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

Maciej Żenczykowski
2020-10-09T23:17:46Z

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;