flow_dissector: document BPF flow dissector environment

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BPF Flow Dissector

==================

Overview

========

Flow dissector is a routine that parses metadata out of the packets. It's

used in the various places in the networking subsystem (RFS, flow hash, etc).

BPF flow dissector is an attempt to reimplement C-based flow dissector logic

in BPF to gain all the benefits of BPF verifier (namely, limits on the

number of instructions and tail calls).

API

===

BPF flow dissector programs operate on an __sk_buff. However, only the

limited set of fields is allowed: data, data_end and flow_keys. flow_keys

is 'struct bpf_flow_keys' and contains flow dissector input and

output arguments.

The inputs are:

  * nhoff - initial offset of the networking header

  * thoff - initial offset of the transport header, initialized to nhoff

  * n_proto - L3 protocol type, parsed out of L2 header

Flow dissector BPF program should fill out the rest of the 'struct

bpf_flow_keys' fields. Input arguments nhoff/thoff/n_proto should be also

adjusted accordingly.

The return code of the BPF program is either BPF_OK to indicate successful

dissection, or BPF_DROP to indicate parsing error.

__sk_buff->data

===============

In the VLAN-less case, this is what the initial state of the BPF flow

dissector looks like:

+------+------+------------+-----------+

| DMAC | SMAC | ETHER_TYPE | L3_HEADER |

+------+------+------------+-----------+

                            ^

                            |

                            +-- flow dissector starts here

skb->data + flow_keys->nhoff point to the first byte of L3_HEADER.

flow_keys->thoff = nhoff

flow_keys->n_proto = ETHER_TYPE

In case of VLAN, flow dissector can be called with the two different states.

Pre-VLAN parsing:

+------+------+------+-----+-----------+-----------+

| DMAC | SMAC | TPID | TCI |ETHER_TYPE | L3_HEADER |

+------+------+------+-----+-----------+-----------+

                      ^

                      |

                      +-- flow dissector starts here

skb->data + flow_keys->nhoff point the to first byte of TCI.

flow_keys->thoff = nhoff

flow_keys->n_proto = TPID

Please note that TPID can be 802.1AD and, hence, BPF program would

have to parse VLAN information twice for double tagged packets.

Post-VLAN parsing:

+------+------+------+-----+-----------+-----------+

| DMAC | SMAC | TPID | TCI |ETHER_TYPE | L3_HEADER |

+------+------+------+-----+-----------+-----------+

                                        ^

                                        |

                                        +-- flow dissector starts here

skb->data + flow_keys->nhoff point the to first byte of L3_HEADER.

flow_keys->thoff = nhoff

flow_keys->n_proto = ETHER_TYPE

In this case VLAN information has been processed before the flow dissector

and BPF flow dissector is not required to handle it.

The takeaway here is as follows: BPF flow dissector program can be called with

the optional VLAN header and should gracefully handle both cases: when single

or double VLAN is present and when it is not present. The same program

can be called for both cases and would have to be written carefully to

handle both cases.

Reference Implementation

========================

See tools/testing/selftests/bpf/progs/bpf_flow.c for the reference

implementation and tools/testing/selftests/bpf/flow_dissector_load.[hc] for

the loader. bpftool can be used to load BPF flow dissector program as well.

The reference implementation is organized as follows:

* jmp_table map that contains sub-programs for each supported L3 protocol

* _dissect routine - entry point; it does input n_proto parsing and does

  bpf_tail_call to the appropriate L3 handler

Since BPF at this point doesn't support looping (or any jumping back),

jmp_table is used instead to handle multiple levels of encapsulation (and

IPv6 options).

Current Limitations

===================

BPF flow dissector doesn't support exporting all the metadata that in-kernel

C-based implementation can export. Notable example is single VLAN (802.1Q)

and double VLAN (802.1AD) tags. Please refer to the 'struct bpf_flow_keys'

for a set of information that's currently can be exported from the BPF context.