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Date:   Fri, 15 May 2020 11:15:08 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Mat Martineau <mathew.j.martineau@...ux.intel.com>
Cc:     netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Christoph Paasch <cpaasch@...le.com>
Subject: Re: [PATCH net-next 1/3] mptcp: add new sock flag to deal with join
 subflows

On Thu, 2020-05-14 at 13:13 -0700, Mat Martineau wrote:
> On Wed, 13 May 2020, Paolo Abeni wrote:
> 
> > MP_JOIN subflows must not land into the accept queue.
> > Currently tcp_check_req() calls an mptcp specific helper
> > to detect such scenario.
> > 
> > Such helper leverages the subflow context to check for
> > MP_JOIN subflows. We need to deal also with MP JOIN
> > failures, even when the subflow context is not available
> > due allocation failure.
> > 
> > A possible solution would be changing the syn_recv_sock()
> > signature to allow returning a more descriptive action/
> > error code and deal with that in tcp_check_req().
> > 
> > Since the above need is MPTCP specific, this patch instead
> > uses a TCP request socket hole to add a MPTCP specific flag.
> > Such flag is used by the MPTCP syn_recv_sock() to tell
> > tcp_check_req() how to deal with the request socket.
> > 
> > This change is a no-op for !MPTCP build, and makes the
> > MPTCP code simpler. It allows also the next patch to deal
> > correctly with MP JOIN failure.
> > 
> > RFC -> v1:
> > - move the drop_req bit inside tcp_request_sock (Eric)
> > 
> > Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> > ---
> > include/linux/tcp.h      |  3 +++
> > include/net/mptcp.h      | 17 ++++++++++-------
> > net/ipv4/tcp_minisocks.c |  2 +-
> > net/mptcp/protocol.c     |  7 -------
> > net/mptcp/subflow.c      |  2 ++
> > 5 files changed, 16 insertions(+), 15 deletions(-)
> > 
> > diff --git a/include/linux/tcp.h b/include/linux/tcp.h
> > index e60db06ec28d..bf44e85d709d 100644
> > --- a/include/linux/tcp.h
> > +++ b/include/linux/tcp.h
> > @@ -120,6 +120,9 @@ struct tcp_request_sock {
> > 	u64				snt_synack; /* first SYNACK sent time */
> > 	bool				tfo_listener;
> > 	bool				is_mptcp;
> > +#if IS_ENABLED(CONFIG_MPTCP)
> > +	bool				drop_req;
> > +#endif
> > 	u32				txhash;
> > 	u32				rcv_isn;
> > 	u32				snt_isn;
> > diff --git a/include/net/mptcp.h b/include/net/mptcp.h
> > index e60275659de6..c4a6ef4ba35b 100644
> > --- a/include/net/mptcp.h
> > +++ b/include/net/mptcp.h
> > @@ -68,6 +68,11 @@ static inline bool rsk_is_mptcp(const struct request_sock *req)
> > 	return tcp_rsk(req)->is_mptcp;
> > }
> > 
> > +static inline bool rsk_drop_req(const struct request_sock *req)
> > +{
> > +	return tcp_rsk(req)->is_mptcp && tcp_rsk(req)->drop_req;
> > +}
> > +
> > void mptcp_space(const struct sock *ssk, int *space, int *full_space);
> > bool mptcp_syn_options(struct sock *sk, const struct sk_buff *skb,
> > 		       unsigned int *size, struct mptcp_out_options *opts);
> > @@ -121,8 +126,6 @@ static inline bool mptcp_skb_can_collapse(const struct sk_buff *to,
> > 				 skb_ext_find(from, SKB_EXT_MPTCP));
> > }
> > 
> > -bool mptcp_sk_is_subflow(const struct sock *sk);
> > -
> > void mptcp_seq_show(struct seq_file *seq);
> > #else
> > 
> > @@ -140,6 +143,11 @@ static inline bool rsk_is_mptcp(const struct request_sock *req)
> > 	return false;
> > }
> > 
> > +static inline bool rsk_drop_req(const struct request_sock *req)
