netdev - Re: [RFC bpf-next v3 6/8] flow_dissector: handle no-skb use case

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Message-ID: <20190326181719.GC7431@mini-arch.hsd1.ca.comcast.net>
Date:   Tue, 26 Mar 2019 11:17:19 -0700
From:   Stanislav Fomichev <sdf@...ichev.me>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, David Miller <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Simon Horman <simon.horman@...ronome.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Petar Penkov <peterpenkov96@...il.com>
Subject: Re: [RFC bpf-next v3 6/8] flow_dissector: handle no-skb use case

On 03/26, Alexei Starovoitov wrote:
> On Tue, Mar 26, 2019 at 10:52 AM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> > The BPF flow dissector should work the same. It is fine to pass the
> > data including ethernet header, but parsing can start at nhoff with
> > proto explicitly passed.
> >
> > We should not assume Ethernet link layer.
> 
> then skb-less dissector has to be different program type
> because semantics are different.
The semantics are the same as for c-based __skb_flow_dissect.
We just need to pass nhoff and proto that has been passed to
__skb_flow_dissect to the bpf program. In case of with-skb,
take this initial data from skb, like __skb_flow_dissect does (and don't
ask BPF program to do it essentially):

https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/tree/net/core/flow_dissector.c#n763

I was thinking of passing proto as flow_keys->n_proto and we already
pass flow_keys->nhoff, so no need to do anything for it. With that,
BPF program doesn't need to look into skb and can parse optional vlan
and L3+ headers. The same way __skb_flow_dissect does that.