netdev - Re: [PATCH bpf-next 1/2] bpf: add BPF_LWT_ENCAP_IP option to bpf_lwt_push

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Date:   Fri, 30 Nov 2018 15:35:56 -0800
From:   Peter Oskolkov <posk@...gle.com>
To:     David Ahern <dsahern@...il.com>
Cc:     ast@...nel.org, daniel@...earbox.net, netdev@...r.kernel.org,
        posk.devel@...il.com
Subject: Re: [PATCH bpf-next 1/2] bpf: add BPF_LWT_ENCAP_IP option to bpf_lwt_push_encap

Thanks, David! This is for egress only, so I'll add an appropriate
check. I'll also address your other comments/concerns in a v2 version
of this patchset.
On Fri, Nov 30, 2018 at 12:08 PM David Ahern <dsahern@...il.com> wrote:
>
> On 11/28/18 6:34 PM, Peter Oskolkov wrote:
> > On Wed, Nov 28, 2018 at 4:47 PM David Ahern <dsahern@...il.com> wrote:
> >>
> >> On 11/28/18 5:22 PM, Peter Oskolkov wrote:
> >>> diff --git a/net/core/filter.c b/net/core/filter.c
> >>> index bd0df75dc7b6..17f3c37218e5 100644
> >>> --- a/net/core/filter.c
> >>> +++ b/net/core/filter.c
> >>> @@ -4793,6 +4793,60 @@ static int bpf_push_seg6_encap(struct sk_buff *skb, u32 type, void *hdr, u32 len
> >>>  }
> >>>  #endif /* CONFIG_IPV6_SEG6_BPF */
> >>>
> >>> +static int bpf_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len)
> >>> +{
> >>> +     struct dst_entry *dst;
> >>> +     struct rtable *rt;
> >>> +     struct iphdr *iph;
> >>> +     struct net *net;
> >>> +     int err;
> >>> +
> >>> +     if (skb->protocol != htons(ETH_P_IP))
> >>> +             return -EINVAL;  /* ETH_P_IPV6 not yet supported */
> >>> +
> >>> +     iph = (struct iphdr *)hdr;
> >>> +
> >>> +     if (unlikely(len < sizeof(struct iphdr) || len > LWTUNNEL_MAX_ENCAP_HSIZE))
> >>> +             return -EINVAL;
> >>> +     if (unlikely(iph->version != 4 || iph->ihl * 4 > len))
> >>> +             return -EINVAL;
> >>> +
> >>> +     if (skb->sk)
> >>> +             net = sock_net(skb->sk);
> >>> +     else {
> >>> +             net = dev_net(skb_dst(skb)->dev);
> >>> +     }
> >>> +     rt = ip_route_output(net, iph->daddr, 0, 0, 0);
> >>
> >> That is a very limited use case. e.g., oif = 0 means you are not
> >> considering any kind of policy routing (e.g., VRF).
> >
> > Hi David! Could you be a bit more specific re: what you would like to
> > see here? Thanks!
> >
>
> Is the encap happening on ingress or egress? Seems like the current code
> does not assume either direction for lwt (BPF_PROG_TYPE_LWT_IN vs
> BPF_PROG_TYPE_LWT_OUT), yet your change does - output only. Basically,
> you should be filling in a flow struct and doing a proper lookup.
>
> When the potentially custom encap header is pushed on, seems to me skb
> marks should still be honored for the route lookup. If not, you should
> handle that in the API.
>
> From there skb->dev at a minimum should be used as either iif (ingress)
> or oif (egress).
>
> The iph is already set so you have quick access to the tos.
>
> Also, this should implement IPv6 as well before going in.