Linux Kernel Selftests¶

The kernel contains a set of “self tests” under the tools/testing/selftests/ directory. These are intended to be small tests to exercise individual code paths in the kernel. Tests are intended to be run after building, installing and booting a kernel.

Kselftest from mainline can be run on older stable kernels. Running tests from mainline offers the best coverage. Several test rings run mainline kselftest suite on stable releases. The reason is that when a new test gets added to test existing code to regression test a bug, we should be able to run that test on an older kernel. Hence, it is important to keep code that can still test an older kernel and make sure it skips the test gracefully on newer releases.

You can find additional information on Kselftest framework, how to write new tests using the framework on Kselftest wiki:

https://kselftest.wiki.kernel.org/

On some systems, hot-plug tests could hang forever waiting for cpu and memory to be ready to be offlined. A special hot-plug target is created to run the full range of hot-plug tests. In default mode, hot-plug tests run in safe mode with a limited scope. In limited mode, cpu-hotplug test is 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)¶

To build the tests:

$ make -C tools/testing/selftests

To run the tests:

$ make -C tools/testing/selftests run_tests

To build and run the tests with a single command, use:

$ make kselftest

Note that some tests will require root privileges.

Kselftest supports saving output files in a separate directory and then running tests. To locate output files in a separate directory two syntaxes are supported. In both cases the working directory must be the root of the kernel src. This is applicable to “Running a subset of selftests” section below.

To build, save output files in a separate directory with O=

$ make O=/tmp/kselftest kselftest

To build, save output files in a separate directory with KBUILD_OUTPUT

$ export KBUILD_OUTPUT=/tmp/kselftest; make kselftest

The O= assignment takes precedence over the KBUILD_OUTPUT environment variable.

The above commands by default run the tests and print full pass/fail report. Kselftest supports “summary” option to make it easier to understand the test results. Please find the detailed individual test results for each test in /tmp/testname file(s) when summary option is specified. This is applicable to “Running a subset of selftests” section below.

To run kselftest with summary option enabled

$ make summary=1 kselftest

Running a subset of selftests¶

You can use the “TARGETS” variable on the make command line to specify single test to run, or a list of tests to run.

To run only tests targeted for a single subsystem:

$ make -C tools/testing/selftests TARGETS=ptrace run_tests

You can specify multiple tests to build and run:

$  make TARGETS="size timers" kselftest

To build, save output files in a separate directory with O=

$ make O=/tmp/kselftest TARGETS="size timers" kselftest

To build, save output files in a separate directory with KBUILD_OUTPUT

$ export KBUILD_OUTPUT=/tmp/kselftest; make TARGETS="size timers" kselftest

Additionally you can use the “SKIP_TARGETS” variable on the make command line to specify one or more targets to exclude from the TARGETS list.

To run all tests but a single subsystem:

$ make -C tools/testing/selftests SKIP_TARGETS=ptrace run_tests

You can specify multiple tests to skip:

$  make SKIP_TARGETS="size timers" kselftest

You can also specify a restricted list of tests to run together with a dedicated skiplist:

$  make TARGETS="bpf breakpoints size timers" SKIP_TARGETS=bpf kselftest

See the top-level tools/testing/selftests/Makefile for the list of all possible targets.

Running the full range hotplug selftests¶

To build the hotplug tests:

$ make -C tools/testing/selftests hotplug

To run the hotplug tests:

$ make -C tools/testing/selftests run_hotplug

Note that some tests will require root privileges.

Install selftests¶

You can use the “install” target of “make” (which calls the kselftest_install.sh tool) to install selftests in the default location (tools/testing/selftests/kselftest_install), or in a user specified location via the INSTALL_PATH “make” variable.

To install selftests in default location:

$ make -C tools/testing/selftests install

To install selftests in a user specified location:

$ make -C tools/testing/selftests install INSTALL_PATH=/some/other/path

Running installed selftests¶

Found in the install directory, as well as in the Kselftest tarball, is a script named run_kselftest.sh to run the tests.

You can simply do the following to run the installed Kselftests. Please note some tests will require root privileges:

$ cd kselftest_install
$ ./run_kselftest.sh

To see the list of available tests, the -l option can be used:

$ ./run_kselftest.sh -l

The -c option can be used to run all the tests from a test collection, or the -t option for specific single tests. Either can be used multiple times:

$ ./run_kselftest.sh -c bpf -c seccomp -t timers:posix_timers -t timer:nanosleep

For other features see the script usage output, seen with the -h option.

