mptcp: add a new sysctl checksum_enabled

This patch added a new sysctl, named checksum_enabled, to control
whether DSS checksum can be enabled.

Acked-by: Paolo Abeni <[e-mail]>
Co-developed-by: Matthieu Baerts <[e-mail]>
Signed-off-by: Matthieu Baerts <[e-mail]>
Signed-off-by: Geliang Tang <[e-mail]>
Signed-off-by: Mat Martineau <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/networking/mptcp-sysctl.rst

···
 	sysctl.
 
 	Default: 120
+
+checksum_enabled - BOOLEAN
+	Control whether DSS checksum can be enabled.
+
+	DSS checksum can be enabled if the value is nonzero. This is a
+	per-namespace sysctl.
+
+	Default: 0

net/mptcp/ctrl.c

···
 
 	u8 mptcp_enabled;
 	unsigned int add_addr_timeout;
+	u8 checksum_enabled;
 };
 
 static struct mptcp_pernet *mptcp_get_pernet(struct net *net)
···
 	return mptcp_get_pernet(net)->add_addr_timeout;
 }
 
+int mptcp_is_checksum_enabled(struct net *net)
+{
+	return mptcp_get_pernet(net)->checksum_enabled;
+}
+
 static void mptcp_pernet_set_defaults(struct mptcp_pernet *pernet)
 {
 	pernet->mptcp_enabled = 1;
 	pernet->add_addr_timeout = TCP_RTO_MAX;
+	pernet->checksum_enabled = 0;
 }
 
 #ifdef CONFIG_SYSCTL
···
 		.mode = 0644,
 		.proc_handler = proc_dointvec_jiffies,
 	},
+	{
+		.procname = "checksum_enabled",
+		.maxlen = sizeof(u8),
+		.mode = 0644,
+		.proc_handler = proc_dou8vec_minmax,
+		.extra1       = SYSCTL_ZERO,
+		.extra2       = SYSCTL_ONE
+	},
 	{}
 };
 
···
 
 	table[0].data = &pernet->mptcp_enabled;
 	table[1].data = &pernet->add_addr_timeout;
+	table[2].data = &pernet->checksum_enabled;
 
 	hdr = register_net_sysctl(net, MPTCP_SYSCTL_PATH, table);
 	if (!hdr)

net/mptcp/protocol.h

···
 
 int mptcp_is_enabled(struct net *net);
 unsigned int mptcp_get_add_addr_timeout(struct net *net);
-static inline int mptcp_is_checksum_enabled(struct net *net) { return false; }
+int mptcp_is_checksum_enabled(struct net *net);
 void mptcp_subflow_fully_established(struct mptcp_subflow_context *subflow,
 				     struct mptcp_options_received *mp_opt);
 bool mptcp_subflow_data_available(struct sock *sk);