diff options
Diffstat (limited to 'Documentation/dev-tools')
| -rw-r--r-- | Documentation/dev-tools/kasan.rst | 228 | ||||
| -rw-r--r-- | Documentation/dev-tools/ktap.rst | 18 | ||||
| -rw-r--r-- | Documentation/dev-tools/kunit/api/index.rst | 5 | ||||
| -rw-r--r-- | Documentation/dev-tools/kunit/api/resource.rst | 13 | ||||
| -rw-r--r-- | Documentation/dev-tools/kunit/architecture.rst | 2 | ||||
| -rw-r--r-- | Documentation/dev-tools/kunit/running_tips.rst | 3 | ||||
| -rw-r--r-- | Documentation/dev-tools/kunit/start.rst | 11 | ||||
| -rw-r--r-- | Documentation/dev-tools/kunit/usage.rst | 19 | ||||
| -rw-r--r-- | Documentation/dev-tools/testing-overview.rst | 63 |
9 files changed, 240 insertions, 122 deletions
diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst index 7614a1fc30fa..1772fd457fed 100644 --- a/Documentation/dev-tools/kasan.rst +++ b/Documentation/dev-tools/kasan.rst @@ -4,39 +4,76 @@ The Kernel Address Sanitizer (KASAN) Overview -------- -KernelAddressSANitizer (KASAN) is a dynamic memory safety error detector -designed to find out-of-bound and use-after-free bugs. KASAN has three modes: +Kernel Address Sanitizer (KASAN) is a dynamic memory safety error detector +designed to find out-of-bounds and use-after-free bugs. -1. generic KASAN (similar to userspace ASan), -2. software tag-based KASAN (similar to userspace HWASan), -3. hardware tag-based KASAN (based on hardware memory tagging). +KASAN has three modes: -Generic KASAN is mainly used for debugging due to a large memory overhead. -Software tag-based KASAN can be used for dogfood testing as it has a lower -memory overhead that allows using it with real workloads. Hardware tag-based -KASAN comes with low memory and performance overheads and, therefore, can be -used in production. Either as an in-field memory bug detector or as a security -mitigation. +1. Generic KASAN +2. Software Tag-Based KASAN +3. Hardware Tag-Based KASAN -Software KASAN modes (#1 and #2) use compile-time instrumentation to insert -validity checks before every memory access and, therefore, require a compiler -version that supports that. +Generic KASAN, enabled with CONFIG_KASAN_GENERIC, is the mode intended for +debugging, similar to userspace ASan. This mode is supported on many CPU +architectures, but it has significant performance and memory overheads. -Generic KASAN is supported in GCC and Clang. With GCC, it requires version -8.3.0 or later. Any supported Clang version is compatible, but detection of -out-of-bounds accesses for global variables is only supported since Clang 11. +Software Tag-Based KASAN or SW_TAGS KASAN, enabled with CONFIG_KASAN_SW_TAGS, +can be used for both debugging and dogfood testing, similar to userspace HWASan. +This mode is only supported for arm64, but its moderate memory overhead allows +using it for testing on memory-restricted devices with real workloads. -Software tag-based KASAN mode is only supported in Clang. +Hardware Tag-Based KASAN or HW_TAGS KASAN, enabled with CONFIG_KASAN_HW_TAGS, +is the mode intended to be used as an in-field memory bug detector or as a +security mitigation. This mode only works on arm64 CPUs that support MTE +(Memory Tagging Extension), but it has low memory and performance overheads and +thus can be used in production. -The hardware KASAN mode (#3) relies on hardware to perform the checks but -still requires a compiler version that supports memory tagging instructions. -This mode is supported in GCC 10+ and Clang 12+. +For details about the memory and performance impact of each KASAN mode, see the +descriptions of the corresponding Kconfig options. -Both software KASAN modes work with SLUB and SLAB memory allocators, -while the hardware tag-based KASAN currently only supports SLUB. +The Generic and the Software Tag-Based modes are commonly referred to as the +software modes. The Software Tag-Based and the Hardware Tag-Based modes are +referred to as the tag-based modes. -Currently, generic KASAN is supported for the x86_64, arm, arm64, xtensa, s390, -and riscv architectures, and tag-based KASAN modes are supported only for arm64. +Support +------- + +Architectures +~~~~~~~~~~~~~ + +Generic KASAN is supported on x86_64, arm, arm64, powerpc, riscv, s390, and +xtensa, and the tag-based KASAN modes are supported only on arm64. + +Compilers +~~~~~~~~~ + +Software KASAN modes use compile-time instrumentation to insert validity checks +before every memory access and thus require a compiler version that provides +support for that. The Hardware Tag-Based mode relies on hardware to perform +these checks but still requires a compiler version that supports the memory +tagging instructions. + +Generic KASAN requires GCC version 8.3.0 or later +or any Clang version supported by the