Packaging selftests¶

In some cases packaging is desired, such as when tests need to run on a different system. To package selftests, run:

$ make -C tools/testing/selftests gen_tar

This generates a tarball in the INSTALL_PATH/kselftest-packages directory. By default, .gz format is used. The tar compression format can be overridden by specifying a FORMAT make variable. Any value recognized by tar’s auto-compress option is supported, such as:

$ make -C tools/testing/selftests gen_tar FORMAT=.xz

make gen_tar invokes make install so you can use it to package a subset of tests by using variables specified in Running a subset of selftests section:

$ make -C tools/testing/selftests gen_tar TARGETS="bpf" FORMAT=.xz

Contributing new tests¶

In general, the rules for selftests are

Do as much as you can if you’re not root;

Don’t take too long;

Don’t break the build on any architecture, and

Don’t cause the top-level “make run_tests” to fail if your feature is unconfigured.

Contributing new tests (details)¶

In your Makefile, use facilities from lib.mk by including it instead of reinventing the wheel. Specify flags and binaries generation flags on need basis before including lib.mk.
CFLAGS = $(KHDR_INCLUDES)
TEST_GEN_PROGS := close_range_test
include ../lib.mk
Use TEST_GEN_XXX if such binaries or files are generated during compiling.

TEST_PROGS, TEST_GEN_PROGS mean it is the executable tested by default.

TEST_CUSTOM_PROGS should be used by tests that require custom build rules and prevent common build rule use.

TEST_PROGS are for test shell scripts. Please ensure shell script has its exec bit set. Otherwise, lib.mk run_tests will generate a warning.

TEST_CUSTOM_PROGS and TEST_PROGS will be run by common run_tests.

TEST_PROGS_EXTENDED, TEST_GEN_PROGS_EXTENDED mean it is the executable which is not tested by default. TEST_FILES, TEST_GEN_FILES mean it is the file which is used by test.

First use the headers inside the kernel source and/or git repo, and then the system headers. Headers for the kernel release as opposed to headers installed by the distro on the system should be the primary focus to be able to find regressions. Use KHDR_INCLUDES in Makefile to include headers from the kernel source.

If a test needs specific kernel config options enabled, add a config file in the test directory to enable them.

e.g: tools/testing/selftests/android/config

Create a .gitignore file inside test directory and add all generated objects in it.
Add new test name in TARGETS in selftests/Makefile:
TARGETS += android
All changes should pass:
kselftest-{all,install,clean,gen_tar}
kselftest-{all,install,clean,gen_tar} O=abo_path
kselftest-{all,install,clean,gen_tar} O=rel_path
make -C tools/testing/selftests {all,install,clean,gen_tar}
make -C tools/testing/selftests {all,install,clean,gen_tar} O=abs_path
make -C tools/testing/selftests {all,install,clean,gen_tar} O=rel_path

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/selftests/kselftest_module.h
tools/testing/selftests/kselftest/module.sh

Note that test modules should taint the kernel with TAINT_TEST. This will happen automatically for modules which are in the tools/testing/ directory, or for modules which use the kselftest_module.h header above. Otherwise, you’ll need to add MODULE_INFO(test, "Y") to your module source. selftests which do not load modules typically should not taint the kernel, but in cases where a non-test module is loaded, TEST_TAINT can be applied from userspace by writing to /proc/sys/kernel/tainted.

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.

Create the test module
Create the test script that will run (load/unload) the module e.g. tools/testing/selftests/lib/printf.sh
Add line to config file e.g. tools/testing/selftests/lib/config
Add test script to makefile e.g. tools/testing/selftests/lib/Makefile
Verify it works:

# 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:

// 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 <jd@fooman.org>");
MODULE_LICENSE("GPL");
MODULE_INFO(test, "Y");

Example test script¶

#!/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 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¶

#include "../kselftest_harness.h"

TEST(standalone_test) {
  do_some_stuff;
  EXPECT_GT(10, stuff) {
     stuff_state_t state;
     enumerate_stuff_state(&state);
     TH_LOG("expectation failed with state: %s", state.msg);
  }
  more_stuff;
  ASSERT_NE(some_stuff, NULL) TH_LOG("how did it happen?!");
  last_stuff;
  EXPECT_EQ(0, last_stuff);
}

FIXTURE(my_fixture) {
  mytype_t *data;
  int awesomeness_level;
};
FIXTURE_SETUP(my_fixture) {
  self->data = mytype_new();
  ASSERT_NE(NULL, self->data);
}
FIXTURE_TEARDOWN(my_fixture) {
  mytype_free(self->data);
}
TEST_F(my_fixture, data_is_good) {
  EXPECT_EQ(1, is_my_data_good(self->data));
}

TEST_HARNESS_MAIN

Helpers¶

TH_LOG¶

TH_LOG (fmt, ...)

