Re: [MPTCP][PATCH v3 mptcp-next 1/8] mptcp: MP_FAIL suboption sending

[Paolo Abeni <pabeni@redhat.com>]

On Wed, 2021-07-14 at 17:16 +0800, Geliang Tang wrote:
> Paolo Abeni <pabeni@redhat.com> 于2021年7月14日周三 下午4:45写道：
> > On Mon, 2021-06-28 at 12:28 +0800, Geliang Tang wrote:
> > > This patch added the MP_FAIL suboption sending support.
> > > 
> > > Add a new flag named send_mp_fail in struct mptcp_subflow_context. If
> > > this flag is set, send out MP_FAIL suboption.
> > > 
> > > Add a new member fail_seq in struct mptcp_out_options to save the data
> > > sequence number to put into the MP_FAIL suboption.
> > > 
> > > Signed-off-by: Geliang Tang <geliangtang@gmail.com>
> > > ---
> > >  include/net/mptcp.h  |  1 +
> > >  net/mptcp/options.c  | 54 ++++++++++++++++++++++++++++++++++++++++++--
> > >  net/mptcp/protocol.h |  4 ++++
> > >  3 files changed, 57 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/include/net/mptcp.h b/include/net/mptcp.h
> > > index cb580b06152f..f48d3b5a3fd4 100644
> > > --- a/include/net/mptcp.h
> > > +++ b/include/net/mptcp.h
> > > @@ -72,6 +72,7 @@ struct mptcp_out_options {
> > >       u32 nonce;
> > >       u64 thmac;
> > >       u32 token;
> > > +     u64 fail_seq;
> > 
> > Since we can't have both a valid mapping and MP_FAIL in the same
> > packet, it would be better to avoid increasing the 'mptcp_out_options'
> > size, e.g. re-using some ext_copy field, or unioning with some other
> > field.
> 
> The RFC says an MP_FAIL option could be included in a RST or on the
> subflow-level ACK. So I think we can't re-using the ext_copy field if it's
> on the subflow-level ACK.
> 
> How about unioning this fail_seq field with ahmac field?
> 
> > mptcp_out_options has grown quite a bit and we should really look into
> > shrinking it.
> > 
> > >       u8 hmac[20];
> > >       struct mptcp_ext ext_copy;
> > >  #endif
> > > diff --git a/net/mptcp/options.c b/net/mptcp/options.c
> > > index b5850afea343..b78defe1aed9 100644
> > > --- a/net/mptcp/options.c
> > > +++ b/net/mptcp/options.c
> > > @@ -771,6 +771,28 @@ static noinline void mptcp_established_options_rst(struct sock *sk, struct sk_bu
> > >       opts->reset_reason = subflow->reset_reason;
> > >  }
> > > 
> > > +static bool mptcp_established_options_mp_fail(struct sock *sk,
> > > +                                           unsigned int *size,
> > > +                                           unsigned int remaining,
> > > +                                           struct mptcp_out_options *opts)
> > > +{
> > > +     struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(sk);
> > > +
> > > +     if (!subflow->send_mp_fail)
> > > +             return false;
> > > +
> > > +     if (remaining < TCPOLEN_MPTCP_FAIL)
> > > +             return false;
> > > +
> > > +     *size = TCPOLEN_MPTCP_FAIL;
> > > +     opts->suboptions |= OPTION_MPTCP_FAIL;
> > > +     opts->fail_seq = subflow->fail_seq;
> > > +
> > > +     pr_debug("MP_FAIL fail_seq=%llu", opts->fail_seq);
> > > +
> > > +     return true;
> > > +}
> > > +
> > >  bool mptcp_established_options(struct sock *sk, struct sk_buff *skb,
> > >                              unsigned int *size, unsigned int remaining,
> > >                              struct mptcp_out_options *opts)
> > > @@ -787,15 +809,29 @@ bool mptcp_established_options(struct sock *sk, struct sk_buff *skb,
> > >               return false;
> > > 
> > >       if (unlikely(skb && TCP_SKB_CB(skb)->tcp_flags & TCPHDR_RST)) {
> > > -             mptcp_established_options_rst(sk, skb, size, remaining, opts);
> > > +             if (mptcp_established_options_mp_fail(sk, &opt_size, remaining, opts)) {
> > > +                     *size += opt_size;
> > > +                     remaining -= opt_size;
> > > +             }
> > > +             mptcp_established_options_rst(sk, skb, &opt_size, remaining, opts);
> > > +             *size += opt_size;
> > > +             remaining -= opt_size;
> > >               return true;
> > >       }
> > > 
> > >       snd_data_fin = mptcp_data_fin_enabled(msk);
> > >       if (mptcp_established_options_mp(sk, skb, snd_data_fin, &opt_size, remaining, opts))
> > >               ret = true;
> > > -     else if (mptcp_established_options_dss(sk, skb, snd_data_fin, &opt_size, remaining, opts))
> > > +     else if (mptcp_established_options_dss(sk, skb, snd_data_fin, &opt_size, remaining, opts)) {
> > >               ret = true;
> > > +             if (opts->ext_copy.use_ack) {
> > 
> > Is the extra check on 'opts->ext_copy.use_ack' necessary? can we just
> > look for mp_fail?
> > 
> 
> I added this check since the RFC says an MP_FAIL option could be included
> on the subflow-level ACK:
> 
> '''
> In this case, if a receiver identifies a checksum failure when there is
> only one path, it will send back an MP_FAIL option on the subflow-level
> ACK, referring to the data-level sequence number of the start of the
> segment on which the checksum error was detected.
> '''
> 
> Do I understand right?

Not a big deal, but I read the above as follow:

the MP_FAIL is included into the next subflow packet/

subflow-level ACK really means TCP-level ACK, I think. Since the
subflow is established (at TCP level) each TCP packet will carry an
ack.

All in all the ' if (opts->ext_copy.use_ack) {' check looks unneeded to
me.

Cheers,

Paolo