1.. SPDX-License-Identifier: GPL-2.0 2 3============================ 4BPF_PROG_TYPE_FLOW_DISSECTOR 5============================ 6 7Overview 8======== 9 10Flow dissector is a routine that parses metadata out of the packets. It's 11used in the various places in the networking subsystem (RFS, flow hash, etc). 12 13BPF flow dissector is an attempt to reimplement C-based flow dissector logic 14in BPF to gain all the benefits of BPF verifier (namely, limits on the 15number of instructions and tail calls). 16 17API 18=== 19 20BPF flow dissector programs operate on an ``__sk_buff``. However, only the 21limited set of fields is allowed: ``data``, ``data_end`` and ``flow_keys``. 22``flow_keys`` is ``struct bpf_flow_keys`` and contains flow dissector input 23and output arguments. 24 25The inputs are: 26 * ``nhoff`` - initial offset of the networking header 27 * ``thoff`` - initial offset of the transport header, initialized to nhoff 28 * ``n_proto`` - L3 protocol type, parsed out of L2 header 29 30Flow dissector BPF program should fill out the rest of the ``struct 31bpf_flow_keys`` fields. Input arguments ``nhoff/thoff/n_proto`` should be 32also adjusted accordingly. 33 34The return code of the BPF program is either BPF_OK to indicate successful 35dissection, or BPF_DROP to indicate parsing error. 36 37__sk_buff->data 38=============== 39 40In the VLAN-less case, this is what the initial state of the BPF flow 41dissector looks like:: 42 43 +------+------+------------+-----------+ 44 | DMAC | SMAC | ETHER_TYPE | L3_HEADER | 45 +------+------+------------+-----------+ 46 ^ 47 | 48 +-- flow dissector starts here 49 50 51.. code:: c 52 53 skb->data + flow_keys->nhoff point to the first byte of L3_HEADER 54 flow_keys->thoff = nhoff 55 flow_keys->n_proto = ETHER_TYPE 56 57In case of VLAN, flow dissector can be called with the two different states. 58 59Pre-VLAN parsing:: 60 61 +------+------+------+-----+-----------+-----------+ 62 | DMAC | SMAC | TPID | TCI |ETHER_TYPE | L3_HEADER | 63 +------+------+------+-----+-----------+-----------+ 64 ^ 65 | 66 +-- flow dissector starts here 67 68.. code:: c 69 70 skb->data + flow_keys->nhoff point the to first byte of TCI 71 flow_keys->thoff = nhoff 72 flow_keys->n_proto = TPID 73 74Please note that TPID can be 802.1AD and, hence, BPF program would 75have to parse VLAN information twice for double tagged packets. 76 77 78Post-VLAN parsing:: 79 80 +------+------+------+-----+-----------+-----------+ 81 | DMAC | SMAC | TPID | TCI |ETHER_TYPE | L3_HEADER | 82 +------+------+------+-----+-----------+-----------+ 83 ^ 84 | 85 +-- flow dissector starts here 86 87.. code:: c 88 89 skb->data + flow_keys->nhoff point the to first byte of L3_HEADER 90 flow_keys->thoff = nhoff 91 flow_keys->n_proto = ETHER_TYPE 92 93In this case VLAN information has been processed before the flow dissector 94and BPF flow dissector is not required to handle it. 95 96 97The takeaway here is as follows: BPF flow dissector program can be called with 98the optional VLAN header and should gracefully handle both cases: when single 99or double VLAN is present and when it is not present. The same program 100can be called for both cases and would have to be written carefully to 101handle both cases. 102 103 104Reference Implementation 105======================== 106 107See ``tools/testing/selftests/bpf/progs/bpf_flow.c`` for the reference 108implementation and ``tools/testing/selftests/bpf/flow_dissector_load.[hc]`` 109for the loader. bpftool can be used to load BPF flow dissector program as well. 110 111The reference implementation is organized as follows: 112 * ``jmp_table`` map that contains sub-programs for each supported L3 protocol 113 * ``_dissect`` routine - entry point; it does input ``n_proto`` parsing and 114 does ``bpf_tail_call`` to the appropriate L3 handler 115 116Since BPF at this point doesn't support looping (or any jumping back), 117jmp_table is used instead to handle multiple levels of encapsulation (and 118IPv6 options). 119 120 121Current Limitations 122=================== 123BPF flow dissector doesn't support exporting all the metadata that in-kernel 124C-based implementation can export. Notable example is single VLAN (802.1Q) 125and double VLAN (802.1AD) tags. Please refer to the ``struct bpf_flow_keys`` 126for a set of information that's currently can be exported from the BPF context. 127