross-synack.pkt
  cross-synack.pkt:13: runtime error in write call: Expected result 100 but got -1 with errno 11 (Resource temporarily unavailable)
  # nstat
  ...
  TcpExtTCPChallengeACK           1                  0.0
  TcpExtTCPSYNChallenge           1                  0.0

The problem is that bpf_skops_established() is triggered by the Challenge ACK instead of SYN+ACK. This causes the bpf prog to miss the chance to check if the peer supports a TCP option that is expected to be exchanged in SYN and SYN+ACK.

Let's accept a bare SYN+ACK for active-open TCP_SYN_RECV sockets to avoid such a situation.

Quoted 6 lines

Note that tcp_ack_snd_check() in tcp_rcv_state_process() is skipped not to send an unnecessary ACK, but this could be a bit risky for net.git, so this targets for net-next.

Link https://www.rfc-editor.org/rfc/rfc9293.html#section-3.5-7 [0]

Signed-off-by Kuniyuki Iwashima <[email protected]>

Thank you for having worked on this patch!

Quoted 27 lines

---
 net/ipv4/tcp_input.c | 9 +++++++++
 1 file changed, 9 insertions(+)

net/ipv4/tcp_input.c +6 -0

index 47dacb575f74..1eddb6b9fb2a 100644
@@ -5989,6 +5989,11 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 	 * RFC 5961 4.2 : Send a challenge ack
 	 */
 	if (th->syn) {
+		if (sk->sk_state == TCP_SYN_RECV && sk->sk_socket && th->ack &&
+		    TCP_SKB_CB(skb)->seq + 1 == TCP_SKB_CB(skb)->end_seq &&
+		    TCP_SKB_CB(skb)->seq + 1 == tp->rcv_nxt &&
+		    TCP_SKB_CB(skb)->ack_seq == tp->snd_nxt)
+			goto pass;
 syn_challenge:
 		if (syn_inerr)
 			TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);

@@ -5998,6 +6003,7 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 		goto discard;
 	}
 
+pass:
 	bpf_skops_parse_hdr(sk, skb);
 

Quoted 14 lines

net/ipv4/tcp_input.c tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb) · line 6815

Quoted 8 lines

net/ipv4/tcp_input.c +3 -0

 	return true;

@@ -6804,6 +6810,9 @@ tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 		tcp_fast_path_on(tp);
 		if (sk->sk_shutdown & SEND_SHUTDOWN)
 			tcp_shutdown(sk, SEND_SHUTDOWN);
+
+		if (sk->sk_socket)
+			goto consume;

It looks like this modification changes the behaviour for MPTCP Join requests for listening sockets: when receiving the 3rd ACK of a request adding a new path (MP_JOIN), sk->sk_socket will be set, and point to the MPTCP sock that has been created when the MPTCP connection got created before with the first path.

Thanks for catching this!

I completely missed how MPTCP sets sk->sk_socket before the 3rd ACK is processed.

I debugged a bit and confirmed mptcp_stream_accept() sets the inflight subflow's sk->sk_socket with newsk->sk_socket.

Quoted 7 lines

This new 'goto' here will then skip the
process of the segment text (step 7) and not go through tcp_data_queue() where the MPTCP options are validated, and some actions are triggered, e.g. sending the MPJ 4th ACK [1].

This doesn't fully break MPTCP, mainly the 4th MPJ ACK that will be delayed,

Yes, the test failure depends on timing. I only reproduced it by running the test many times on non-kvm qemu.

Quoted 20 lines

but it looks like it affects the MPTFO feature as well -- probably in case of retransmissions I suppose -- and being the reason why the selftests started to be unstable the last few days [2].

[1] https://datatracker.ietf.org/doc/html/rfc8684#fig_tokens
[2]
https://netdev.bots.linux.dev/contest.html?executor=vmksft-mptcp-dbg&test=mptcp-connect-sh

Looking at what this patch here is trying to fix, I wonder if it would not be enough to apply this patch:

Quoted 8 lines

net/ipv4/tcp_input.c +0 -0

index ff9ab3d01ced..ff981d7776c3 100644
@@ -6820,7 +6820,7 @@ tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
                if (sk->sk_shutdown & SEND_SHUTDOWN)
                        tcp_shutdown(sk, SEND_SHUTDOWN);
 

Quoted 10 lines

net/ipv4/tcp_input.c tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb) · line 6826

Quoted 5 lines

net/ipv4/tcp_input.c +1 -1

-               if (sk->sk_socket)
+               if (sk->sk_socket && !sk_is_mptcp(sk))
                        goto consume;
                break;
 

But I still need to investigate how the issue that is being addressed by your patch can be translated to the MPTCP case. I guess we could add additional checks for MPTCP: new connection or additional path? etc. Or maybe that's not needed.

