netdev - RE: [PATCH bpf-next V11 5/7] bpf: drop MTU check when doing TC-BPF redirect to ingress

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Date:   Thu, 14 Jan 2021 01:03:33 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>, bpf@...r.kernel.org
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>, netdev@...r.kernel.org,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        maze@...gle.com, lmb@...udflare.com, shaun@...era.io,
        Lorenzo Bianconi <lorenzo@...nel.org>, marek@...udflare.com,
        John Fastabend <john.fastabend@...il.com>,
        Jakub Kicinski <kuba@...nel.org>, eyal.birger@...il.com,
        colrack@...il.com
Subject: RE: [PATCH bpf-next V11 5/7] bpf: drop MTU check when doing TC-BPF
 redirect to ingress

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/
> 
> V9:
>  - Make net_device "up" (IFF_UP) check explicit in skb_do_redirect
> 
> V4:
>  - Keep net_device "up" (IFF_UP) check.
>  - Adjustment to handle bpf_redirect_peer() helper
> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.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/net/core/filter.c b/net/core/filter.c
> index 3f2e593244ca..1908800b671c 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);
> +		ret = netif_rx(skb);
> +	}
> +
> +	return ret;

How about putting above block into a dev.c routine call it

 dev_forward_skb_nomtu(...)

or something like that. Then we keep this code next to its pair
with mtu check, dev_forward_skb().

dev_forward_skb() also uses netif_rx_internal() looks like maybe we should
just do the same here?

Thanks,
John