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

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Message-ID: <20190326185456.GD7431@mini-arch.hsd1.ca.comcast.net>
Date:   Tue, 26 Mar 2019 11:54:56 -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 11:17:19AM -0700, Stanislav Fomichev wrote:
> > 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.
> 
> makes sense. then I'd also prefer for proto to be in flow_keys to
> high light this difference.
Maybe rename existing flow_keys->n_proto to flow_keys->proto?
That would match __skb_flow_dissect and remove ambiguity with both proto
and n_proto in flow_keys.

> may be add vlan_proto/present/tci there as well?
> At least on the kernel side ctx rewriter will be the same for w/ & w/o skb cases.
Why do you think we need them? My understanding was that when
skb_vlan_tag_present(skb) (or skb->vlan_present) returns true, that means
that vlan info has been already parsed out of the packet and stored in
the vlan_tci/vlan_proto (where vlan_proto is 8021Q/8021AD); skb data
points to proper L3 header.

If that's correct, BPF flow dissector should not care about that. For
example, look at how C-based flow dissector does that:

https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/tree/net/core/flow_dissector.c#n944

If skb_vlan_tag_present(skb) returns true, we set proto to skb->protocol
and move on.

But, we would need vlan_proto/present/tci in the flow_keys in the future.
We don't currently return parsed vlan data from the BPF flow dissector.
But it feels like it's getting into bpf-next territory :-)