mptcp: add new sock flag to deal with join subflows

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.

v1 -> v2:
 - be more conservative on drop_req initialization (Mat)

RFC -> v1:
 - move the drop_req bit inside tcp_request_sock (Eric)

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Reviewed-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 13 insertions, 7 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 */