[PATCH 5.15.y] mptcp: don't account accept() of non-MPC client as fallback to TCP

["Matthieu Baerts (NGI0)" <matttbe@kernel.org>]

From: Davide Caratti <dcaratti@redhat.com>

Current MPTCP servers increment MPTcpExtMPCapableFallbackACK when they
accept non-MPC connections. As reported by Christoph, this is "surprising"
because the counter might become greater than MPTcpExtMPCapableSYNRX.

MPTcpExtMPCapableFallbackACK counter's name suggests it should only be
incremented when a connection was seen using MPTCP options, then a
fallback to TCP has been done. Let's do that by incrementing it when
the subflow context of an inbound MPC connection attempt is dropped.
Also, update mptcp_connect.sh kselftest, to ensure that the
above MIB does not increment in case a pure TCP client connects to a
MPTCP server.

Fixes: fc518953bc9c ("mptcp: add and use MIB counter infrastructure")
Cc: stable@vger.kernel.org
Reported-by: Christoph Paasch <cpaasch@apple.com>
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/449
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240329-upstream-net-20240329-fallback-mib-v1-1-324a8981da48@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit 7a1b3490f47e88ec4cbde65f1a77a0f4bc972282)
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Notes:
  - Conflicts in protocol.c: because commit 8e2b8a9fa512 ("mptcp: don't
    overwrite sock_ops in mptcp_is_tcpsk()") is not in this version, but
    it depends on new features, making it hard to be backported, while
    the conflict resolution is easy: just remove the MIB incrementation
    from the previous location.
  - Conflicts in subflow.c: because commit a88d0092b24b ("mptcp:
    simplify subflow_syn_recv_sock()") is not in this version, but it
    depends on new features, making it hard to be backported, while the
    conflict resolution is easy: just move the MIB incrementation where
    the subflow context is dropped (fallback to TCP).
  - Conflicts in mptcp_connect.sh: because commit e3aae1098f10
    ("selftests: mptcp: connect: fix shellcheck warnings") and commit
    e7c42bf4d320 ("selftests: mptcp: use += operator to append strings")
    are not in this version. The dependency chain looks too long, while
    resolving the conflicts was not difficult:
      - using get_mib_counter() instead of the new
        mptcp_lib_get_counter()
      - writing the error message with 'printf' instead of the new
        mptcp_lib_pr_fail().
---
 net/mptcp/protocol.c                               | 3 ---
 net/mptcp/subflow.c                                | 3 +++
 tools/testing/selftests/net/mptcp/mptcp_connect.sh | 7 +++++++
 3 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 5db1c0142abb..cde62dafda49 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -3018,9 +3018,6 @@ static struct sock *mptcp_accept(struct sock *sk, int flags, int *err,
 		sock_hold(new_mptcp_sock);
 		newsk = new_mptcp_sock;
 		MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_MPCAPABLEPASSIVEACK);
-	} else {
-		MPTCP_INC_STATS(sock_net(sk),
-				MPTCP_MIB_MPCAPABLEPASSIVEFALLBACK);
 	}
 
 out:
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 80230787554e..ff7239fe3d2c 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -725,6 +725,9 @@ static struct sock *subflow_syn_recv_sock(const struct sock *sk,
 				goto dispose_child;
 			}
 
+			if (fallback)
+				SUBFLOW_REQ_INC_STATS(req, MPTCP_MIB_MPCAPABLEPASSIVEFALLBACK);
+
 			subflow_drop_ctx(child);
 			goto out;
 		}
diff --git a/tools/testing/selftests/net/mptcp/mptcp_connect.sh b/tools/testing/selftests/net/mptcp/mptcp_connect.sh
index 8efff3f9c52a..5a1277d17286 100755
--- a/tools/testing/selftests/net/mptcp/mptcp_connect.sh
+++ b/tools/testing/selftests/net/mptcp/mptcp_connect.sh
@@ -451,6 +451,7 @@ do_transfer()
 	local stat_cookierx_last=$(get_mib_counter "${listener_ns}" "TcpExtSyncookiesRecv")
 	local stat_csum_err_s=$(get_mib_counter "${listener_ns}" "MPTcpExtDataCsumErr")
 	local stat_csum_err_c=$(get_mib_counter "${connector_ns}" "MPTcpExtDataCsumErr")
+	local stat_tcpfb_last_l=$(get_mib_counter "${listener_ns}" "MPTcpExtMPCapableFallbackACK")
 
 	timeout ${timeout_test} \
 		ip netns exec ${listener_ns} \
@@ -516,6 +517,7 @@ do_transfer()
 	local stat_cookietx_now=$(get_mib_counter "${listener_ns}" "TcpExtSyncookiesSent")
 	local stat_cookierx_now=$(get_mib_counter "${listener_ns}" "TcpExtSyncookiesRecv")
 	local stat_ooo_now=$(get_mib_counter "${listener_ns}" "TcpExtTCPOFOQueue")
+	local stat_tcpfb_now_l=$(get_mib_counter "${listener_ns}" "MPTcpExtMPCapableFallbackACK")
 
 	expect_synrx=$((stat_synrx_last_l))
 	expect_ackrx=$((stat_ackrx_last_l))
@@ -560,6 +562,11 @@ do_transfer()
 		fi
 	fi
 
+	if [ ${stat_ooo_now} -eq 0 ] && [ ${stat_tcpfb_last_l} -ne ${stat_tcpfb_now_l} ]; then
+		printf "[ FAIL ]\nunexpected fallback to TCP"
+		rets=1
+	fi
+
 	if [ $retc -eq 0 ] && [ $rets -eq 0 ]; then
 		printf "[ OK ]"
 	fi
-- 
2.43.0