Re: [PATCH net v2 1/2] tcp: process the 3rd ACK with sk_socket for TFO/MPTCP

[Matthieu Baerts <matttbe@kernel.org>]

Hi Eric,

+cc Neal
-cc Jerry (NoSuchUser)

On 23/07/2024 16:37, Eric Dumazet wrote:
> On Thu, Jul 18, 2024 at 12:34 PM Matthieu Baerts (NGI0)
> <matttbe@kernel.org> wrote:
>>
>> The 'Fixes' commit recently changed the behaviour of TCP by skipping the
>> processing of the 3rd ACK when a sk->sk_socket is set. The goal was to
>> skip tcp_ack_snd_check() in tcp_rcv_state_process() not to send an
>> unnecessary ACK in case of simultaneous connect(). Unfortunately, that
>> had an impact on TFO and MPTCP.
>>
>> I started to look at the impact on MPTCP, because the MPTCP CI found
>> some issues with the MPTCP Packetdrill tests [1]. Then Paolo suggested
>> me to look at the impact on TFO with "plain" TCP.
>>
>> For MPTCP, 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 established before with
>> the first path. The newly added 'goto' will then skip the processing 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 [2] as demonstrated by the new errors when
>> running a packetdrill test [3] establishing a second subflow.
>>
>> This doesn't fully break MPTCP, mainly the 4th MPJ ACK that will be
>> delayed. Still, we don't want to have this behaviour as it delays the
>> switch to the fully established mode, and invalid MPTCP options in this
>> 3rd ACK will not be caught any more. This modification also affects the
>> MPTCP + TFO feature as well, and being the reason why the selftests
>> started to be unstable the last few days [4].
>>
>> For TFO, the existing 'basic-cookie-not-reqd' test [5] was no longer
>> passing: if the 3rd ACK contains data, and the connection is accept()ed
>> before receiving them, these data would no longer be processed, and thus
>> not ACKed.
>>
>> One last thing about MPTCP, in case of simultaneous connect(), a
>> fallback to TCP will be done, which seems fine:
>>
>>   `../common/defaults.sh`
>>
>>    0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_MPTCP) = 3
>>   +0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)
>>
>>   +0 > S  0:0(0)                 <mss 1460, sackOK, TS val 100 ecr 0,   nop, wscale 8, mpcapable v1 flags[flag_h] nokey>
>>   +0 < S  0:0(0) win 1000        <mss 1460, sackOK, TS val 407 ecr 0,   nop, wscale 8, mpcapable v1 flags[flag_h] nokey>
>>   +0 > S. 0:0(0) ack 1           <mss 1460, sackOK, TS val 330 ecr 0,   nop, wscale 8, mpcapable v1 flags[flag_h] nokey>
>>   +0 < S. 0:0(0) ack 1 win 65535 <mss 1460, sackOK, TS val 700 ecr 100, nop, wscale 8, mpcapable v1 flags[flag_h] key[skey=2]>
>>
>>   +0 write(3, ..., 100) = 100
>>   +0 >  . 1:1(0)     ack 1 <nop, nop, TS val 845707014 ecr 700, nop, nop, sack 0:1>
>>   +0 > P. 1:101(100) ack 1 <nop, nop, TS val 845958933 ecr 700>
>>
>> Simultaneous SYN-data crossing is also not supported by TFO, see [6].
>>
>> Link: https://github.com/multipath-tcp/mptcp_net-next/actions/runs/9936227696 [1]
>> Link: https://datatracker.ietf.org/doc/html/rfc8684#fig_tokens [2]
>> Link: https://github.com/multipath-tcp/packetdrill/blob/mptcp-net-next/gtests/net/mptcp/syscalls/accept.pkt#L28 [3]
>> Link: https://netdev.bots.linux.dev/contest.html?executor=vmksft-mptcp-dbg&test=mptcp-connect-sh [4]
>> Link: https://github.com/google/packetdrill/blob/master/gtests/net/tcp/fastopen/server/basic-cookie-not-reqd.pkt#L21 [5]
>> Link: https://github.com/google/packetdrill/blob/master/gtests/net/tcp/fastopen/client/simultaneous-fast-open.pkt [6]
>> Fixes: 23e89e8ee7be ("tcp: Don't drop SYN+ACK for simultaneous connect().")
>> Suggested-by: Paolo Abeni <pabeni@redhat.com>
>> Suggested-by: Kuniyuki Iwashima <kuniyu@amazon.com>
>> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
>> ---
>> Notes:
>>  - We could also drop this 'goto consume', and send the unnecessary ACK
>>    in this simultaneous connect case, which doesn't seem to be a "real"
>>    case, more something for fuzzers. But that's not what the RFC 9293
>>    recommends to do.
>>  - v2:
>>    - Check if the SYN bit is set instead of looking for TFO and MPTCP
>>      specific attributes, as suggested by Kuniyuki.
>>    - Updated the comment above
>>    - Please note that the v2 has been sent mainly to satisfy the CI (to
>>      be able to catch new bugs with MPTCP), and because the suggestion
>>      from Kuniyuki looks better. It has not been sent to urge TCP
>>      maintainers to review it quicker than it should, please take your
>>      time and enjoy netdev.conf :)
>> ---
>>  net/ipv4/tcp_input.c | 7 ++++++-
>>  1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
>> index ff9ab3d01ced..bfe1bc69dc3e 100644
>> --- a/net/ipv4/tcp_input.c
>> +++ b/net/ipv4/tcp_input.c
>> @@ -6820,7 +6820,12 @@ tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
>>                 if (sk->sk_shutdown & SEND_SHUTDOWN)
>>                         tcp_shutdown(sk, SEND_SHUTDOWN);
>>
>> -               if (sk->sk_socket)
>> +               /* For crossed SYN cases, not to send an unnecessary ACK.
>> +                * Note that sk->sk_socket can be assigned in other cases, e.g.
>> +                * with TFO (if accept()'ed before the 3rd ACK) and MPTCP (MPJ:
>> +                * sk_socket is the parent MPTCP sock).
>> +                */
>> +               if (sk->sk_socket && th->syn)
>>                         goto consume;
> 
> I think we should simply remove this part completely, because we
> should send an ack anyway.

