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We want the fixes in here as well as this resolves an iio driver merge issue. Signed-off-by: Greg Kroah-Hartman <[email protected]>
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@@ -156,6 +156,8 @@ Morten Welinder <[email protected]> | |
Morten Welinder <[email protected]> | ||
Mythri P K <[email protected]> | ||
Nguyen Anh Quynh <[email protected]> | ||
Nicolas Pitre <[email protected]> <[email protected]> | ||
Nicolas Pitre <[email protected]> <[email protected]> | ||
Paolo 'Blaisorblade' Giarrusso <[email protected]> | ||
Patrick Mochel <[email protected]> | ||
Paul Burton <[email protected]> <[email protected]> | ||
|
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.. SPDX-License-Identifier: GPL-2.0 | ||
================== | ||
BPF Flow Dissector | ||
================== | ||
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Overview | ||
======== | ||
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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). | ||
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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). | ||
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API | ||
=== | ||
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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. | ||
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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 | ||
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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. | ||
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The return code of the BPF program is either BPF_OK to indicate successful | ||
dissection, or BPF_DROP to indicate parsing error. | ||
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__sk_buff->data | ||
=============== | ||
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In the VLAN-less case, this is what the initial state of the BPF flow | ||
dissector looks like:: | ||
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+------+------+------------+-----------+ | ||
| DMAC | SMAC | ETHER_TYPE | L3_HEADER | | ||
+------+------+------------+-----------+ | ||
^ | ||
| | ||
+-- flow dissector starts here | ||
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.. 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. | ||
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Pre-VLAN parsing:: | ||
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+------+------+------+-----+-----------+-----------+ | ||
| DMAC | SMAC | TPID | TCI |ETHER_TYPE | L3_HEADER | | ||
+------+------+------+-----+-----------+-----------+ | ||
^ | ||
| | ||
+-- flow dissector starts here | ||
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.. 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. | ||
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Post-VLAN parsing:: | ||
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+------+------+------+-----+-----------+-----------+ | ||
| DMAC | SMAC | TPID | TCI |ETHER_TYPE | L3_HEADER | | ||
+------+------+------+-----+-----------+-----------+ | ||
^ | ||
| | ||
+-- flow dissector starts here | ||
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.. 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. | ||
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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. | ||
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Reference Implementation | ||
======================== | ||
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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. | ||
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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 | ||
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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). | ||
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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. |
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