RE: [PATCH bpf-next V5 4/5] bpf: drop MTU check when doing TC-BPF redirect to ingress

[John Fastabend <john.fastabend@gmail.com>]

Jesper Dangaard Brouer wrote:
> The use-case for dropping the MTU check when TC-BPF does redirect to
> ingress, is described by Eyal Birger in email[0]. The summary is the
> ability to increase packet size (e.g. with IPv6 headers for NAT64) and
> ingress redirect packet and let normal netstack fragment packet as needed.
> 
> [0] https://lore.kernel.org/netdev/CAHsH6Gug-hsLGHQ6N0wtixdOa85LDZ3HNRHVd0opR=19Qo4W4Q@mail.gmail.com/
> 
> V4:
>  - Keep net_device "up" (IFF_UP) check.
>  - Adjustment to handle bpf_redirect_peer() helper
> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> ---
>  include/linux/netdevice.h |   31 +++++++++++++++++++++++++++++--
>  net/core/dev.c            |   19 ++-----------------
>  net/core/filter.c         |   14 +++++++++++---
>  3 files changed, 42 insertions(+), 22 deletions(-)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 964b494b0e8d..bd02ddab8dfe 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -3891,11 +3891,38 @@ int dev_forward_skb(struct net_device *dev, struct sk_buff *skb);
>  bool is_skb_forwardable(const struct net_device *dev,
>  			const struct sk_buff *skb);
>  
> +static __always_inline bool __is_skb_forwardable(const struct net_device *dev,
> +						 const struct sk_buff *skb,
> +						 const bool check_mtu)

It looks like if check_mtu=false then this is just an interface up check.
Can we leave is_skb_forwardable logic alone and just change the spots where
this is called with false to something with a name that describes the check,
such as is_dev_up(dev). I think it will make this change smaller and the
code easier to read. Did I miss something?

> +{
> +	const u32 vlan_hdr_len = 4; /* VLAN_HLEN */
> +	unsigned int len;
> +
> +	if (!(dev->flags & IFF_UP))
> +		return false;
> +
> +	if (!check_mtu)
> +		return true;
> +
> +	len = dev->mtu + dev->hard_header_len + vlan_hdr_len;
> +	if (skb->len <= len)
> +		return true;
> +
> +	/* if TSO is enabled, we don't care about the length as the packet
> +	 * could be forwarded without being segmented before
> +	 */
> +	if (skb_is_gso(skb))
> +		return true;
> +
> +	return false;
> +}
> +
>  static __always_inline int ____dev_forward_skb(struct net_device *dev,
> -					       struct sk_buff *skb)
> +					       struct sk_buff *skb,
> +					       const bool check_mtu)
>  {

I guess you will get some duplication here if you have a dev_forward_skb()
and a dev_forward_skb_nocheck() or something. Take it or leave it. I know
I've added my share of bool swivel bits like this, but better to avoid
it if possible IMO.

Other than style aspects it looks correct to me.

>  	if (skb_orphan_frags(skb, GFP_ATOMIC) ||
> -	    unlikely(!is_skb_forwardable(dev, skb))) {
> +	    unlikely(!__is_skb_forwardable(dev, skb, check_mtu))) {
>  		atomic_long_inc(&dev->rx_dropped);
>  		kfree_skb(skb);
>  		return NET_RX_DROP;
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 9499a414d67e..445ccf92c149 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -2188,28 +2188,13 @@ static inline void net_timestamp_set(struct sk_buff *skb)
>