netdev - Re: [PATCH][v2] dev : fix mtu check when TSO is enabled

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Date:	Mon, 14 Mar 2011 21:02:12 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Daniel Lezcano <daniel.lezcano@...e.fr>
Cc:	davem@...emloft.net, kaber@...sh.net, nightnord@...il.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH][v2] dev : fix mtu check when TSO is enabled

Le lundi 14 mars 2011 à 20:10 +0100, Daniel Lezcano a écrit :
> In case the device where is coming from the packet has TSO enabled,
> we should not check the mtu size value as this one could be bigger
> than the expected value.
> 
> This is the case for the macvlan driver when the lower device has
> TSO enabled. The macvlan inherit this feature and forward the packets
> without fragmenting them. Then the packets go through dev_forward_skb
> and are dropped. This patch fix this by checking TSO is not enabled
> when we want to check the mtu size.
> 
> Signed-off-by: Daniel Lezcano <daniel.lezcano@...e.fr>
> Cc: Eric Dumazet <eric.dumazet@...il.com>
> Cc: Patrick McHardy <kaber@...sh.net>
> Cc: Andrian Nord <nightnord@...il.com>
> ---
>  net/core/dev.c |   24 ++++++++++++++++++++++--
>  1 files changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 6561021..f1607a0 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -1503,6 +1503,27 @@ static inline void net_timestamp_check(struct sk_buff *skb)
>  		__net_timestamp(skb);
>  }
>  
> +static inline bool is_skb_forwardable(struct net_device *dev,
> +				      struct sk_buff *skb)
> +{
> +	unsigned int len;
> +
> +	if (!dev->flags & IFF_UP)
> +		return false;

	if (!(dev->flags & IFF_UP))
		return false;

> +
> +	/* we should not check the mtu size if TSO is enabled otherwise
> +	 * we may have a packet with a length bigger than the expected
> +	 * one as it was not segmented before */
> +	if (skb->dev && skb->dev->features & NETIF_F_TSO)
> +		return true;
> +
> +	len = dev->mtu + dev->hard_header_len + VLAN_HLEN;
> +	if (skb->len > len)
> +		return false;
> +


I suggest you reorder tests so that the first one is discriminant most
of the time

	maxlen = dev->mtu + dev->hard_header_len + VLAN_HLEN;
	if (skb->len < maxlen)
		return true;
	
	if (skb->dev && skb->dev->features & NETIF_F_TSO)
		return true;

	return false;

> +	return true;
> +}
> +
>  /**
>   * dev_forward_skb - loopback an skb to another netif
>   *
> @@ -1526,8 +1547,7 @@ int dev_forward_skb(struct net_device *dev, struct sk_buff *skb)
>  	skb_orphan(skb);
>  	nf_reset(skb);
>  
> -	if (unlikely(!(dev->flags & IFF_UP) ||
> -		     (skb->len > (dev->mtu + dev->hard_header_len + VLAN_HLEN)))) {
> +	if (unlikely(!is_skb_forwardable(dev, skb))) {
>  		atomic_long_inc(&dev->rx_dropped);
>  		kfree_skb(skb);
>  		return NET_RX_DROP;


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