net: make dev_set_mtu() honor notification return code

Currently, after changing the MTU for a device, dev_set_mtu() calls
NETDEV_CHANGEMTU notification, however doesn't verify it's return code -
which can be NOTIFY_BAD - i.e. some of the net notifier blocks refused this
change, and continues nevertheless.

To fix this, verify the return code, and if it's an error - then revert the
MTU to the original one, notify again and pass the error code.

CC: Jiri Pirko <jiri@resnulli.us>
CC: "David S. Miller" <davem@davemloft.net>
CC: Eric Dumazet <edumazet@google.com>
CC: Alexander Duyck <alexander.h.duyck@intel.com>
CC: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Reviewed-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 25 insertions, 9 deletions

diff --git a/net/core/dev.c b/net/core/dev.c
index a8280154c42a..87312dcf0aa8 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5300,6 +5300,17 @@ int dev_change_flags(struct net_device *dev, unsigned int flags)
 }
 EXPORT_SYMBOL(dev_change_flags);
 
+static int __dev_set_mtu(struct net_device *dev, int new_mtu)
+{
+        const struct net_device_ops *ops = dev->netdev_ops;
+
+        if (ops->ndo_change_mtu)
+                return ops->ndo_change_mtu(dev, new_mtu);
+
+        dev->mtu = new_mtu;
+        return 0;
+}
+
 /**
  *      dev_set_mtu - Change maximum transfer unit
  *      @dev: device
@@ -5309,8 +5320,7 @@ EXPORT_SYMBOL(dev_change_flags);
  */
 int dev_set_mtu(struct net_device *dev, int new_mtu)
 {
-        const struct net_device_ops *ops = dev->netdev_ops;
-        int err;
+        int err, orig_mtu;
 
         if (new_mtu == dev->mtu)
                 return 0;
@@ -5322,14 +5332,20 @@ int dev_set_mtu(struct net_device *dev, int new_mtu)
         if (!netif_device_present(dev))
                 return -ENODEV;
 
-        err = 0;
-        if (ops->ndo_change_mtu)
-                err = ops->ndo_change_mtu(dev, new_mtu);
-        else
-                dev->mtu = new_mtu;
+        orig_mtu = dev->mtu;
+        err = __dev_set_mtu(dev, new_mtu);
 
-        if (!err)
-                call_netdevice_notifiers(NETDEV_CHANGEMTU, dev);
+        if (!err) {
+                err = call_netdevice_notifiers(NETDEV_CHANGEMTU, dev);
+                err = notifier_to_errno(err);
+                if (err) {
+                        /* setting mtu back and notifying everyone again,
+                         * so that they have a chance to revert changes.
+                         */
+                        __dev_set_mtu(dev, orig_mtu);
+                        call_netdevice_notifiers(NETDEV_CHANGEMTU, dev);
+                }
+        }
         return err;
 }
 EXPORT_SYMBOL(dev_set_mtu);