kselftest: Add test module framework header

kselftest runs as a userspace process. Sometimes we need to test things
from kernel space. One way of doing this is by creating a test module.
Currently doing so requires developers to write a bunch of boiler plate
in the module if kselftest is to be used to run the tests. This means
we currently have a load of duplicate code to achieve these ends. If we
have a uniform method for implementing test modules then we can reduce
code duplication, ensure uniformity in the test framework, ease code
maintenance, and reduce the work required to create tests. This all
helps to encourage developers to write and run tests.

Add a C header file that can be included in test modules. This provides
a single point for common test functions/macros. Implement a few macros
that make up the start of the test framework.

Add documentation for new kselftest header to kselftest documentation.

Acked-by: Kees Cook <[e-mail]>
Signed-off-by: Tobin C. Harding <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

Documentation/dev-tools/kselftest.rst

···
 run on a single cpu as opposed to all hotplug capable cpus, and memory
 hotplug test is run on 2% of hotplug capable memory instead of 10%.
 
+kselftest runs as a userspace process.  Tests that can be written/run in
+userspace may wish to use the `Test Harness`_.  Tests that need to be
+run in kernel space may wish to use a `Test Module`_.
+
 Running the selftests (hotplug tests are run in limited mode)
 =============================================================
 
···
 
    e.g: tools/testing/selftests/android/config
 
+Test Module
+===========
+
+Kselftest tests the kernel from userspace.  Sometimes things need
+testing from within the kernel, one method of doing this is to create a
+test module.  We can tie the module into the kselftest framework by
+using a shell script test runner.  ``kselftest_module.sh`` is designed
+to facilitate this process.  There is also a header file provided to
+assist writing kernel modules that are for use with kselftest:
+
+- ``tools/testing/kselftest/kselftest_module.h``
+- ``tools/testing/kselftest/kselftest_module.sh``
+
+How to use
+----------
+
+Here we show the typical steps to create a test module and tie it into
+kselftest.  We use kselftests for lib/ as an example.
+
+1. Create the test module
+
+2. Create the test script that will run (load/unload) the module
+   e.g. ``tools/testing/selftests/lib/printf.sh``
+
+3. Add line to config file e.g. ``tools/testing/selftests/lib/config``
+
+4. Add test script to makefile  e.g. ``tools/testing/selftests/lib/Makefile``
+
+5. Verify it works:
+
+.. code-block:: sh
+
+   # Assumes you have booted a fresh build of this kernel tree
+   cd /path/to/linux/tree
+   make kselftest-merge
+   make modules
+   sudo make modules_install
+   make TARGETS=lib kselftest
+
+Example Module
+--------------
+
+A bare bones test module might look like this:
+
+.. code-block:: c
+
+   // SPDX-License-Identifier: GPL-2.0+
+
+   #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+   #include "../tools/testing/selftests/kselftest_module.h"
+
+   KSTM_MODULE_GLOBALS();
+
+   /*
+    * Kernel module for testing the foobinator
+    */
+
+   static int __init test_function()
+   {
+           ...
+   }
+
+   static void __init selftest(void)
+   {
+           KSTM_CHECK_ZERO(do_test_case("", 0));
+   }
+
+   KSTM_MODULE_LOADERS(test_foo);
+   MODULE_AUTHOR("John Developer <[e-mail]>");
+   MODULE_LICENSE("GPL");
+
+Example test script
+-------------------
+
+.. code-block:: sh
+
+    #!/bin/bash
+    # SPDX-License-Identifier: GPL-2.0+
+    $(dirname $0)/../kselftest_module.sh "foo" test_foo
+
+
 Test Harness
 ============
 
-The kselftest_harness.h file contains useful helpers to build tests.  The tests
-from tools/testing/selftests/seccomp/seccomp_bpf.c can be used as example.
+The kselftest_harness.h file contains useful helpers to build tests.  The
+test harness is for userspace testing, for kernel space testing see `Test
+Module`_ above.
+
+The tests from tools/testing/selftests/seccomp/seccomp_bpf.c can be used as
+example.
 
 Example
 -------

tools/testing/selftests/kselftest_module.h

···
+/* SPDX-License-Identifier: GPL-2.0+ */
+#ifndef __KSELFTEST_MODULE_H
+#define __KSELFTEST_MODULE_H
+
+#include <linux/module.h>
+
+/*
+ * Test framework for writing test modules to be loaded by kselftest.
+ * See Documentation/dev-tools/kselftest.rst for an example test module.
+ */
+
+#define KSTM_MODULE_GLOBALS()			\
+static unsigned int total_tests __initdata;	\
+static unsigned int failed_tests __initdata
+
+#define KSTM_CHECK_ZERO(x) do {						\
+	total_tests++;							\
+	if (x) {							\
+		pr_warn("TC failed at %s:%d\n", __func__, __LINE__);	\
+		failed_tests++;						\
+	}								\
+} while (0)
+
+static inline int kstm_report(unsigned int total_tests, unsigned int failed_tests)
+{
+	if (failed_tests == 0)
+		pr_info("all %u tests passed\n", total_tests);
+	else
+		pr_warn("failed %u out of %u tests\n", failed_tests, total_tests);
+
+	return failed_tests ? -EINVAL : 0;
+}
+
+#define KSTM_MODULE_LOADERS(__module)			\
+static int __init __module##_init(void)			\
+{							\
+	pr_info("loaded.\n");				\
+	selftest();					\
+	return kstm_report(total_tests, failed_tests);	\
+}							\
+static void __exit __module##_exit(void)		\
+{							\
+	pr_info("unloaded.\n");				\
+}							\
+module_init(__module##_init);				\
+module_exit(__module##_exit)
+
+#endif	/* __KSELFTEST_MODULE_H */