My first intention was not to drop SYN+ACK in tcp_validate_incoming(), and the goto is added in v2, which is rather to be more compliant with RFC not to send an unnecessary ACK for simultaneous connect().

So, we could rewrite the condition as this,

  if (sk->sk_socket && !th->syn)

but I think your patch is better to give a hint that MPTCP has a different logic.

Also, a similar check done before the goto, and this could be improved ?

  if (sk->sk_socket)
    sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);

Hi Kuniyuki,

Thank you for your reply!

Quoted 106 lines · Kuniyuki Iwashima

Hi Matthieu,

From: Matthieu Baerts <[email protected]>
Date: Mon, 15 Jul 2024 17:58:49 +0200

Quoted 87 lines

Hi Kuniyuki,

Quoted 48 lines · Kuniyuki Iwashima

RFC 9293 states that in the case of simultaneous connect(), the connection gets established when SYN+ACK is received. [0]

TCP Peer A TCP Peer B

  1.  CLOSED                                           CLOSED
  2.  SYN-SENT     --> <SEQ=100><CTL=SYN>              ...
  3.  SYN-RECEIVED <-- <SEQ=300><CTL=SYN>              <-- SYN-SENT
  4.               ... <SEQ=100><CTL=SYN>              --> SYN-RECEIVED

5. SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...
6. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
7. ... <SEQ=100><ACK=301><CTL=SYN,ACK> --> ESTABLISHED

However, since commit 0c24604b68fc ("tcp: implement RFC 5961 4.2"), such a SYN+ACK is dropped in tcp_validate_incoming() and responded with Challenge ACK.

For example, the write() syscall in the following packetdrill script fails with -EAGAIN, and wrong SNMP stats get incremented.

0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_TCP) = 3
+0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)

+0 > S 0:0(0) <mss 1460,sackOK,TS val 1000 ecr 0,nop,wscale 8> +0 < S 0:0(0) win 1000 <mss 1000>
+0 > S. 0:0(0) ack 1 <mss 1460,sackOK,TS val 3308134035 ecr 0,nop,wscale 8> +0 < S. 0:0(0) ack 1 win 1000

+0 write(3, ..., 100) = 100
+0 > P. 1:101(100) ack 1

--

  # packetdrill cross-synack.pkt
  cross-synack.pkt:13: runtime error in write call: Expected result 100 but got -1 with errno 11 (Resource temporarily unavailable)
  # nstat
  ...
  TcpExtTCPChallengeACK           1                  0.0
  TcpExtTCPSYNChallenge           1                  0.0

The problem is that bpf_skops_established() is triggered by the Challenge ACK instead of SYN+ACK. This causes the bpf prog to miss the chance to check if the peer supports a TCP option that is expected to be exchanged in SYN and SYN+ACK.

Let's accept a bare SYN+ACK for active-open TCP_SYN_RECV sockets to avoid such a situation.

Quoted 6 lines

Note that tcp_ack_snd_check() in tcp_rcv_state_process() is skipped not to send an unnecessary ACK, but this could be a bit risky for net.git, so this targets for net-next.

Link https://www.rfc-editor.org/rfc/rfc9293.html#section-3.5-7 [0]

Signed-off-by Kuniyuki Iwashima <[email protected]>

Thank you for having worked on this patch!

Quoted 27 lines

---
 net/ipv4/tcp_input.c | 9 +++++++++
 1 file changed, 9 insertions(+)

net/ipv4/tcp_input.c +6 -0

index 47dacb575f74..1eddb6b9fb2a 100644
@@ -5989,6 +5989,11 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 	 * RFC 5961 4.2 : Send a challenge ack
 	 */
 	if (th->syn) {
+		if (sk->sk_state == TCP_SYN_RECV && sk->sk_socket && th->ack &&
+		    TCP_SKB_CB(skb)->seq + 1 == TCP_SKB_CB(skb)->end_seq &&
+		    TCP_SKB_CB(skb)->seq + 1 == tp->rcv_nxt &&
+		    TCP_SKB_CB(skb)->ack_seq == tp->snd_nxt)
+			goto pass;
 syn_challenge:
 		if (syn_inerr)
 			TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);

