af_unix: Do not call kmemdup() for init_net's sysctl table.

While setting up init_net's sysctl table, we need not duplicate the
global table and can use it directly as ipv4_sysctl_init_net() does.

Unlike IPv4, AF_UNIX does not have a huge sysctl table for now, so it
cannot be a problem, but this patch makes code consistent.
Acked-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20220627233627.51646-1-kuniyu@amazon.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/unix/sysctl_net_unix.c

@@ -26,11 +26,16 @@ int __net_init unix_sysctl_register(struct net *net)
 {
 	struct ctl_table *table;
 
-	table = kmemdup(unix_table, sizeof(unix_table), GFP_KERNEL);
-	if (table == NULL)
-		goto err_alloc;
+	if (net_eq(net, &init_net)) {
+		table = unix_table;
+	} else {
+		table = kmemdup(unix_table, sizeof(unix_table), GFP_KERNEL);
+		if (!table)
+			goto err_alloc;
+
+		table[0].data = &net->unx.sysctl_max_dgram_qlen;
+	}
 
-	table[0].data = &net->unx.sysctl_max_dgram_qlen;
 	net->unx.ctl = register_net_sysctl(net, "net/unix", table);
 	if (net->unx.ctl == NULL)
 		goto err_reg;
@@ -38,7 +43,8 @@ int __net_init unix_sysctl_register(struct net *net)
 	return 0;
 
 err_reg:
-	kfree(table);
+	if (net_eq(net, &init_net))
+		kfree(table);
 err_alloc:
 	return -ENOMEM;
 }
@@ -49,5 +55,6 @@ void unix_sysctl_unregister(struct net *net)
 
 	table = net->unx.ctl->ctl_table_arg;
 	unregister_net_sysctl_table(net->unx.ctl);
-	kfree(table);
+	if (!net_eq(net, &init_net))
+		kfree(table);
 }