[kernel-hardening] [PATCH 3/3] Documentation for the GCC plugin infrastructure

[Emese Revfy <re.emese@gmail.com>]

This is the GCC infrastructure documentation about its operation, how to add
and use a new plugin with an example.

---
 Documentation/example_gcc_plugin.c | 103 +++++++++++++++++++++++++++++++++++++
 Documentation/gcc-plugins.txt      |  76 +++++++++++++++++++++++++++
 2 files changed, 179 insertions(+)
 create mode 100644 Documentation/example_gcc_plugin.c
 create mode 100644 Documentation/gcc-plugins.txt

diff --git a/Documentation/example_gcc_plugin.c b/Documentation/example_gcc_plugin.c
new file mode 100644
index 0000000..950ff78
--- /dev/null
+++ b/Documentation/example_gcc_plugin.c
@@ -0,0 +1,103 @@
+/*
+ * Copyright 2011-2016 by Emese Revfy <re.emese@gmail.com>
+ * Licensed under the GPL v2, or (at your option) v3
+ */
+
+#include "gcc-common.h"
+
+int plugin_is_GPL_compatible;
+
+static struct plugin_info example_plugin_info = {
+	.version	= "0",
+	.help		= "example plugin\n",
+};
+
+static unsigned int handle_function(void)
+{
+	gimple_stmt_iterator gsi;
+	basic_block bb;
+
+	FOR_ALL_BB_FN(bb, cfun) {
+		for (gsi = gsi_start_bb(bb); !gsi_end_p(gsi); gsi_next(&gsi))
+			debug_gimple_stmt(gsi_stmt(gsi));
+	}
+	return 0;
+}
+
+#if BUILDING_GCC_VERSION >= 4009
+namespace {
+static const struct pass_data example_plugin_pass_data = {
+#else
+static struct gimple_opt_pass example_plugin_pass = {
+	.pass = {
+#endif
+		.type			= GIMPLE_PASS,
+		.name			= "example_plugin",
+#if BUILDING_GCC_VERSION >= 4008
+		.optinfo_flags		= OPTGROUP_NONE,
+#endif
+#if BUILDING_GCC_VERSION >= 5000
+#elif BUILDING_GCC_VERSION >= 4009
+		.has_gate		= false,
+		.has_execute		= true,
+#else
+		.gate			= NULL,
+		.execute		= handle_function,
+		.sub			= NULL,
+		.next			= NULL,
+		.static_pass_number	= 0,
+#endif
+		.tv_id			= TV_NONE,
+		.properties_required	= 0,
+		.properties_provided	= 0,
+		.properties_destroyed	= 0,
+		.todo_flags_start	= 0,
+		.todo_flags_finish	= TODO_dump_func
+#if BUILDING_GCC_VERSION < 4009
+	}
+#endif
+};
+
+#if BUILDING_GCC_VERSION >= 4009
+class example_plugin_pass : public gimple_opt_pass {
+public:
+	example_plugin_pass() : gimple_opt_pass(example_plugin_pass_data, g) {}
+#if BUILDING_GCC_VERSION >= 5000
+	virtual unsigned int execute(function *) { return handle_function(); }
+#else
+	unsigned int execute() { return handle_function(); }
+#endif
+};
+}
+
+static opt_pass *make_example_plugin_pass(void)
+{
+	return new example_plugin_pass();
+}
+#else
+static struct opt_pass *make_example_plugin_pass(void)
+{
+	return &example_plugin_pass.pass;
+}
+#endif
+
+int plugin_init(struct plugin_name_args *plugin_info, struct plugin_gcc_version *version)
+{
+	const char * const plugin_name = plugin_info->base_name;
+	struct register_pass_info example_plugin_pass_info;
+
+	example_plugin_pass_info.pass				= make_example_plugin_pass();
+	example_plugin_pass_info.reference_pass_name		= "ssa";
+	example_plugin_pass_info.ref_pass_instance_number	= 1;
+	example_plugin_pass_info.pos_op				= PASS_POS_INSERT_AFTER;
+
+	if (!plugin_default_version_check(version, &gcc_version)) {
+		error(G_("incompatible gcc/plugin versions"));
+		return 1;
+	}
+
+	register_callback(plugin_name, PLUGIN_INFO, NULL, &example_plugin_info);
+	register_callback(plugin_name, PLUGIN_PASS_MANAGER_SETUP, NULL, &example_plugin_pass_info);
+
+	return 0;
+}
diff --git a/Documentation/gcc-plugins.txt b/Documentation/gcc-plugins.txt
new file mode 100644
index 0000000..e669d83
--- /dev/null
+++ b/Documentation/gcc-plugins.txt
@@ -0,0 +1,76 @@
+GCC plugin infrastructure
+=========================
+
+
+1. 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 all gcc versions from
+4.5 to 6.0, building out-of-tree modules, cross-compilation and building in a
+separate directory.
+
+Currently the GCC plugin infrastructure supports only the x86 architecture.
+
+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/
+
+
+2. Files
+========
+
+$(src)/tools/gcc
+	This is the directory of the GCC plugins.
+
+$(src)/tools/gcc/gcc-common.h
+	This is a compatibility header for GCC plugins.
+	It should be always included instead of individual gcc headers.
+
+$(src)/scripts/gcc-plugin.sh
+	This script checks the availability of the included headers in
+	gcc-common.h and chooses the proper host compiler to build the plugins
+	(gcc-4.7 can be built by either gcc or g++).
+
+
+3. Usage
+========
+
+Enable a GCC plugin based feature in the kernel config:
+
+	CONFIG_GCC_PLUGIN_CYC_COMPLEXITY = y
+
+To compile only the plugin(s):
+
+	make gcc-plugins
+
+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 $(src)/tools/gcc/. You can use a file or a directory
+here. It must be added to $(src)/tools/gcc/Makefile, $(src)/Makefile and
+$(src)/arch/Kconfig.
+See the cyc_complexity_plugin.c (CONFIG_GCC_PLUGIN_CYC_COMPLEXITY) GCC plugin.
+
+
+5. Example GCC plugin
+=====================
+
+You can find an example plugin under $(src)/Documentation/example_gcc_plugin.c .
+This plugin has a GIMPLE pass that is inserted after the ssa GCC pass.
+It prints out all the GIMPLE statements in a translation unit.