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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Mar 27, 2019 at 12:58:20PM -0700, Stanislav Fomichev wrote:
> On 03/27, Alexei Starovoitov wrote:
> > 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@gmail.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.
> For with-skb, nhoff would be zero, and ctx->data would point to
> TCI/L3.
> For skb-less, ctx->data would point to L2 (DMAC), and nhoff would be
> non-zero (TCI/L3 offset).
> 
> If you want, for skb-less case, when calling BPF program we can do the math
> ourselves and set ctx->data to data + nhoff, and pass nhoff = 0.
> But I'm not sure whether we need to do that; flow dissector is supposed
> to look at ctx->data + nhoff, it should not matter what each individual
> value is, they only make sense together.

My strong preference is to have data to point to L2 in all cases.
Semantics of requiring bpf prog to start processing from a tuple
(data + nhoff) where both point to random places is very confusing.