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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:
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 headers
$ 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
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
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="breakpoints size timers" SKIP_TARGETS=size 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 ``
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 `` 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
$ ./
To see the list of available tests, the `-l` option can be used::
$ ./ -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::
$ ./ -c size -c seccomp -t timers:posix_timers -t timer:nanosleep
For other features see the script usage output, seen with the `-h` option.
Timeout for selftests
Selftests are designed to be quick and so a default timeout is used of 45
seconds for each test. Tests can override the default timeout by adding
a settings file in their directory and set a timeout variable there to the
configured a desired upper timeout for the test. Only a few tests override
the timeout with a value higher than 45 seconds, selftests strives to keep
it that way. Timeouts in selftests are not considered fatal because the
system under which a test runs may change and this can also modify the
expected time it takes to run a test. If you have control over the systems
which will run the tests you can configure a test runner on those systems to
use a greater or lower timeout on the command line as with the `-o` or
the `--override-timeout` argument. For example to use 165 seconds instead
one would use::
$ ./ --override-timeout 165
You can look at the TAP output to see if you ran into the timeout. Test
runners which know a test must run under a specific time can then optionally
treat these timeouts then as fatal.
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`_
$ make -C tools/testing/selftests gen_tar TARGETS="size" FORMAT=.xz
.. _tar's auto-compress:
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
Contributing new tests (details)
* In your Makefile, use facilities from by including it instead of
reinventing the wheel. Specify flags and binaries generation flags on
need basis before including ::
TEST_GEN_PROGS := close_range_test
include ../
* Use TEST_GEN_XXX if such binaries or files are generated during
TEST_PROGS, TEST_GEN_PROGS mean it is the executable tested by
TEST_GEN_MODS_DIR should be used by tests that require modules to be built
before the test starts. The variable will contain the name of the directory
containing the modules.
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, run_tests will generate a warning.
TEST_CUSTOM_PROGS and TEST_PROGS will be run by common run_tests.
executable which is not tested by default.
TEST_FILES, TEST_GEN_FILES mean it is the file which is used by
TEST_INCLUDES is similar to TEST_FILES, it lists files which should be
included when exporting or installing the tests, with the following
* symlinks to files in other directories are preserved
* the part of paths below tools/testing/selftests/ is preserved when
copying the files to the output directory
TEST_INCLUDES is meant to list dependencies located in other directories of
the selftests hierarchy.
* 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} 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/`` 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/``
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.
1. Create the test module
2. Create the test script that will run (load/unload) the module
e.g. ``tools/testing/selftests/lib/``
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"
* Kernel module for testing the foobinator
static int __init test_function()
static void __init selftest(void)
KSTM_CHECK_ZERO(do_test_case("", 0));
MODULE_AUTHOR("John Developer <>");
MODULE_INFO(test, "Y");
Example test script
.. code-block:: sh
# SPDX-License-Identifier: GPL-2.0+
$(dirname $0)/../kselftest/ "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
.. kernel-doc:: tools/testing/selftests/kselftest_harness.h
:doc: example
.. kernel-doc:: tools/testing/selftests/kselftest_harness.h
.. kernel-doc:: tools/testing/selftests/kselftest_harness.h
:doc: operators
.. kernel-doc:: tools/testing/selftests/kselftest_harness.h