Message-ID: <20190328041715.GG7431@mini-arch.hsd1.ca.comcast.net>
Date:   Wed, 27 Mar 2019 21:17:15 -0700
From:   Stanislav Fomichev <sdf@...ichev.me>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
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 03/27, Alexei Starovoitov wrote:
> On Wed, Mar 27, 2019 at 11:14:46PM -0400, Willem de Bruijn wrote:
> > On Wed, Mar 27, 2019 at 9:26 PM Alexei Starovoitov
> > <alexei.starovoitov@...il.com> wrote:
> > >
> > > 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@...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.
> > > > 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.
> > 
> > Since flow dissection starts at the network layer, I would then
> > suggest data always at L3 and nhoff 0.
For eth_get_headlen we need to manually parse 802.1q header. And for RFS
case as well (unless I'm missing something).

> > This can be derived in the same manner as __skb_flow_dissect
> > already does if !data, using only skb_network_offset.
> > 
> > From a quick scan, skb_mac_offset should also be valid in all cases
> > where the flow dissector is called today, so the other can be computed, too.
> > 
> > But this is less obvious. For instance, tun_get_user calls into the flow
> > dissector up to three times (wow) and IFF_TUN has no link layer
> > (ARPHRD_NONE). And then there are also fun variable length link layer
> > protocols to deal with..
> 
> ahh. ok. Can we guarantee some stable position?
I don't think so. Pre RFS ctx->data+nhoff can point to 802.1q header,
post RFS it will point to L3. The only thing we can do is to have
nhoff=0 (and adjust ctx->data accordingly) when the main bpf
flow dissector procedure is called. But that would require bringing
this new kernel context (bpf_flow_dissector) into bpf/stable.
(And it's not clear what's the benefit, since tail calls would still
have to look at that offset).

> Current bpf_flow_dissect_get_header assumes that
> ctx->data + ctx->flow_keys->thoff point to IP, right?
Yes, mostly, except that if skb->protocol is 802.1q/ad, it's 802.1q header.
And it's only for the "main" call; bpf program adjusts this thoff
to make sure that tail calls preserve some sense of progress (so it
eventually points to L4 and that's what we export back).

> Based on what Stanislav saying above even that is not a guarantee?
> I'm struggling to see how users can wrap their heads around this.
> It seems bpf_flow.c will become the only prog that can deal with
> this range of possible inputs.
> 
> I propose to start with the doc that describes all cases, where
> things point to and how prog suppose to parse that.
Yeah, that is what I was going to propose - add a doc along with the
patch series. I don't see how we can make it simple(r) at this point :-(
I can try to document everything so users don't have to read the
kernel code to understand how to write the bpf flow dissector programs.

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