[PATCH net-next v4 1/6] net: introduce rstreason to detect why the RST is sent

[Jason Xing <kerneljasonxing@gmail.com>]

From: Jason Xing <kernelxing@tencent.com>

Add a new standalone file for the easy future extension to support
both active reset and passive reset in the TCP/DCCP/MPTCP protocols.

This patch only does the preparations for reset reason mechanism,
nothing else changes.

The reset reasons are divided into three parts:
1) reuse drop reasons for passive reset in TCP
2) reuse MP_TCPRST option for MPTCP
3) our own reasons

I will implement the basic codes of active/passive reset reason in
those three protocols, which is not complete for this moment. But
it provides a new chance to let other people add more reasons into
it:)

Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
 include/net/rstreason.h | 93 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 93 insertions(+)
 create mode 100644 include/net/rstreason.h

diff --git a/include/net/rstreason.h b/include/net/rstreason.h
new file mode 100644
index 000000000000..38c39d32a961
--- /dev/null
+++ b/include/net/rstreason.h
@@ -0,0 +1,93 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#ifndef _LINUX_RSTREASON_H
+#define _LINUX_RSTREASON_H
+#include <net/dropreason-core.h>
+
+#define DEFINE_RST_REASON(FN, FNe)	\
+	FN(MPTCP_RST_EUNSPEC)		\
+	FN(MPTCP_RST_EMPTCP)		\
+	FN(MPTCP_RST_ERESOURCE)		\
+	FN(MPTCP_RST_EPROHIBIT)		\
+	FN(MPTCP_RST_EWQ2BIG)		\
+	FN(MPTCP_RST_EBADPERF)		\
+	FN(MPTCP_RST_EMIDDLEBOX)	\
+	FN(NOT_SPECIFIED)		\
+	FNe(MAX)
+
+#define RST_REASON_START (SKB_DROP_REASON_MAX + 1)
+
+/* There are three parts in order:
+ * 1) 0 - SKB_DROP_REASON_MAX: rely on drop reasons for passive reset in TCP
+ * 2) SKB_DROP_REASON_MAX + 1 - MPTCP_RST_EMIDDLEBOX: for MPTCP use
+ * 3) MPTCP_RST_EMIDDLEBOX - SK_RST_REASON_MAX: independent reset reason
+ */
+enum sk_rst_reason {
+	/* Leave this 'blank' part (0-SKB_DROP_REASON_MAX) for the reuse
+	 * of skb drop reason because rst reason relies on what drop reason
+	 * indicates exactly why it could happen.
+	 */
+
+	/* Copy from include/uapi/linux/mptcp.h.
+	 * These reset fields will not be changed since they adhere to
+	 * RFC 8684. So do not touch them. I'm going to list each definition
+	 * of them respectively.
+	 */
+	/* Unspecified error.
+	 * This is the default error; it implies that the subflow is no
+	 * longer available. The presence of this option shows that the
+	 * RST was generated by an MPTCP-aware device.
+	 */
+	SK_RST_REASON_MPTCP_RST_EUNSPEC = RST_REASON_START,
+	/* MPTCP-specific error.
+	 * An error has been detected in the processing of MPTCP options.
+	 * This is the usual reason code to return in the cases where a RST
+	 * is being sent to close a subflow because of an invalid response.
+	 */
+	SK_RST_REASON_MPTCP_RST_EMPTCP,
+	/* Lack of resources.
+	 * This code indicates that the sending host does not have enough
+	 * resources to support the terminated subflow.
+	 */
+	SK_RST_REASON_MPTCP_RST_ERESOURCE,
+	/* Administratively prohibited.
+	 * This code indicates that the requested subflow is prohibited by
+	 * the policies of the sending host.
+	 */
+	SK_RST_REASON_MPTCP_RST_EPROHIBIT,
+	/* Too much outstanding data.
+	 * This code indicates that there is an excessive amount of data
+	 * that needs to be transmitted over the terminated subflow while
+	 * having already been acknowledged over one or more other subflows.
+	 * This may occur if a path has been unavailable for a short period
+	 * and it is more efficient to reset and start again than it is to
+	 * retransmit the queued data.
+	 */
+	SK_RST_REASON_MPTCP_RST_EWQ2BIG,
+	/* Unacceptable performance.
+	 * This code indicates that the performance of this subflow was
+	 * too low compared to the other subflows of this Multipath TCP
+	 * connection.
+	 */
+	SK_RST_REASON_MPTCP_RST_EBADPERF,
+	/* Middlebox interference.
+	 * Middlebox interference has been detected over this subflow,
+	 * making MPTCP signaling invalid. For example, this may be sent
+	 * if the checksum does not validate.
+	 */
+	SK_RST_REASON_MPTCP_RST_EMIDDLEBOX,
+
+	/* For the real standalone socket reset reason, we start from here */
+	SK_RST_REASON_NOT_SPECIFIED,
+
+	/* Maximum of socket reset reasons.
+	 * It shouldn't be used as a real 'reason'.
+	 */
+	SK_RST_REASON_MAX,
+};
+
+static inline enum sk_rst_reason convert_mptcp_reason(int reason)
+{
+	return reason += RST_REASON_START;
+}
+#endif
-- 
2.37.3