@@ -5998,6 +6003,7 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
 		goto discard;
 	}
 
+pass:
 	bpf_skops_parse_hdr(sk, skb);
 

Quoted 14 lines

net/ipv4/tcp_input.c tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb) · line 6815

Quoted 8 lines

net/ipv4/tcp_input.c +3 -0

 	return true;

@@ -6804,6 +6810,9 @@ tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 		tcp_fast_path_on(tp);
 		if (sk->sk_shutdown & SEND_SHUTDOWN)
 			tcp_shutdown(sk, SEND_SHUTDOWN);
+
+		if (sk->sk_socket)
+			goto consume;

It looks like this modification changes the behaviour for MPTCP Join requests for listening sockets: when receiving the 3rd ACK of a request adding a new path (MP_JOIN), sk->sk_socket will be set, and point to the MPTCP sock that has been created when the MPTCP connection got created before with the first path.

Quoted 4 lines

Thanks for catching this!

I completely missed how MPTCP sets sk->sk_socket before the 3rd ACK is processed.

No problem. That's a shame there was not a clear error in the selftests :)

Quoted 2 lines

I debugged a bit and confirmed mptcp_stream_accept() sets the inflight subflow's sk->sk_socket with newsk->sk_socket.

Yes, that's correct.

Quoted 10 lines

Quoted 7 lines

This new 'goto' here will then skip the
process of the segment text (step 7) and not go through tcp_data_queue() where the MPTCP options are validated, and some actions are triggered, e.g. sending the MPJ 4th ACK [1].

This doesn't fully break MPTCP, mainly the 4th MPJ ACK that will be delayed,

Yes, the test failure depends on timing. I only reproduced it by running the test many times on non-kvm qemu.

Thank you for having checked!

Quoted 35 lines

Quoted 20 lines

but it looks like it affects the MPTFO feature as well -- probably in case of retransmissions I suppose -- and being the reason why the selftests started to be unstable the last few days [2].

[1] https://datatracker.ietf.org/doc/html/rfc8684#fig_tokens
[2]
https://netdev.bots.linux.dev/contest.html?executor=vmksft-mptcp-dbg&test=mptcp-connect-sh

Looking at what this patch here is trying to fix, I wonder if it would not be enough to apply this patch:

Quoted 8 lines

net/ipv4/tcp_input.c +0 -0

index ff9ab3d01ced..ff981d7776c3 100644
@@ -6820,7 +6820,7 @@ tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
                if (sk->sk_shutdown & SEND_SHUTDOWN)
                        tcp_shutdown(sk, SEND_SHUTDOWN);
 

Quoted 10 lines

net/ipv4/tcp_input.c tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb) · line 6826

Quoted 5 lines

net/ipv4/tcp_input.c +1 -1

-               if (sk->sk_socket)
+               if (sk->sk_socket && !sk_is_mptcp(sk))
                        goto consume;
                break;
 

But I still need to investigate how the issue that is being addressed by your patch can be translated to the MPTCP case. I guess we could add additional checks for MPTCP: new connection or additional path? etc. Or maybe that's not needed.

My first intention was not to drop SYN+ACK in tcp_validate_incoming(), and the goto is added in v2, which is rather to be more compliant with RFC not to send an unnecessary ACK for simultaneous connect().

Quoted 3 lines

So, we could rewrite the condition as this,

  if (sk->sk_socket && !th->syn)

(Just to be sure, do you mean the opposite with th->syn?)

  if (sk->sk_socket && th->syn)
      goto consume;

That's a good idea!

I sent my patch a couple of minutes ago [1], then I saw your suggestion here. It looks like it should work for the TFO case as well. Maybe your suggestion is more generic and will cover more cases?

[1]
https://lore.kernel.org/all/20240716-upstream-net-next-20240716-tcp-3rd-ack-consume-sk_socket-v1-1-4e61d0b79233@kernel.org/

Quoted 2 lines

but I think your patch is better to give a hint that MPTCP has a different logic.

Because TFO has also a different logic, it might be good to have a clear comment about that.

Quoted 6 lines

Also, a similar check done before the goto, and this could be improved ?

  if (sk->sk_socket)
    sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);

It is a bit late for me, but I think it can be kept as it is: for MPTCP, it will not wake up the userspace as the subflow is managed by the kernel. I would need to check if we could avoid that. Also, will this wakeup not be useful for TFO?