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Date:   Thu, 14 May 2020 13:13:31 -0700 (PDT)
From:   Mat Martineau <mathew.j.martineau@...ux.intel.com>
To:     Paolo Abeni <pabeni@...hat.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


Paolo -

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?

> 	}
>
> -- 
> 2.21.3
>
>

--
Mat Martineau
Intel

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