> > +{
> > +	return false;
> > +}
> > +
> > static inline void mptcp_parse_option(const struct sk_buff *skb,
> > 				      const unsigned char *ptr, int opsize,
> > 				      struct tcp_options_received *opt_rx)
> > @@ -190,11 +198,6 @@ static inline bool mptcp_skb_can_collapse(const struct sk_buff *to,
> > 	return true;
> > }
> > 
> > -static inline bool mptcp_sk_is_subflow(const struct sock *sk)
> > -{
> > -	return false;
> > -}
> > -
> > static inline void mptcp_space(const struct sock *ssk, int *s, int *fs) { }
> > static inline void mptcp_seq_show(struct seq_file *seq) { }
> > #endif /* CONFIG_MPTCP */
> > diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
> > index 7e40322cc5ec..495dda2449fe 100644
> > --- a/net/ipv4/tcp_minisocks.c
> > +++ b/net/ipv4/tcp_minisocks.c
> > @@ -774,7 +774,7 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
> > 	if (!child)
> > 		goto listen_overflow;
> > 
> > -	if (own_req && sk_is_mptcp(child) && mptcp_sk_is_subflow(child)) {
> > +	if (own_req && rsk_drop_req(req)) {
> > 		reqsk_queue_removed(&inet_csk(sk)->icsk_accept_queue, req);
> > 		inet_csk_reqsk_queue_drop_and_put(sk, req);
> > 		return child;
> > diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> > index 6a812dd8b6b6..b974898eb6b5 100644
> > --- a/net/mptcp/protocol.c
> > +++ b/net/mptcp/protocol.c
> > @@ -1687,13 +1687,6 @@ bool mptcp_finish_join(struct sock *sk)
> > 	return ret;
> > }
> > 
> > -bool mptcp_sk_is_subflow(const struct sock *sk)
> > -{
> > -	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(sk);
> > -
> > -	return subflow->mp_join == 1;
> > -}
> > -
> > static bool mptcp_memory_free(const struct sock *sk, int wake)
> > {
> > 	struct mptcp_sock *msk = mptcp_sk(sk);
> > diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
> > index 009d5c478062..42a8a650ff20 100644
> > --- a/net/mptcp/subflow.c
> > +++ b/net/mptcp/subflow.c
> > @@ -500,6 +500,7 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
> > 			ctx->remote_key = mp_opt.sndr_key;
> > 			ctx->fully_established = mp_opt.mp_capable;
> > 			ctx->can_ack = mp_opt.mp_capable;
> > +			tcp_rsk(req)->drop_req = false;
> > 		} else if (ctx->mp_join) {
> > 			struct mptcp_sock *owner;
> > 
> > @@ -512,6 +513,7 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
> > 				goto close_child;
> > 
> > 			SUBFLOW_REQ_INC_STATS(req, MPTCP_MIB_JOINACKRX);
> > +			tcp_rsk(req)->drop_req = true;
> > 		}
> 
> It looks like tcp_rsk(req)->drop_req gets initialized when the 
> ctx->mp_capable and ctx->mp_join values are set as expected, but it's hard 
> to tell if it's guaranteed to be initialized in every corner case (any 
> allocation or token failures, unexpected input, etc.).
> 
> Patch 3 will set drop_req for some cases, but does it makes sense to 
> set tcp_rsk(req)->drop_req in subflow_v{4,6}_init_req() here as an 
> additional 'else' clause?

We already have such check in place in subflow_ulp_clone(): if both
subflow_req->mp_join and subflow_req->mp_capable are cleared we
fallback, elsewhere we set either subflow->mp_capable or subflow-
>mp_join.

Later in subflow_ulp_clone() we have similar scenario, where do:

	} else if (subflow_req->mp_join) {

while we could simply use a plain 'else'.

To make things clearer, I can replace both 'else if'... with plain else
(possibly in a separate patch), WDYT?

Thanks,

Paolo

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