From patchwork Sat Jan 28 07:49:18 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: David Gow X-Patchwork-Id: 648362 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id DE54EC38142 for ; Sat, 28 Jan 2023 07:49:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232813AbjA1Hth (ORCPT ); Sat, 28 Jan 2023 02:49:37 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42112 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232898AbjA1Htc (ORCPT ); Sat, 28 Jan 2023 02:49:32 -0500 Received: from mail-yw1-x1149.google.com (mail-yw1-x1149.google.com [IPv6:2607:f8b0:4864:20::1149]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CAD85783D1 for ; Fri, 27 Jan 2023 23:49:30 -0800 (PST) Received: by mail-yw1-x1149.google.com with SMTP id 00721157ae682-5005ef73cf3so78979697b3.2 for ; Fri, 27 Jan 2023 23:49:30 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=771NPyRF1PEt8cs2hCV18puTF3LGONEcEJnkC6VrUwM=; b=Xe48EWeSfcJugp9Fqys1a4HBtDNLooqtWsIE1RteRtRyRQICJL3sy+3N7Zd4gnyZs1 e/EUgOTUYLEp5BkjcI6i9SUMhSMXOhT8oLZgu+sqtx0UIByQw8jS3a122xO78VhL3N/e EChvjI0sUTHFErwRPLCUjeHBs7JQjdZCe9aVq/3fNeIjhCXqvmh74lSeFZnSMSZBNB7V fgHzKjRCLZWuMPoIgWErpolwxxvTD88wGxF2SPq7bhhtDSUEf6MlOnNzDMcBKXGCOwNB oDWZ1t3n20GM/SHg9RgTa9rb6ssbWS/XZGpsaVVxQwGp9WCPcIRfktO5olwH3gviBknV M8YA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=771NPyRF1PEt8cs2hCV18puTF3LGONEcEJnkC6VrUwM=; b=ecuj70X6/tJ02BIH/D9ZonJ9ZJ81vE+9cSuhDPt1reydNcQZHEWCtw9mg2wVetjiO7 4sCJtGC2n+GBee35iVECjyOBb48QIGs9mgqqMZieVd/QUxo10IJ68EgyG9vyIdvnev3z HGAKksQkQMxtKTUjoHxsdgWSrVik75Myu+sZCKsL2lilsVRTmDQtyYc1GzqjMP0Fio3Z eeLsjnv3gwyDRbtuz4L4br5EV5upHHDygmxyQle7v/almm/5fVGjTE0Y497DM/L9BfVg BrBJqJvAGh5LWn9+AvSM0K/d27UKBaLDTACnRk4v2kvoi6qA+6xJehSRVE2zxJOcEoXu /DQg== X-Gm-Message-State: AFqh2kpfBQeKmkQfovoklliqO9gNjmnqtQxATr22qmmOwVCJdyE2aiJD Y/nGbNQcuqJOmbY2vCTG55uJxlm727HHbQ== X-Google-Smtp-Source: AMrXdXuMV3wsThXD7wbG0TCsvlinfj8R6omUbC0OplNqe0/ZHUXG3RUNA/0fvUzpGeyBv6A3b5h5vFP1jYFGpg== X-Received: from slicestar.c.googlers.com ([fda3:e722:ac3:cc00:4f:4b78:c0a8:20a1]) (user=davidgow job=sendgmr) by 2002:a81:79d0:0:b0:4fe:276a:baf4 with SMTP id u199-20020a8179d0000000b004fe276abaf4mr4068084ywc.401.1674892170006; Fri, 27 Jan 2023 23:49:30 -0800 (PST) Date: Sat, 28 Jan 2023 15:49:18 +0800 In-Reply-To: <20230128074918.1180523-1-davidgow@google.com> Mime-Version: 1.0 References: <20230128074918.1180523-1-davidgow@google.com> X-Mailer: git-send-email 2.39.1.456.gfc5497dd1b-goog Message-ID: <20230128074918.1180523-2-davidgow@google.com> Subject: [PATCH v2 2/2] Documentation: Add Function Redirection API docs From: David Gow To: Brendan Higgins , Kees Cook , Shuah Khan , Daniel Latypov , Rae Moar , Sadiya Kazi , Steven Rostedt , Joe Fradley , Steve Muckle , Jonathan Corbet Cc: kunit-dev@googlegroups.com, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, David Gow , Brendan Higgins Precedence: bulk List-ID: X-Mailing-List: linux-kselftest@vger.kernel.org From: Sadiya Kazi Added a new page (functionredirection.rst) that describes the Function Redirection (static stubbing) API. This page will be expanded if we add, for example, ftrace-based stubbing. In addition, 1. Updated the api/index.rst page to create an entry for function redirection api 2. Updated the toctree to be hidden, reducing redundancy on the generated page. Signed-off-by: Sadiya Kazi Co-developed-by: Daniel Latypov Signed-off-by: Daniel Latypov Co-developed-by: David Gow Signed-off-by: David Gow Reviewed-by: Brendan Higgins --- Note that this document reworks some elements of the KUnit website's "mocking" page at http://kunit.dev/mocking.html written by Daniel Latypov, and used with his permission. Changes since v1: https://lore.kernel.org/all/20221208061841.2186447-3-davidgow@google.com/ - Fix a bunch of typos (Thanks, Daniel) - Remove a redundant comment (Thanks, Daniel) - Reword a few things to be clearer, especially about global state. --- .../kunit/api/functionredirection.rst | 162 ++++++++++++++++++ Documentation/dev-tools/kunit/api/index.rst | 13 +- 2 files changed, 172 insertions(+), 3 deletions(-) create mode 100644 Documentation/dev-tools/kunit/api/functionredirection.rst diff --git a/Documentation/dev-tools/kunit/api/functionredirection.rst b/Documentation/dev-tools/kunit/api/functionredirection.rst new file mode 100644 index 000000000000..3791efc2fcca --- /dev/null +++ b/Documentation/dev-tools/kunit/api/functionredirection.rst @@ -0,0 +1,162 @@ +.. SPDX-License-Identifier: GPL-2.0 + +======================== +Function Redirection API +======================== + +Overview +======== + +When writing unit tests, it's important to be able to isolate the code being +tested from other parts of the kernel. This ensures the reliability of the test +(it won't be affected by external factors), reduces dependencies on specific +hardware or config options (making the test easier to run), and protects the +stability of the rest of the system (making it less likely for test-specific +state to interfere with the rest of the system). + +While for some code (typically generic data structures, helpers, and other +"pure functions") this is trivial, for others (like device drivers, +filesystems, core subsystems) the code is heavily coupled with other parts of +the kernel. + +This coupling is often due to global