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

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Date:   Tue, 26 Mar 2019 18:41:23 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Stanislav Fomichev <sdf@...ichev.me>
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 Tue, Mar 26, 2019 at 11:54:56AM -0700, Stanislav Fomichev wrote:
> 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.

disabling useless fields in ctx is one thing, since probability of breaking users
is low, but renaming n_proto is imo too much.

> > 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 :-)

Whether ctx->data points to L2 or L3 is uapi regardless whether
progs/bpf_flow.c is relying on that or not.
So far I think you're saying that in all three cases:
no-skb, skb befor rfs, skb after rfs ctx->data points to L2, right?
This has to be preserved.
Only now after reading bpf_flow.c for Nth time I realized what semantics
you gave to skb->vlan* and skb->protocol fields. All of them have
to be kept as-is.
For no-skb cases all of them should be available with the same logic
and it has to documented, since it's different from other bpf progs
that access these fields.