Parameters

fmt: format string
...: optional arguments

Description

TH_LOG(format, ...)

Optional debug logging function available for use in tests. Logging may be enabled or disabled by defining TH_LOG_ENABLED. E.g., #define TH_LOG_ENABLED 1

If no definition is provided, logging is enabled by default.

If there is no way to print an error message for the process running the test (e.g. not allowed to write to stderr), it is still possible to get the ASSERT_* number for which the test failed. This behavior can be enabled by writing _metadata->no_print = true; before the check sequence that is unable to print. When an error occur, instead of printing an error message and calling abort(3), the test process call _exit(2) with the assert number as argument, which is then printed by the parent process.

TEST¶

TEST (test_name)

Defines the test function and creates the registration stub

Parameters

test_name: test name

Description

TEST(name) { implementation }

Defines a test by name. Names must be unique and tests must not be run in parallel. The implementation containing block is a function and scoping should be treated as such. Returning early may be performed with a bare “return;” statement.

EXPECT_* and ASSERT_* are valid in a TEST() { } context.

TEST_SIGNAL¶

TEST_SIGNAL (test_name, signal)

Parameters

test_name: test name
signal: signal number

Description

TEST_SIGNAL(name, signal) { implementation }

Defines a test by name and the expected term signal. Names must be unique and tests must not be run in parallel. The implementation containing block is a function and scoping should be treated as such. Returning early may be performed with a bare “return;” statement.

EXPECT_* and ASSERT_* are valid in a TEST() { } context.

FIXTURE_DATA¶

FIXTURE_DATA (datatype_name)

Wraps the struct name so we have one less argument to pass around

Parameters

datatype_name: datatype name

Description

FIXTURE_DATA(datatype_name)

Almost always, you want just FIXTURE() instead (see below). This call may be used when the type of the fixture data is needed. In general, this should not be needed unless the self is being passed to a helper directly.

FIXTURE¶

FIXTURE (fixture_name)

Called once per fixture to setup the data and register

Parameters

fixture_name: fixture name

Description

FIXTURE(fixture_name) {
  type property1;
  ...
};

Defines the data provided to TEST_F()-defined tests as self. It should be populated and cleaned up using FIXTURE_SETUP() and FIXTURE_TEARDOWN().

FIXTURE_SETUP¶

FIXTURE_SETUP (fixture_name)

Prepares the setup function for the fixture. _metadata is included so that EXPECT_* and ASSERT_* work correctly.

Parameters

fixture_name: fixture name

Description

FIXTURE_SETUP(fixture_name) { implementation }

Populates the required “setup” function for a fixture. An instance of the datatype defined with FIXTURE_DATA() will be exposed as self for the implementation.

ASSERT_* are valid for use in this context and will prempt the execution of any dependent fixture tests.

A bare “return;” statement may be used to return early.

FIXTURE_TEARDOWN¶

FIXTURE_TEARDOWN (fixture_name)

Parameters

fixture_name: fixture name

Description

_metadata is included so that EXPECT_* and ASSERT_* work correctly.

FIXTURE_TEARDOWN(fixture_name) { implementation }

Populates the required “teardown” function for a fixture. An instance of the datatype defined with FIXTURE_DATA() will be exposed as self for the implementation to clean up.

A bare “return;” statement may be used to return early.

FIXTURE_VARIANT¶

FIXTURE_VARIANT (fixture_name)

Optionally called once per fixture to declare fixture variant

Parameters

fixture_name: fixture name

Description

FIXTURE_VARIANT(fixture_name) {
  type property1;
  ...
};

Defines type of constant parameters provided to FIXTURE_SETUP(), TEST_F() and FIXTURE_TEARDOWN as variant. Variants allow the same tests to be run with different arguments.

FIXTURE_VARIANT_ADD¶

FIXTURE_VARIANT_ADD (fixture_name, variant_name)

Called once per fixture variant to setup and register the data

Parameters

fixture_name: fixture name
variant_name: name of the parameter set

Description

FIXTURE_VARIANT_ADD(fixture_name, variant_name) {
  .property1 = val1,
  ...
};

Defines a variant of the test fixture, provided to FIXTURE_SETUP() and TEST_F() as variant. Tests of each fixture will be run once for each variant.

