From: Stanislav Fomichev <sdf@fomichev.me>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
Stanislav Fomichev <sdf@google.com>,
Network Development <netdev@vger.kernel.org>,
bpf <bpf@vger.kernel.org>, David Miller <davem@davemloft.net>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Simon Horman <simon.horman@netronome.com>,
Willem de Bruijn <willemb@google.com>,
Petar Penkov <peterpenkov96@gmail.com>
Subject: Re: [RFC bpf-next v3 6/8] flow_dissector: handle no-skb use case
Date: Wed, 27 Mar 2019 12:58:20 -0700 [thread overview]
Message-ID: <20190327195820.GF7431@mini-arch.hsd1.ca.comcast.net> (raw)
In-Reply-To: <20190327175535.ewpc6a7gpfoxmxys@ast-mbp>
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.
> > 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.
Sure, we can do that, see above, we can manually move ctx->data to nhoff
and set nhoff to zero.
> 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.
Ack, that's what we are trying to do here as well :-) Address all
existing shortcomings before it's too late.
next prev parent reply other threads:[~2019-03-27 19:58 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-22 19:58 [RFC bpf-next v3 0/8] net: flow_dissector: trigger BPF hook when called from eth_get_headlen Stanislav Fomichev
2019-03-22 19:58 ` [RFC bpf-next v3 1/8] flow_dissector: allow access only to a subset of __sk_buff fields Stanislav Fomichev
2019-03-22 19:58 ` [RFC bpf-next v3 2/8] flow_dissector: switch kernel context to struct bpf_flow_dissector Stanislav Fomichev
2019-03-22 19:58 ` [RFC bpf-next v3 3/8] flow_dissector: fix clamping of BPF flow_keys for non-zero nhoff Stanislav Fomichev
2019-03-22 19:58 ` [RFC bpf-next v3 4/8] bpf: when doing BPF_PROG_TEST_RUN for flow dissector use no-skb mode Stanislav Fomichev
2019-03-22 19:59 ` [RFC bpf-next v3 5/8] net: plumb network namespace into __skb_flow_dissect Stanislav Fomichev
2019-03-22 19:59 ` [RFC bpf-next v3 6/8] flow_dissector: handle no-skb use case Stanislav Fomichev
2019-03-23 1:00 ` Alexei Starovoitov
2019-03-23 1:19 ` Stanislav Fomichev
2019-03-23 1:41 ` Alexei Starovoitov
2019-03-23 16:05 ` Stanislav Fomichev
2019-03-26 0:35 ` Alexei Starovoitov
2019-03-26 16:45 ` Stanislav Fomichev
2019-03-26 17:48 ` Alexei Starovoitov
2019-03-26 17:51 ` Willem de Bruijn
2019-03-26 18:08 ` Alexei Starovoitov
2019-03-26 18:17 ` Stanislav Fomichev
2019-03-26 18:30 ` Alexei Starovoitov
2019-03-26 18:54 ` Stanislav Fomichev
2019-03-27 1:41 ` Alexei Starovoitov
2019-03-27 2:44 ` Stanislav Fomichev
2019-03-27 17:55 ` Alexei Starovoitov
2019-03-27 19:58 ` Stanislav Fomichev [this message]
2019-03-28 1:26 ` Alexei Starovoitov
2019-03-28 3:14 ` Willem de Bruijn
2019-03-28 3:32 ` Alexei Starovoitov
2019-03-28 4:17 ` Stanislav Fomichev
2019-03-28 12:58 ` Willem de Bruijn
2019-04-01 16:30 ` Stanislav Fomichev
2019-03-22 19:59 ` [RFC bpf-next v3 7/8] net: pass net argument to the eth_get_headlen Stanislav Fomichev
2019-03-22 19:59 ` [RFC bpf-next v3 8/8] selftests/bpf: add flow dissector bpf_skb_load_bytes helper test Stanislav Fomichev
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