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@xxxxxxxxx> 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@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. > > > > > > 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.
