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

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Date:   Wed, 27 Mar 2019 10:55:37 -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 07:44:21PM -0700, Stanislav Fomichev wrote:
> On 03/26, Alexei Starovoitov wrote:
> > 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.
> It points to L3 (or vlan). And this will be preserved, I have no
> intention to change that.
> 
> Just to make sure, we are on the same page, here is what
> __skb_flow_dissect (and BPF prog) is seeing in nhoff.
> 
> NO-VLAN is always the same for both with-skb/no-skb:
> +----+----+-----+--+
> |DMAC|SMAC|PROTO|L3|
> +----+----+-----+--+
>                  ^
>                  +-- nhoff
>                      proto = PROTO
> 
> VLAN no-skb (eth_get_headlen):
> +----+----+----+---+-----+--+
> |DMAC|SMAC|TPID|TCI|PROTO|L3|
> +----+----+----+---+-----+--+
>                 ^
>                 +-- nhoff
>                     proto = TPID

where ctx->data will point to ?
These nhoff differences are fine.
I want to make sure that ctx->data is the same for all.

> And in the last case, networking stack sets:
>  * skb->vlan_present to true
>  * skb->vlan_proto to TPID
>  * skb->vlan_tci to TCI
>  * skb->protocol to PROTO
>  * pulls vlan header, so skb->data points to L3 header

then skb-less should point after L2 as well
or point to L2 for all.

I'm arguing about it because early on during tc-bpf days
we had this discrepancy and it caused issues.
Thankfully we addressed it before it became long term headache.