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

From: Eric Dumazet
Date: Tue Jul 23 2024 - 11:38:49 EST

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On Tue, Jul 23, 2024 at 4:58 PM Matthieu Baerts <matttbe@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx> 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@xxxxxxxxxx>
> >> Suggested-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
> >> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>
> >> ---
> >> 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.

Problem of this 'goto consume' is that we are not properly sending a
DUPACK in this case.

+.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>

+.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 <nop,nop,sack 0:1> // See here

Not sending a dupack seems wrong and could hurt.

I had reservations about this part, if you look back to the discussion.

This is why Kuniyuki added in his changelog :

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.

Next message: Andi Kleen: "Re: [PATCH v1 2/2] perf script: Fix for `perf script +F metric` with leader sampling"
Previous message: Takero Funaki: "Re: [PATCH v3 1/2] mm: zswap: fix global shrinker memcg iteration"
In reply to: Matthieu Baerts: "Re: [PATCH net v2 1/2] tcp: process the 3rd ACK with sk_socket for TFO/MPTCP"
Next in thread: Matthieu Baerts: "Re: [PATCH net v2 1/2] tcp: process the 3rd ACK with sk_socket for TFO/MPTCP"
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