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Documentation/bpf/redirect.rst

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.. SPDX-License-Identifier: GPL-2.0-only .. Copyright (C) 2022 Red Hat, Inc.

======== Redirect

XDP_REDIRECT ############ Supported maps

XDP_REDIRECT works with the following map types:

  • BPF_MAP_TYPE_DEVMAP
  • BPF_MAP_TYPE_DEVMAP_HASH
  • BPF_MAP_TYPE_CPUMAP
  • BPF_MAP_TYPE_XSKMAP

For more information on these maps, please see the specific map documentation.

Process

.. kernel-doc:: net/core/filter.c :doc: xdp redirect

.. note:: Not all drivers support transmitting frames after a redirect, and for those that do, not all of them support non-linear frames. Non-linear xdp bufs/frames are bufs/frames that contain more than one fragment.

Debugging packet drops

Silent packet drops for XDP_REDIRECT can be debugged using:

  • bpf_trace
  • perf_record

bpf_trace ^^^^^^^^^ The following bpftrace command can be used to capture and count all XDP tracepoints:

.. code-block:: none

sudo bpftrace -e 'tracepoint:xdp:* { @cnt[probe] = count(); }'
Attaching 12 probes...
^C

@cnt[tracepoint:xdp:mem_connect]: 18
@cnt[tracepoint:xdp:mem_disconnect]: 18
@cnt[tracepoint:xdp:xdp_exception]: 19605
@cnt[tracepoint:xdp:xdp_devmap_xmit]: 1393604
@cnt[tracepoint:xdp:xdp_redirect]: 22292200

.. note:: The various xdp tracepoints can be found in source/include/trace/events/xdp.h

The following bpftrace command can be used to extract the ERRNO being returned as part of the err parameter:

.. code-block:: none

sudo bpftrace -e \
'tracepoint:xdp:xdp_redirect*_err {@redir_errno[-args->err] = count();}
tracepoint:xdp:xdp_devmap_xmit {@devmap_errno[-args->err] = count();}'

perf record ^^^^^^^^^^^ The perf tool also supports recording tracepoints:

.. code-block:: none

perf record -a -e xdp:xdp_redirect_err \
    -e xdp:xdp_redirect_map_err \
    -e xdp:xdp_exception \
    -e xdp:xdp_devmap_xmit

References