Subject: Re: [PATCH RFC 2/2] veth: propagate bridge GSO to peer

From: Eric Dumazet <[email-masked]>
Date: Thu, 30 Nov 2017 09:59:23 -0800

On Thu, 2017-11-30 at 09:49 -0800, Stephen Hemminger wrote:
> On Thu, 30 Nov 2017 09:26:39 -0800
> Eric Dumazet <[email-masked]> wrote:
> 
> > On Thu, 2017-11-30 at 09:10 -0800, Stephen Hemminger wrote:
> > > 
> > > 
> > > The problem goes back into the core GSO networking code.
> > > Something like this is needed.
> > > 
> > > static inline bool netif_needs_gso(struct sk_buff *skb,
> > > 				   const struct net_device *dev,
> > > 				   netdev_features_t features)
> > > {
> > > 	return skb_is_gso(skb) &&
> > > 		(!skb_gso_ok(skb, features) ||
> > > 		 unlikely(skb_shinfo(skb)->gso_segs > dev-  
> > > > gso_max_segs) ||  << new  
> > > 
> > > 		 unlikely(skb_shinfo(skb)->gso_size > dev-  
> > > > gso_max_size) ||  << new  
> > > 
> > > 		 unlikely((skb->ip_summed != CHECKSUM_PARTIAL) &&
> > > 			  (skb->ip_summed != CHECKSUM_UNNECESSARY)));
> > > }
> > > 
> > > What that will do is split up the monster GSO packets if they
> > > ever
> > > bleed
> > > across from one device to another through the twisty mazes of
> > > packet
> > > processing paths.  
> > 
> > 
> > Since very few drivers have these gso_max_segs / gso_max_size,
> > check
> > could be done in their ndo_features_check()
> 
> Actually, we already check for max_segs, just missing check for size
> here:
> 
> From 71a134f41c4aae8947241091300d21745aa237f2 Mon Sep 17 00:00:00
> 2001
> From: Stephen Hemminger <[email-masked]>
> Date: Thu, 30 Nov 2017 09:45:11 -0800
> Subject: [PATCH] net: do not GSO if frame is too large
> 
> This adds an additional check to breakup skb's that exceed a devices
> GSO maximum size. The code was already checking for too many segments
> but did not check size.
> 
> This has been observed to be a problem when using containers on
> Hyper-V/Azure where the allowed GSO maximum size is less than
> maximum and skb's have gone through multiple layers to arrive
> at the virtual device.
> 
> Signed-off-by: Stephen Hemminger <[email-masked]>
> ---
>  net/core/dev.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 07ed21d64f92..0bb398f3bfa3 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -2918,9 +2918,11 @@ static netdev_features_t
> gso_features_check(const struct sk_buff *skb,
>  					    struct net_device *dev,
>  					    netdev_features_t
> features)
>  {
> +	unsigned int gso_size = skb_shinfo(skb)->gso_size;
>  	u16 gso_segs = skb_shinfo(skb)->gso_segs;
>  
> -	if (gso_segs > dev->gso_max_segs)
> +	if (gso_segs > dev->gso_max_segs ||
> +	    gso_size > dev->gso_max_size)
>  		return features & ~NETIF_F_GSO_MASK;
>  
>  	/* Support for GSO partial features requires software

Yes, but check commit 743b03a83297690f0bd38c452a3bbb47d2be300a
("net: remove netdevice gso_min_segs")

Plan was to get rid of the existing check, not adding new ones :/