kunit: test-bug.h: include `stddef.h` for `NULL`

The header uses `NULL` in both `CONFIG_KUNIT=y` and `=n` cases,
but does not include it explicitly.

When `CONFIG_KUNIT=y`, the header is already getting included via
the other headers, so it is not a problem for users.

However, when `CONFIG_KUNIT=n`, it is not, and thus a user could hit
a build error when including `kunit/test-bug.h`, like we are doing
later in this series [1].

Thus include `linux/stddef.h`, and do so outside the `#if`, since it
is used in both cases.

Reported-by: Boqun Feng <boqun.feng@gmail.com>
Closes: https://lore.kernel.org/rust-for-linux/ZJ8cNUW3oR2p+gL1@boqun-archlinux/ [1]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

kunit: Expose 'static stub' API to redirect functions

Add a simple way of redirecting calls to functions by including a
special prologue in the "real" function which checks to see if the
replacement function should be called (and, if so, calls it).

To redirect calls to a function, make the first (non-declaration) line
of the function:

KUNIT_STATIC_STUB_REDIRECT(function_name, [function arguments]);

(This will compile away to nothing if KUnit is not enabled, otherwise it
will check if a redirection is active, call the replacement function,
and return. This check is protected by a static branch, so has very
little overhead when there are no KUnit tests running.)

Calls to the real function can be redirected to a replacement using:

kunit_activate_static_stub(test, real_fn, replacement_fn);

The redirection will only affect calls made from within the kthread of
the current test, and will be automatically disabled when the test
completes. It can also be manually disabled with
kunit_deactivate_static_stub().

The 'example' KUnit test suite has a more complete example.

Co-developed-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

kunit: Add "hooks" to call into KUnit when it's built as a module

KUnit has several macros and functions intended for use from non-test
code. These hooks, currently the kunit_get_current_test() and
kunit_fail_current_test() macros, didn't work when CONFIG_KUNIT=m.

In order to support this case, the required functions and static data
need to be available unconditionally, even when KUnit itself is not
built-in. The new 'hooks.c' file is therefore always included, and has
both the static key required for kunit_get_current_test(), and a table
of function pointers in struct kunit_hooks_table. This is filled in with
the real implementations by kunit_install_hooks(), which is kept in
hooks-impl.h and called when the kunit module is loaded.

This can be extended for future features which require similar
"hook" behaviour, such as static stubs, by simply adding new entries to
the struct, and the appropriate code to set them.

Fixed white-space errors during commit:
Shuah Khan <skhan@linuxfoundation.org>

Resolved merge conflicts with:
db105c37a4d6 ("kunit: Export kunit_running()")
This patch supersedes the above.
Shuah Khan <skhan@linuxfoundation.org>

Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Rae Moar <rmoar@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

kunit: Use the static key when retrieving the current test

In order to detect if a KUnit test is running, and to access its
context, the 'kunit_test' member of the current task_struct is used.
Usually, this is accessed directly or via the kunit_fail_current_task()
function.

In order to speed up the case where no test is running, add a wrapper,
kunit_get_current_test(), which uses the static key to fail early.
Equally, Speed up kunit_fail_current_test() by using the static key.

This should make it convenient for code to call this
unconditionally in fakes or error paths, without worrying that this will
slow the code down significantly.

If CONFIG_KUNIT=n (or m), this compiles away to nothing. If
CONFIG_KUNIT=y, it will compile down to a NOP (on most architectures) if
no KUnit test is currently running.

Note that kunit_get_current_test() does not work if KUnit is built as a
module. This mirrors the existing restriction on kunit_fail_current_test().

Note that the definition of kunit_fail_current_test() still wraps an
empty, inline function if KUnit is not built-in. This is to ensure that
the printf format string __attribute__ will still work.

Also update the documentation to suggest users use the new
kunit_get_current_test() function, update the example, and to describe
the behaviour when KUnit is disabled better.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Sadiya Kazi <sadiyakazi@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

kunit: fix -Wunused-function warning for __kunit_fail_current_test

When CONFIG_KUNIT is not enabled, __kunit_fail_current_test() an empty
static function.

But GCC complains about unused static functions, *unless* they're static
inline. So add inline to make GCC happy.

Fixes: 359a376081d4 ("kunit: support failure from dynamic analysis tools")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

kunit: support failure from dynamic analysis tools

Add a kunit_fail_current_test() function to fail the currently running
test, if any, with an error message.

This is largely intended for dynamic analysis tools like UBSAN and for
fakes.
E.g. say I had a fake ops struct for testing and I wanted my `free`
function to complain if it was called with an invalid argument, or
caught a double-free. Most return void and have no normal means of
signalling failure (e.g. super_operations, iommu_ops, etc.).

Key points:
* Always update current->kunit_test so anyone can use it.
* commit 83c4e7a0363b ("KUnit: KASAN Integration") only updated it for
CONFIG_KASAN=y

* Create a new header <kunit/test-bug.h> so non-test code doesn't have
to include all of <kunit/test.h> (e.g. lib/ubsan.c)

* Forward the file and line number to make it easier to track down
failures

* Declare the helper function for nice __printf() warnings about mismatched
format strings even when KUnit is not enabled.

Example output from kunit_fail_current_test("message"):
[15:19:34] [FAILED] example_simple_test
[15:19:34] # example_simple_test: initializing
[15:19:34] # example_simple_test: lib/kunit/kunit-example-test.c:24: message
[15:19:34] not ok 1 - example_simple_test

Fixed minor check patch with checkpatch --fix option:
Shuah Khan <skhan@linuxfoundation.org>

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Signed-off-by: Uriel Guajardo <urielguajardo@google.com>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>