mirror of
https://github.com/edk2-porting/linux-next.git
synced 2024-12-24 21:24:00 +08:00
8069594673
In commit da70314917 ("bpf: Document BPF_PROG_TYPE_CGROUP_SYSCTL")
Andrey proposes to put per-prog type docs under Documentation/bpf/
Let's move flow dissector documentation there as well.
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
127 lines
4.3 KiB
ReStructuredText
127 lines
4.3 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
============================
|
|
BPF_PROG_TYPE_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
|
|
|
|
|
|
.. 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 |
|
|
+------+------+------+-----+-----------+-----------+
|
|
^
|
|
|
|
|
+-- flow dissector starts here
|
|
|
|
.. 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 |
|
|
+------+------+------+-----+-----------+-----------+
|
|
^
|
|
|
|
|
+-- flow dissector starts here
|
|
|
|
.. 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.
|
|
|
|
|
|
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.
|