[bpf-next,V3,5/6] bpf: drop MTU check when doing TC-BPF redirect to ingress

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/

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 include/linux/netdevice.h |    5 +++--
 net/core/dev.c            |    2 +-
 net/core/filter.c         |   12 ++++++++++--
 3 files changed, 14 insertions(+), 5 deletions(-)

On Thu, Oct 8, 2020 at 7:09 AM Jesper Dangaard Brouer <brouer@redhat.com> 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/
>
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> ---
>  include/linux/netdevice.h |    5 +++--
>  net/core/dev.c            |    2 +-
>  net/core/filter.c         |   12 ++++++++++--
>  3 files changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 28cfa53daf72..58fb7b4869ba 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -3866,10 +3866,11 @@ bool is_skb_forwardable(const struct net_device *dev,
>                         const struct sk_buff *skb);
>
>  static __always_inline int ____dev_forward_skb(struct net_device *dev,
> -                                              struct sk_buff *skb)
> +                                              struct sk_buff *skb,
> +                                              const bool mtu_check)

check_mtu might be a better arg name then 'mtu_check'

>  {
>         if (skb_orphan_frags(skb, GFP_ATOMIC) ||
> -           unlikely(!is_skb_forwardable(dev, skb))) {
> +           (mtu_check && unlikely(!is_skb_forwardable(dev, skb)))) {
>                 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 b433098896b2..96b455f15872 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -2209,7 +2209,7 @@ EXPORT_SYMBOL_GPL(is_skb_forwardable);
>
>  int __dev_forward_skb(struct net_device *dev, struct sk_buff *skb)
>  {
> -       int ret = ____dev_forward_skb(dev, skb);
> +       int ret = ____dev_forward_skb(dev, skb, true);
>
>         if (likely(!ret)) {
>                 skb->protocol = eth_type_trans(skb, dev);
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 5986156e700e..a8e24092e4f5 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -2083,13 +2083,21 @@ static const struct bpf_func_proto bpf_csum_level_proto = {
>
>  static inline int __bpf_rx_skb(struct net_device *dev, struct sk_buff *skb)
>  {
> -       return dev_forward_skb(dev, skb);
> +       int ret = ____dev_forward_skb(dev, skb, false);
> +
> +       if (likely(!ret)) {
> +               skb->protocol = eth_type_trans(skb, dev);
> +               skb_postpull_rcsum(skb, eth_hdr(skb), ETH_HLEN);

this blindly assumes eth header size in a function that does (by name)
seem ethernet specific...
could this use dev->hard_header_len?  or change func name to be
__bpf_ethernet_rx_skb or something

> +               ret = netif_rx(skb);
> +       }
> +
> +       return ret;
>  }
>
>  static inline int __bpf_rx_skb_no_mac(struct net_device *dev,
>                                       struct sk_buff *skb)
>  {
> -       int ret = ____dev_forward_skb(dev, skb);
> +       int ret = ____dev_forward_skb(dev, skb, false);
>
>         if (likely(!ret)) {
>                 skb->dev = dev;