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  * ``flags`` - optional flags
30
31Flow dissector BPF program should fill out the rest of the ``struct
32bpf_flow_keys`` fields. Input arguments ``nhoff/thoff/n_proto`` should be
33also adjusted accordingly.
34
35The return code of the BPF program is either BPF_OK to indicate successful
36dissection, or BPF_DROP to indicate parsing error.
37
38__sk_buff->data
39===============
40
41In the VLAN-less case, this is what the initial state of the BPF flow
42dissector looks like::
43
44  +------+------+------------+-----------+
45  | DMAC | SMAC | ETHER_TYPE | L3_HEADER |
46  +------+------+------------+-----------+
47                              ^
48                              |
49                              +-- flow dissector starts here
50
51
52.. code:: c
53
54  skb->data + flow_keys->nhoff point to the first byte of L3_HEADER
55  flow_keys->thoff = nhoff
56  flow_keys->n_proto = ETHER_TYPE
57
58In case of VLAN, flow dissector can be called with the two different states.
59
60Pre-VLAN parsing::
61
62  +------+------+------+-----+-----------+-----------+
63  | DMAC | SMAC | TPID | TCI |ETHER_TYPE | L3_HEADER |
64  +------+------+------+-----+-----------+-----------+
65                        ^
66                        |
67                        +-- flow dissector starts here
68
69.. code:: c
70
71  skb->data + flow_keys->nhoff point the to first byte of TCI
72  flow_keys->thoff = nhoff
73  flow_keys->n_proto = TPID
74
75Please note that TPID can be 802.1AD and, hence, BPF program would
76have to parse VLAN information twice for double tagged packets.
77
78
79Post-VLAN parsing::
80
81  +------+------+------+-----+-----------+-----------+
82  | DMAC | SMAC | TPID | TCI |ETHER_TYPE | L3_HEADER |
83  +------+------+------+-----+-----------+-----------+
84                                          ^
85                                          |
86                                          +-- flow dissector starts here
87
88.. code:: c
89
90  skb->data + flow_keys->nhoff point the to first byte of L3_HEADER
91  flow_keys->thoff = nhoff
92  flow_keys->n_proto = ETHER_TYPE
93
94In this case VLAN information has been processed before the flow dissector
95and BPF flow dissector is not required to handle it.
96
97
98The takeaway here is as follows: BPF flow dissector program can be called with
99the optional VLAN header and should gracefully handle both cases: when single
100or double VLAN is present and when it is not present. The same program
101can be called for both cases and would have to be written carefully to
102handle both cases.
103
104
105Flags
106=====
107
108``flow_keys->flags`` might contain optional input flags that work as follows:
109
110* ``BPF_FLOW_DISSECTOR_F_PARSE_1ST_FRAG`` - tells BPF flow dissector to
111  continue parsing first fragment; the default expected behavior is that
112  flow dissector returns as soon as it finds out that the packet is fragmented;
113  used by ``eth_get_headlen`` to estimate length of all headers for GRO.
114* ``BPF_FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL`` - tells BPF flow dissector to
115  stop parsing as soon as it reaches IPv6 flow label; used by
116  ``___skb_get_hash`` and ``__skb_get_hash_symmetric`` to get flow hash.
117* ``BPF_FLOW_DISSECTOR_F_STOP_AT_ENCAP`` - tells BPF flow dissector to stop
118  parsing as soon as it reaches encapsulated headers; used by routing
119  infrastructure.
120
121
122Reference Implementation
123========================
124
125See ``tools/testing/selftests/bpf/progs/bpf_flow.c`` for the reference
126implementation and ``tools/testing/selftests/bpf/flow_dissector_load.[hc]``
127for the loader. bpftool can be used to load BPF flow dissector program as well.
128
129The reference implementation is organized as follows:
130  * ``jmp_table`` map that contains sub-programs for each supported L3 protocol
131  * ``_dissect`` routine - entry point; it does input ``n_proto`` parsing and
132    does ``bpf_tail_call`` to the appropriate L3 handler
133
134Since BPF at this point doesn't support looping (or any jumping back),
135jmp_table is used instead to handle multiple levels of encapsulation (and
136IPv6 options).
137
138
139Current Limitations
140===================
141BPF flow dissector doesn't support exporting all the metadata that in-kernel
142C-based implementation can export. Notable example is single VLAN (802.1Q)
143and double VLAN (802.1AD) tags. Please refer to the ``struct bpf_flow_keys``
144for a set of information that's currently can be exported from the BPF context.
145