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doc: quick HOWTO on python/xrelfo.py

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Just a few paragraphs on what it does and how to invoke it.

Signed-off-by: David Lamparter <equinox@diac24.net>
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David Lamparter 2021-02-23 10:35:08 +01:00
parent a971f0718d
commit 9e6c14a46b
1 changed files with 47 additions and 2 deletions
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49
doc/developer/xrefs.rst
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@ -20,8 +20,6 @@ To verify xrefs have been included in a binary or dynamic library, run
``readelf -n binary``. For individual object files, it's
``readelf -S object.o | grep xref_array`` instead.
An extraction tool will be added in a future commit.
Structure and contents
----------------------
@ -168,3 +166,50 @@ entry point.
for C++ code when compiled by GCC. A workaround is present for runtime
functionality, but to extract the xrefs from a C++ source file, it needs
to be built with clang (or a future fixed version of GCC) instead.
Extraction tool
---------------
The FRR source contains a matching tool to extract xref data from compiled ELF
binaries in ``python/xrelfo.py``. This tool uses CPython extensions
implemented in ``clippy`` and must therefore be executed with that.
``xrelfo.py`` processes input from one or more ELF file (.o, .so, executable),
libtool object (.lo, .la, executable wrapper script) or JSON (output from
``xrelfo.py``) and generates an output JSON file. During standard FRR build,
it is invoked on all binaries and libraries and the result is combined into
``frr.json``.
ELF files from any operating system, CPU architecture and endianness can be
processed on any host. Any issues with this are bugs in ``xrelfo.py``
(or clippy's ELF code.)
``xrelfo.py`` also performs some sanity checking, particularly on log
messages. The following options are available:
.. option:: -o OUTPUT
Filename to write JSON output to. As a convention, a ``.xref`` filename
extension is used.
.. option:: -Wlog-format
Performs extra checks on log message format strings, particularly checks
for ``\t`` and ``\n`` characters (which should not be used in log messages).
.. option:: -Wlog-args
Generates cleanup hints for format string arguments where
:c:func:`printfrr()` extensions could be used, e.g. replacing ``inet_ntoa``
with ``%pI4``.
.. option:: --profile
Runs the Python profiler to identify hotspots in the ``xrelfo.py`` code.
``xrelfo.py`` uses information about C structure definitions saved in
``python/xrefstructs.json``. This file is included with the FRR sources and
only needs to be regenerated when some of the ``struct xref_*`` definitions
are changed (which should be almost never). The file is written by
``python/tiabwarfo.py``, which uses ``pahole`` to extract the necessary data
from DWARF information.