ipv6 addrconf: remove addrconf_sysctl_hop_limit()

This is an effective no-op in terms of user observable behaviour.

By preventing the overwrite of non-null extra1/extra2 fields
in addrconf_sysctl() we can enable the use of proc_dointvec_minmax().

This allows us to eliminate the constant min/max (1..255) trampoline
function that is addrconf_sysctl_hop_limit().

This is nice because it simplifies the code, and allows future
sysctls with constant min/max limits to also not require trampolines.

We still can't eliminate the trampoline for mtu because it isn't
actually a constant (it depends on other tunables of the device)
and thus requires at-write-time logic to enforce range.
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Acked-by: Erik Kline <ek@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -5496,20 +5496,6 @@ int addrconf_sysctl_forward(struct ctl_table *ctl, int write,
 	return ret;
 }
 
-static
-int addrconf_sysctl_hop_limit(struct ctl_table *ctl, int write,
-                              void __user *buffer, size_t *lenp, loff_t *ppos)
-{
-	struct ctl_table lctl;
-	int min_hl = 1, max_hl = 255;
-
-	lctl = *ctl;
-	lctl.extra1 = &min_hl;
-	lctl.extra2 = &max_hl;
-
-	return proc_dointvec_minmax(&lctl, write, buffer, lenp, ppos);
-}
-
 static
 int addrconf_sysctl_mtu(struct ctl_table *ctl, int write,
 			void __user *buffer, size_t *lenp, loff_t *ppos)
@@ -5743,6 +5729,9 @@ int addrconf_sysctl_ignore_routes_with_linkdown(struct ctl_table *ctl,
 	return ret;
 }
 
+static const int one = 1;
+static const int two_five_five = 255;
+
 static const struct ctl_table addrconf_sysctl[] = {
 	{
 		.procname	= "forwarding",
@@ -5756,7 +5745,9 @@ static const struct ctl_table addrconf_sysctl[] = {
 		.data		= &ipv6_devconf.hop_limit,
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
-		.proc_handler	= addrconf_sysctl_hop_limit,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= (void *)&one,
+		.extra2		= (void *)&two_five_five,
 	},
 	{
 		.procname	= "mtu",
@@ -6081,9 +6072,15 @@ static int __addrconf_sysctl_register(struct net *net, char *dev_name,
 
 	for (i = 0; table[i].data; i++) {
 		table[i].data += (char *)p - (char *)&ipv6_devconf;
+		/* If one of these is already set, then it is not safe to
+		 * overwrite either of them: this makes proc_dointvec_minmax
+		 * usable.
+		 */
+		if (!table[i].extra1 && !table[i].extra2) {
 			table[i].extra1 = idev; /* embedded; no ref */
 			table[i].extra2 = net;
 		}
+	}
 
 	snprintf(path, sizeof(path), "net/ipv6/conf/%s", dev_name);