netdev - Re: [PATCH net-next 07/15] ipv6: add GRO_IPV6_MAX

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Date:   Thu, 03 Feb 2022 11:44:59 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Coco Li <lixiaoyan@...gle.com>
Subject: Re: [PATCH net-next 07/15] ipv6: add GRO_IPV6_MAX_SIZE

On Wed, 2022-02-02 at 17:51 -0800, Eric Dumazet wrote:
> From: "Signed-off-by: Coco Li" <lixiaoyan@...gle.com>
> 
> Enable GRO to have IPv6 specific limit for max packet size.
> 
> This patch introduces new dev->gro_ipv6_max_size
> that is modifiable through ip link.
> 
> ip link set dev eth0 gro_ipv6_max_size 185000
> 
> Note that this value is only considered if bigger than
> gro_max_size, and for non encapsulated TCP/ipv6 packets.
> 
> Signed-off-by: Coco Li <lixiaoyan@...gle.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  include/linux/netdevice.h          | 10 ++++++++++
>  include/uapi/linux/if_link.h       |  1 +
>  net/core/dev.c                     |  1 +
>  net/core/gro.c                     | 20 ++++++++++++++++++--
>  net/core/rtnetlink.c               | 15 +++++++++++++++
>  tools/include/uapi/linux/if_link.h |  1 +
>  6 files changed, 46 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 2a563869ba44f7d48095d36b1395e3fbd8cfff87..a3a61cffd953add6f272a53f551a49a47d200c68 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -1944,6 +1944,8 @@ enum netdev_ml_priv_type {
>   *			keep a list of interfaces to be deleted.
>   *	@gro_max_size:	Maximum size of aggregated packet in generic
>   *			receive offload (GRO)
> + *	@gro_ipv6_max_size:	Maximum size of aggregated packet in generic
> + *				receive offload (GRO), for IPv6
>   *
>   *	@dev_addr_shadow:	Copy of @dev_addr to catch direct writes.
>   *	@linkwatch_dev_tracker:	refcount tracker used by linkwatch.
> @@ -2137,6 +2139,7 @@ struct net_device {
>  	int			napi_defer_hard_irqs;
>  #define GRO_MAX_SIZE		65536
>  	unsigned int		gro_max_size;
> +	unsigned int		gro_ipv6_max_size;
>  	rx_handler_func_t __rcu	*rx_handler;
>  	void __rcu		*rx_handler_data;
>  
> @@ -4840,6 +4843,13 @@ static inline void netif_set_gso_ipv6_max_size(struct net_device *dev,
>  	WRITE_ONCE(dev->gso_ipv6_max_size, size);
>  }
>  
> +static inline void netif_set_gro_ipv6_max_size(struct net_device *dev,
> +					       unsigned int size)
> +{
> +	/* This pairs with the READ_ONCE() in skb_gro_receive() */
> +	WRITE_ONCE(dev->gro_ipv6_max_size, size);
> +}
> +
>  static inline void skb_gso_error_unwind(struct sk_buff *skb, __be16 protocol,
>  					int pulled_hlen, u16 mac_offset,
>  					int mac_len)
> diff --git a/include/uapi/linux/if_link.h b/include/uapi/linux/if_link.h
> index 024b3bd0467e1360917001dba6bcfd1f30391894..48fe85bed4a629df0dd7cc0ee3a5139370e2c94d 100644
> --- a/include/uapi/linux/if_link.h
> +++ b/include/uapi/linux/if_link.h
> @@ -350,6 +350,7 @@ enum {
>  	IFLA_GRO_MAX_SIZE,
>  	IFLA_TSO_IPV6_MAX_SIZE,
>  	IFLA_GSO_IPV6_MAX_SIZE,
> +	IFLA_GRO_IPV6_MAX_SIZE,
>  
>  	__IFLA_MAX
>  };
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 53c947e6fdb7c47e6cc92fd4e38b71e9b90d921c..e7df5c3f53d6e96d01ff06d081cef77d0c6d9d29 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -10190,6 +10190,7 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
>  	dev->gro_max_size = GRO_MAX_SIZE;
>  	dev->tso_ipv6_max_size = GSO_MAX_SIZE;
>  	dev->gso_ipv6_max_size = GSO_MAX_SIZE;
> +	dev->gro_ipv6_max_size = GRO_MAX_SIZE;
>  
>  	dev->upper_level = 1;
>  	dev->lower_level = 1;
> diff --git a/net/core/gro.c b/net/core/gro.c
> index a11b286d149593827f1990fb8d06b0295fa72189..005a05468418f0373264e8019384e2daa13176eb 100644
> --- a/net/core/gro.c
> +++ b/net/core/gro.c
> @@ -136,11 +136,27 @@ int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb)
>  	unsigned int new_truesize;
>  	struct sk_buff *lp;
>  
> +	if (unlikely(NAPI_GRO_CB(skb)->flush))
> +		return -E2BIG;
> +
>  	/* pairs with WRITE_ONCE() in netif_set_gro_max_size() */
>  	gro_max_size = READ_ONCE(p->dev->gro_max_size);
>  
> -	if (unlikely(p->len + len >= gro_max_size || NAPI_GRO_CB(skb)->flush))
> -		return -E2BIG;
> +	if (unlikely(p->len + len >= gro_max_size)) {
> +		/* pairs with WRITE_ONCE() in netif_set_gro_ipv6_max_size() */
> +		unsigned int gro6_max_size = READ_ONCE(p->dev->gro_ipv6_max_size);
> +
> +		if (gro6_max_size > gro_max_size &&
> +		    p->protocol == htons(ETH_P_IPV6) &&
> +		    skb_headroom(p) >= sizeof(struct hop_jumbo_hdr) &&
> +		    ipv6_hdr(p)->nexthdr == IPPROTO_TCP &&
> +		    !p->encapsulation)
> +			gro_max_size = gro6_max_size;
> +
> +		if (p->len + len >= gro_max_size)
> +			return -E2BIG;
> +	}
> +
>  
>  	lp = NAPI_GRO_CB(p)->last;
>  	pinfo = skb_shinfo(lp);

If I read correctly, a big GRO packet could be forwarded and/or
redirected to an egress device not supporting LSOv2 or with a lower
tso_ipv6_max_size. Don't we need to update netif_needs_gso() to take
care of such scenario? 
AFAICS we are not enforcing gso_max_size, so I'm wondering if that is
really a problem?!?

Thanks!

Paolo