Documentation/kbuild/gcc-plugins.rst

=========================
GCC plugin infrastructure
=========================


Introduction
============

GCC plugins are loadable modules that provide extra features to the
compiler [1]_. They are useful for runtime instrumentation and static analysis.
We can analyse, change and add further code during compilation via
callbacks [2]_, GIMPLE [3]_, IPA [4]_ and RTL passes [5]_.

The GCC plugin infrastructure of the kernel supports building out-of-tree
modules, cross-compilation and building in a separate directory.
Plugin source files have to be compilable by a C++ compiler.

Currently the GCC plugin infrastructure supports only some architectures.
Grep "select HAVE_GCC_PLUGINS" to find out which architectures support
GCC plugins.

This infrastructure was ported from grsecurity [6]_ and PaX [7]_.

--

.. [1] https://gcc.gnu.org/onlinedocs/gccint/Plugins.html
.. [2] https://gcc.gnu.org/onlinedocs/gccint/Plugin-API.html#Plugin-API
.. [3] https://gcc.gnu.org/onlinedocs/gccint/GIMPLE.html
.. [4] https://gcc.gnu.org/onlinedocs/gccint/IPA.html
.. [5] https://gcc.gnu.org/onlinedocs/gccint/RTL.html
.. [6] https://grsecurity.net/
.. [7] https://pax.grsecurity.net/


Files
=====

**$(src)/scripts/gcc-plugins**

	This is the directory of the GCC plugins.

**$(src)/scripts/gcc-plugins/gcc-common.h**

	This is a compatibility header for GCC plugins.
	It should be always included instead of individual gcc headers.

**$(src)/scripts/gcc-plugins/gcc-generate-gimple-pass.h,
$(src)/scripts/gcc-plugins/gcc-generate-ipa-pass.h,
$(src)/scripts/gcc-plugins/gcc-generate-simple_ipa-pass.h,
$(src)/scripts/gcc-plugins/gcc-generate-rtl-pass.h**

	These headers automatically generate the registration structures for
	GIMPLE, SIMPLE_IPA, IPA and RTL passes.
	They should be preferred to creating the structures by hand.


Usage
=====

You must install the gcc plugin headers for your gcc version,
e.g., on Ubuntu for gcc-10::

	apt-get install gcc-10-plugin-dev

Or on Fedora::

	dnf install gcc-plugin-devel

Enable the GCC plugin infrastructure and some plugin(s) you want to use
in the kernel config::

	CONFIG_GCC_PLUGINS=y
	CONFIG_GCC_PLUGIN_CYC_COMPLEXITY=y
	CONFIG_GCC_PLUGIN_LATENT_ENTROPY=y
	...

To compile the minimum tool set including the plugin(s)::

	make scripts

or just run the kernel make and compile the whole kernel with
the cyclomatic complexity GCC plugin.


4. How to add a new GCC plugin
==============================

The GCC plugins are in scripts/gcc-plugins/. You need to put plugin source files
right under scripts/gcc-plugins/. Creating subdirectories is not supported.
It must be added to scripts/gcc-plugins/Makefile, scripts/Makefile.gcc-plugins
and a relevant Kconfig file.
See the cyc_complexity_plugin.c (CONFIG_GCC_PLUGIN_CYC_COMPLEXITY) GCC plugin.