kernel. + +Software Tag-Based KASAN requires GCC 11+ +or any Clang version supported by the kernel. + +Hardware Tag-Based KASAN requires GCC 10+ or Clang 12+. + +Memory types +~~~~~~~~~~~~ + +Generic KASAN supports finding bugs in all of slab, page_alloc, vmap, vmalloc, +stack, and global memory. + +Software Tag-Based KASAN supports slab, page_alloc, vmalloc, and stack memory. + +Hardware Tag-Based KASAN supports slab, page_alloc, and non-executable vmalloc +memory. + +For slab, both software KASAN modes support SLUB and SLAB allocators, while +Hardware Tag-Based KASAN only supports SLUB. Usage ----- @@ -45,18 +82,59 @@ To enable KASAN, configure the kernel with:: CONFIG_KASAN=y -and choose between ``CONFIG_KASAN_GENERIC`` (to enable generic KASAN), -``CONFIG_KASAN_SW_TAGS`` (to enable software tag-based KASAN), and -``CONFIG_KASAN_HW_TAGS`` (to enable hardware tag-based KASAN). +and choose between ``CONFIG_KASAN_GENERIC`` (to enable Generic KASAN), +``CONFIG_KASAN_SW_TAGS`` (to enable Software Tag-Based KASAN), and +``CONFIG_KASAN_HW_TAGS`` (to enable Hardware Tag-Based KASAN). -For software modes, also choose between ``CONFIG_KASAN_OUTLINE`` and +For the software modes, also choose between ``CONFIG_KASAN_OUTLINE`` and ``CONFIG_KASAN_INLINE``. Outline and inline are compiler instrumentation types. -The former produces a smaller binary while the latter is 1.1-2 times faster. +The former produces a smaller binary while the latter is up to 2 times faster. To include alloc and free stack traces of affected slab objects into reports, enable ``CONFIG_STACKTRACE``. To include alloc and free stack traces of affected physical pages, enable ``CONFIG_PAGE_OWNER`` and boot with ``page_owner=on``. +Boot parameters +~~~~~~~~~~~~~~~ + +KASAN is affected by the generic ``panic_on_warn`` command line parameter. +When it is enabled, KASAN panics the kernel after printing a bug report. + +By default, KASAN prints a bug report only for the first invalid memory access. +With ``kasan_multi_shot``, KASAN prints a report on every invalid access. This +effectively disables ``panic_on_warn`` for KASAN reports. + +Alternatively, independent of ``panic_on_warn``, the ``kasan.fault=`` boot +parameter can be used to control panic and reporting behaviour: + +- ``kasan.fault=report`` or ``=panic`` controls whether to only print a KASAN + report or also panic the kernel (default: ``report``). The panic happens even + if ``kasan_multi_shot`` is enabled. + +Hardware Tag-Based KASAN mode (see the section about various modes below) is +intended for use in production as a security mitigation. Therefore, it supports +additional boot parameters that allow disabling KASAN or controlling features: + +- ``kasan=off`` or ``=on`` controls whether KASAN is enabled (default: ``on``). + +- ``kasan.mode=sync``, ``=async`` or ``=asymm`` controls whether KASAN + is configured in synchronous, asynchronous or asymmetric mode of + execution (default: ``sync``). + Synchronous mode: a bad access is detected immediately when a tag + check fault occurs. + Asynchronous mode: a bad access detection is delayed. When a tag check + fault occurs, the information is stored in hardware (in the TFSR_EL1 + register for arm64). The kernel periodically checks the hardware and + only reports tag faults during these checks. + Asymmetric mode: a bad access is detected synchronously on reads and + asynchronously on writes. + +- ``kasan.vmalloc=off`` or ``=on`` disables or enables tagging of vmalloc + allocations (default: ``on``). + +- ``kasan.stacktrace=off`` or ``=on`` disables or enables alloc and free stack + traces collection (default: ``on``). + Error reports ~~~~~~~~~~~~~ @@ -146,7 +224,7 @@ is either 8 or 16 aligned bytes depending on KASAN mode. Each number in the memory state section of the report shows the state of one of the memory granules that surround the accessed address. -For generic KASAN, the size of each memory granule is 8. The state of each +For Generic KASAN, the size of each memory granule is 8. The state of each granule is encoded in one shadow byte. Those 8 bytes can be accessible, partially accessible, freed, or be a part of a redzone. KASAN uses the following encoding for each shadow byte: 00 means that all 8 bytes of the corresponding @@ -171,47 +249,6 @@ traces point to places in code that interacted with the object but that are not directly present in the bad access stack trace. Currently, this includes call_rcu() and workqueue queuing. -Boot parameters -~~~~~~~~~~~~~~~ - -KASAN is affected by the generic ``panic_on_warn`` command line parameter. -When it is enabled, KASAN panics the kernel after printing a bug report. - -By default, KASAN prints a bug report only for the first invalid memory access. -With ``kasan_multi_shot``, KASAN