TEST_F¶

TEST_F (fixture_name, test_name)

Emits test registration and helpers for fixture-based test cases

Parameters

fixture_name: fixture name
test_name: test name

Description

TEST_F(fixture, name) { implementation }

Defines a test that depends on a fixture (e.g., is part of a test case). Very similar to TEST() except that self is the setup instance of fixture’s datatype exposed for use by the implementation.

TEST_HARNESS_MAIN¶

TEST_HARNESS_MAIN ()

Simple wrapper to run the test harness

Parameters

Description

TEST_HARNESS_MAIN

Use once to append a main() to the test file.

Operators¶

Operators for use in TEST() and TEST_F(). ASSERT_* calls will stop test execution immediately. EXPECT_* calls will emit a failure warning, note it, and continue.

ASSERT_EQ¶

ASSERT_EQ (expected, seen)

Parameters

expected: expected value
seen: measured value

Description

ASSERT_EQ(expected, measured): expected == measured

ASSERT_NE¶

ASSERT_NE (expected, seen)

Parameters

expected: expected value
seen: measured value

Description

ASSERT_NE(expected, measured): expected != measured

ASSERT_LT¶

ASSERT_LT (expected, seen)

Parameters

expected: expected value
seen: measured value

Description

ASSERT_LT(expected, measured): expected < measured

ASSERT_LE¶

ASSERT_LE (expected, seen)

Parameters

expected: expected value
seen: measured value

Description

ASSERT_LE(expected, measured): expected <= measured

ASSERT_GT¶

ASSERT_GT (expected, seen)

Parameters

expected: expected value
seen: measured value

Description

ASSERT_GT(expected, measured): expected > measured

ASSERT_GE¶

ASSERT_GE (expected, seen)

Parameters

expected: expected value
seen: measured value

Description

ASSERT_GE(expected, measured): expected >= measured

ASSERT_NULL¶

ASSERT_NULL (seen)

Parameters

seen: measured value

Description

ASSERT_NULL(measured): NULL == measured

ASSERT_TRUE¶

ASSERT_TRUE (seen)

Parameters

seen: measured value

Description

ASSERT_TRUE(measured): measured != 0

ASSERT_FALSE¶

ASSERT_FALSE (seen)

Parameters

seen: measured value

Description

ASSERT_FALSE(measured): measured == 0

ASSERT_STREQ¶

ASSERT_STREQ (expected, seen)

Parameters

expected: expected value
seen: measured value

Description

ASSERT_STREQ(expected, measured): !strcmp(expected, measured)

ASSERT_STRNE¶

ASSERT_STRNE (expected, seen)

Parameters

expected: expected value
seen: measured value

Description

ASSERT_STRNE(expected, measured): strcmp(expected, measured)

EXPECT_EQ¶

EXPECT_EQ (expected, seen)

Parameters

expected: expected value
seen: measured value

Description

EXPECT_EQ(expected, measured): expected == measured

EXPECT_NE¶

EXPECT_NE (expected, seen)

Parameters

expected: expected value
seen: measured value

Description

EXPECT_NE(expected, measured): expected != measured

EXPECT_LT¶

EXPECT_LT (expected, seen)

Parameters

expected: expected value
seen: measured value

Description

EXPECT_LT(expected, measured): expected < measured

EXPECT_LE¶

EXPECT_LE (expected, seen)

Parameters

expected: expected value
seen: measured value

Description

EXPECT_LE(expected, measured): expected <= measured

EXPECT_GT¶

EXPECT_GT (expected, seen)

Parameters

expected: expected value
seen: measured value

Description

EXPECT_GT(expected, measured): expected > measured

EXPECT_GE¶

EXPECT_GE (expected, seen)

Parameters

expected: expected value
seen: measured value

Description

EXPECT_GE(expected, measured): expected >= measured

EXPECT_NULL¶

EXPECT_NULL (seen)

Parameters

seen: measured value

Description

EXPECT_NULL(measured): NULL == measured

EXPECT_TRUE¶

EXPECT_TRUE (seen)

Parameters

seen: measured value

Description

EXPECT_TRUE(measured): 0 != measured

EXPECT_FALSE¶

EXPECT_FALSE (seen)

Parameters

seen: measured value

Description

EXPECT_FALSE(measured): 0 == measured

EXPECT_STREQ¶

EXPECT_STREQ (expected, seen)

Parameters

expected: expected value
seen: measured value

Description

EXPECT_STREQ(expected, measured): !strcmp(expected, measured)

EXPECT_STRNE¶

EXPECT_STRNE (expected, seen)

Parameters

expected: expected value
seen: measured value

Description

EXPECT_STRNE(expected, measured): strcmp(expected, measured)