flow_dissector: rst'ify documentation

Rename bpf_flow_dissector.txt to bpf_flow_dissector.rst and fix
formatting. Also, link it from the Documentation/networking/index.rst.

Tested with 'make htmldocs' to make sure it looks reasonable.

Fixes: ae82899b ("flow_dissector: document BPF flow dissector environment")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

Documentation/networking/bpf_flow_dissector.txt → Documentation/networking/bpf_flow_dissector.rst

+.. SPDX-License-Identifier: GPL-2.0
+
 ==================
 BPF Flow Dissector
 ==================
@@ -15,19 +17,19 @@ 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.
+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
+  * ``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.
+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.
@@ -36,48 +38,57 @@ __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 |
-+------+------+------------+-----------+
+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
 
+.. code:: c
+
+  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 |
-+------+------+------+-----+-----------+-----------+
+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
+.. code:: c
+
+  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 |
-+------+------+------+-----+-----------+-----------+
+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
+.. code:: c
+
+  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.
@@ -93,14 +104,14 @@ 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.
+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
+  * ``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
@@ -111,5 +122,5 @@ 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'
+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.

Documentation/networking/index.rst

@@ -9,6 +9,7 @@ Contents:
    netdev-FAQ
    af_xdp
    batman-adv
+   bpf_flow_dissector
    can
    can_ucan_protocol
    device_drivers/freescale/dpaa2/index