mptcp: restrict values of 'enabled' sysctl

To avoid confusions, it seems better to parse this sysctl parameter as a
boolean. We use it as a boolean, no need to parse an integer and bring
confusions if we see a value different from 0 and 1, especially with
this parameter name: enabled.

It seems fine to do this modification because the default value is 1
(enabled). Then the only other interesting value to set is 0 (disabled).
All other values would not have changed the default behaviour.
Suggested-by: Florian Westphal <fw@strlen.de>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/mptcp-sysctl.rst

@@ -7,13 +7,13 @@ MPTCP Sysfs variables
 /proc/sys/net/mptcp/* Variables
 ===============================
 
-enabled - INTEGER
+enabled - BOOLEAN
 	Control whether MPTCP sockets can be created.
 
-	MPTCP sockets can be created if the value is nonzero. This is
-	a per-namespace sysctl.
+	MPTCP sockets can be created if the value is 1. This is a
+	per-namespace sysctl.
 
-	Default: 1
+	Default: 1 (enabled)
 
 add_addr_timeout - INTEGER (seconds)
 	Set the timeout after which an ADD_ADDR control message will be

net/mptcp/ctrl.c

@@ -21,7 +21,7 @@ struct mptcp_pernet {
 	struct ctl_table_header *ctl_table_hdr;
 #endif
 
-	int mptcp_enabled;
+	u8 mptcp_enabled;
 	unsigned int add_addr_timeout;
 };
 
@@ -50,12 +50,14 @@ static void mptcp_pernet_set_defaults(struct mptcp_pernet *pernet)
 static struct ctl_table mptcp_sysctl_table[] = {
 	{
 		.procname = "enabled",
-		.maxlen = sizeof(int),
+		.maxlen = sizeof(u8),
 		.mode = 0644,
 		/* users with CAP_NET_ADMIN or root (not and) can change this
 		 * value, same as other sysctl or the 'net' tree.
 		 */
-		.proc_handler = proc_dointvec,
+		.proc_handler = proc_dou8vec_minmax,
+		.extra1       = SYSCTL_ZERO,
+		.extra2       = SYSCTL_ONE
 	},
 	{
 		.procname = "add_addr_timeout",