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@xxxxxxxxx> 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.