state in some way: be it a global list of +devices, the filesystem, or some hardware state. Tests need to either carefully +manage, isolate, and restore state, or they can avoid it altogether by +replacing access to and mutation of this state with a "fake" or "mock" variant. + +By refactoring access to such state, such as by introducing a layer of +indirection which can use or emulate a separate set of test state. However, +such refactoring comes with its own costs (and undertaking significant +refactoring before being able to write tests is suboptimal). + +A simpler way to intercept and replace some of the function calls is to use +function redirection via static stubs. + + +Static Stubs +============ + +Static stubs are a way of redirecting calls to one function (the "real" +function) to another function (the "replacement" function). + +It works by adding a macro to the "real" function which checks to see if a test +is running, and if a replacement function is available. If so, that function is +called in place of the original. + +Using static stubs is pretty straightforward: + +1. Add the KUNIT_STATIC_STUB_REDIRECT() macro to the start of the "real" + function. + + This should be the first statement in the function, after any variable + declarations. KUNIT_STATIC_STUB_REDIRECT() takes the name of the + function, followed by all of the arguments passed to the real function. + + For example: + + .. code-block:: c + + void send_data_to_hardware(const char *str) + { + KUNIT_STATIC_STUB_REDIRECT(send_data_to_hardware, str); + /* real implementation */ + } + +2. Write one or more replacement functions. + + These functions should have the same function signature as the real function. + In the event they need to access or modify test-specific state, they can use + kunit_get_current_test() to get a struct kunit pointer. This can then + be passed to the expectation/assertion macros, or used to look up KUnit + resources. + + For example: + + .. code-block:: c + + void fake_send_data_to_hardware(const char *str) + { + struct kunit *test = kunit_get_current_test(); + KUNIT_EXPECT_STREQ(test, str, "Hello World!"); + } + +3. Activate the static stub from your test. + + From within a test, the redirection can be enabled with + kunit_activate_static_stub(), which accepts a struct kunit pointer, + the real function, and the replacement function. You can call this several + times with different replacement functions to swap out implementations of the + function. + + In our example, this would be + + .. code-block:: c + + kunit_activate_static_stub(test, + send_data_to_hardware, + fake_send_data_to_hardware); + +4. Call (perhaps indirectly) the real function. + + Once the redirection is activated, any call to the real function will call + the replacement function instead. Such calls may be buried deep in the + implementation of another function, but must occur from the test's kthread. + + For example: + + .. code-block:: c + + send_data_to_hardware("Hello World!"); /* Succeeds */ + send_data_to_hardware("Something else"); /* Fails the test. */ + +5. (Optionally) disable the stub. + + When you no longer need it, disable the redirection (and hence resume the + original behaviour of the 'real' function) using + kunit_deactivate_static_stub(). Otherwise, it will be automatically disabled + when the test exits. + + For example: + + .. code-block:: c + + kunit_deactivate_static_stub(test, send_data_to_hardware); + + +It's also possible to use these replacement functions to test to see if a +function is called at all, for example: + +.. code-block:: c + + void send_data_to_hardware(const char *str) + { + KUNIT_STATIC_STUB_REDIRECT(send_data_to_hardware, str); + /* real implementation */ + } + + /* In test file */ + int times_called = 0; + void fake_send_data_to_hardware(const char *str) + { + times_called++; + } + ... + /* In the test case, redirect calls for the duration of the test */ + kunit_activate_static_stub(test, send_data_to_hardware, fake_send_data_to_hardware); + + send_data_to_hardware("hello"); + KUNIT_EXPECT_EQ(test, times_called, 1); + + /* Can also deactivate the stub early, if wanted */ + kunit_deactivate_static_stub(test, send_data_to_hardware); + + send_data_to_hardware("hello again"); + KUNIT_EXPECT_EQ(test, times_called, 1); + + + +API Reference +============= + +.. kernel-doc:: include/kunit/static_stub.h + :internal: diff --git a/Documentation/dev-tools/kunit/api/index.rst b/Documentation/dev-tools/kunit/api/index.rst index 45ce04823f9f..2d8f756aab56 100644 --- a/Documentation/dev-tools/kunit/api/index.rst +++ b/Documentation/dev-tools/kunit/api/index.rst @@ -4,17 +4,24 @@ API Reference ============= .. toctree:: + :hidden: test resource + functionredirection -This section documents the KUnit kernel testing API. It is divided into the + +This page documents the KUnit kernel testing API. It is divided into the following sections: Documentation/dev-tools/kunit/api/test.rst - - documents all of the standard testing API + - Documents all of the standard testing API Documentation/dev-tools/kunit/api/resource.rst - - documents the KUnit resource API + - Documents the KUnit resource API + +Documentation/dev-tools/kunit/api/functionredirection.rst + + - Documents the KUnit Function Redirection API