Subject: Re: [PATCH net-next] net: make dev_set_mtu() honor notification return code

From: Alexander Duyck <[email-masked]>
Date: Fri, 10 Jan 2014 07:36:45 -0800

On 01/10/2014 04:48 AM, Veaceslav Falico wrote:
> 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, and pass the error code.
>
> CC: Jiri Pirko <[email-masked]>
> CC: "David S. Miller" <[email-masked]>
> CC: Eric Dumazet <[email-masked]>
> CC: Alexander Duyck <[email-masked]>
> CC: Nicolas Dichtel <[email-masked]>
> Signed-off-by: Veaceslav Falico <[email-masked]>
> ---
>  net/core/dev.c | 29 ++++++++++++++++++++---------
>  1 file changed, 20 insertions(+), 9 deletions(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index ce01847..1c570ff 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5287,6 +5287,17 @@ 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
> @@ -5296,8 +5307,7 @@ 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;
> @@ -5309,14 +5319,15 @@ 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)
> +			__dev_set_mtu(dev, orig_mtu);
> +	}
>  	return err;
>  }
>  EXPORT_SYMBOL(dev_set_mtu);

So what about the netdevices that succeeded in changing the MTU based on
the notifiers?  It seems like you still  have an inconsistent state
after the failure unless you issue a second call with a notification
that you reverted to the old MTU.

Thanks,

Alex