flow_dissector: document BPF flow dissector environment

Short doc on what BPF flow dissector should expect in the input
__sk_buff and flow_keys.
Signed-off-by: NStanislav Fomichev <sdf@google.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

Documentation/networking/bpf_flow_dissector.txt

+==================
+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.