Thank you for having looked, and ran the full packetdrill test suite!

> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index ff9ab3d01ced89570903d3a9f649a637c5e07a90..91357d4713182078debd746a224046cba80ea3ce
> 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -6820,8 +6820,6 @@ tcp_rcv_state_process(struct sock *sk, struct
> sk_buff *skb)
>                 if (sk->sk_shutdown & SEND_SHUTDOWN)
>                         tcp_shutdown(sk, SEND_SHUTDOWN);
> 
> -               if (sk->sk_socket)
> -                       goto consume;
>                 break;
> 
>         case TCP_FIN_WAIT1: {
> 
> 
> I have a failing packetdrill test after  Kuniyuki  patch :
> 
> 
> 
> //
> // Test the simultaneous open scenario that both end sends
> // SYN/data. Although we don't support that the connection should
> // still be established.
> //
> `../../common/defaults.sh
>  ../../common/set_sysctls.py /proc/sys/net/ipv4/tcp_timestamps=0`
> 
> // Cache warmup: send a Fast Open cookie request
>     0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
>    +0 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0
>    +0 sendto(3, ..., 0, MSG_FASTOPEN, ..., ...) = -1 EINPROGRESS
> (Operation is now in progress)
>    +0 > S 0:0(0) <mss 1460,nop,nop,sackOK,nop,wscale 8,FO,nop,nop>
>  +.01 < S. 123:123(0) ack 1 win 14600 <mss
> 1460,nop,nop,sackOK,nop,wscale 6,FO abcd1234,nop,nop>
>    +0 > . 1:1(0) ack 1
>  +.01 close(3) = 0
>    +0 > F. 1:1(0) ack 1
>  +.01 < F. 1:1(0) ack 2 win 92
>    +0 > .  2:2(0) ack 2
> 
> 
> //
> // Test: simulatenous fast open
> //
>  +.01 socket(..., SOCK_STREAM, IPPROTO_TCP) = 4
>    +0 fcntl(4, F_SETFL, O_RDWR|O_NONBLOCK) = 0
>    +0 sendto(4, ..., 1000, MSG_FASTOPEN, ..., ...) = 1000
>    +0 > S 0:1000(1000) <mss 1460,nop,nop,sackOK,nop,wscale 8,FO
> abcd1234,nop,nop>
> // Simul. SYN-data crossing: we don't support that yet so ack only remote ISN
> +.005 < S 1234:1734(500) win 14600 <mss 1040,nop,nop,sackOK,nop,wscale
> 6,FO 87654321,nop,nop>
>    +0 > S. 0:0(0) ack 1235 <mss 1460,nop,nop,sackOK,nop,wscale 8>
> 
> // SYN data is never retried.
> +.045 < S. 1234:1234(0) ack 1001 win 14600 <mss
> 940,nop,nop,sackOK,nop,wscale 6,FO 12345678,nop,nop>
>    +0 > . 1001:1001(0) ack 1

I recently sent a PR -- already applied -- to Neal to remove this line:

  https://github.com/google/packetdrill/pull/86

I thought it was the intension of Kuniyuki's patch not to send this ACK
in this case to follow the RFC 9293's recommendation. This TFO test
looks a bit similar to the example from Kuniyuki's patch:


--------------- 8< ---------------
 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

  /* No ACK here */

+0 write(3, ..., 100) = 100
+0 > P. 1:101(100) ack 1
--------------- 8< ---------------



But maybe here that should be different for TFO?

For my case with MPTCP (and TFO), it is fine to drop this 'goto consume'
but I don't know how "strict" we want to be regarding the RFC and this
marginal case.


> // The other end retries
>   +.1 < P. 1:501(500) ack 1000 win 257
>    +0 > . 1001:1001(0) ack 501
>    +0 read(4, ..., 4096) = 500
>    +0 close(4) = 0
>    +0 > F. 1001:1001(0) ack 501
>  +.05 < F. 501:501(0) ack 1002 win 257
>    +0 > . 1002:1002(0) ack 502
> 
> `/tmp/sysctl_restore_${PPID}.sh`
> 

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.