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

[Stanislav Fomichev <sdf@fomichev.me>]

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

VLAN with-skb, RFS (pre __netif_receive_skb_core):
+----+----+----+---+-----+--+
|DMAC|SMAC|TPID|TCI|PROTO|L3|
+----+----+----+---+-----+--+
                ^
                +-- nhoff
                    proto = TPID

VLAN with-skb, post RFS (post __netif_receive_skb_core / skb_vlan_untag):
+----+----+----+---+-----+--+
|DMAC|SMAC|TPID|TCI|PROTO|L3|
+----+----+----+---+-----+--+
                          ^
                          +-- nhoff
			      proto = PROTO

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

> 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.
Don't read too much into current bpf_flow.c, I don't think it really
works with vlans in all the cases :-/

It always looks back, assuming post RFS situation; that needs to be
changed by dropping that "if (!skb->vlan_present)" and just looking
into input 'proto' (and optionally parsing vlan hdr if proto ==
802.1q/ad, which we already, sort of, do).

I'm gonna add a small testcase for BPF_PROG_TEST_RUN.

> 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.
I feel like dropping those vlan_{present,proto,tci} from bpf flow dissector.
It should not care what's in the skb and should just rely on the input 'proto'
to optionally parse vlan header.

+1 on documenting all of that