prints a report on every invalid access. This -effectively disables ``panic_on_warn`` for KASAN reports. - -Alternatively, independent of ``panic_on_warn`` the ``kasan.fault=`` boot -parameter can be used to control panic and reporting behaviour: - -- ``kasan.fault=report`` or ``=panic`` controls whether to only print a KASAN - report or also panic the kernel (default: ``report``). The panic happens even - if ``kasan_multi_shot`` is enabled. - -Hardware tag-based KASAN mode (see the section about various modes below) is -intended for use in production as a security mitigation. Therefore, it supports -additional boot parameters that allow disabling KASAN or controlling features: - -- ``kasan=off`` or ``=on`` controls whether KASAN is enabled (default: ``on``). - -- ``kasan.mode=sync``, ``=async`` or ``=asymm`` controls whether KASAN - is configured in synchronous, asynchronous or asymmetric mode of - execution (default: ``sync``). - Synchronous mode: a bad access is detected immediately when a tag - check fault occurs. - Asynchronous mode: a bad access detection is delayed. When a tag check - fault occurs, the information is stored in hardware (in the TFSR_EL1 - register for arm64). The kernel periodically checks the hardware and - only reports tag faults during these checks. - Asymmetric mode: a bad access is detected synchronously on reads and - asynchronously on writes. - -- ``kasan.vmalloc=off`` or ``=on`` disables or enables tagging of vmalloc - allocations (default: ``on``). - -- ``kasan.stacktrace=off`` or ``=on`` disables or enables alloc and free stack - traces collection (default: ``on``). - Implementation details ---------------------- @@ -250,49 +287,46 @@ outline-instrumented kernel. Generic KASAN is the only mode that delays the reuse of freed objects via quarantine (see mm/kasan/quarantine.c for implementation). -Software tag-based KASAN +Software Tag-Based KASAN ~~~~~~~~~~~~~~~~~~~~~~~~ -Software tag-based KASAN uses a software memory tagging approach to checking +Software Tag-Based KASAN uses a software memory tagging approach to checking access validity. It is currently only implemented for the arm64 architecture. -Software tag-based KASAN uses the Top Byte Ignore (TBI) feature of arm64 CPUs +Software Tag-Based KASAN uses the Top Byte Ignore (TBI) feature of arm64 CPUs to store a pointer tag in the top byte of kernel pointers. It uses shadow memory to store memory tags associated with each 16-byte memory cell (therefore, it dedicates 1/16th of the kernel memory for shadow memory). -On each memory allocation, software tag-based KASAN generates a random tag, tags +On each memory allocation, Software Tag-Based KASAN generates a random tag, tags the allocated memory with this tag, and embeds the same tag into the returned pointer. -Software tag-based KASAN uses compile-time instrumentation to insert checks +Software Tag-Based KASAN uses compile-time instrumentation to insert checks before each memory access. These checks make sure that the tag of the memory that is being accessed is equal to the tag of the pointer that is used to access -this memory. In case of a tag mismatch, software tag-based KASAN prints a bug +this memory. In case of a tag mismatch, Software Tag-Based KASAN prints a bug report. -Software tag-based KASAN also has two instrumentation modes (outline, which +Software Tag-Based KASAN also has two instrumentation modes (outline, which emits callbacks to check memory accesses; and inline, which performs the shadow memory checks inline). With outline instrumentation mode, a bug report is printed from the function that performs the access check. With inline instrumentation, a ``brk`` instruction is emitted by the compiler, and a dedicated ``brk`` handler is used to print bug reports. -Software tag-based KASAN uses 0xFF as a match-all pointer tag (accesses through +Software Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses through pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently reserved to tag freed memory regions. -Software tag-based KASAN currently only supports tagging of slab, page_alloc, -and vmalloc memory. - -Hardware tag-based KASAN +Hardware Tag-Based KASAN ~~~~~~~~~~~~~~~~~~~~~~~~ -Hardware tag-based KASAN is similar to the software mode in concept but uses +Hardware Tag-Based KASAN is similar to the software mode in concept but uses hardware memory tagging support instead of compiler instrumentation and shadow memory. -Hardware tag-based KASAN is currently only implemented for arm64 architecture +Hardware Tag-Based KASAN is currently only implemented for arm64 architecture and based on both arm64 Memory Tagging Extension (MTE) introduced in ARMv8.5 Instruction Set Architecture and Top Byte Ignore (TBI). @@ -302,21 +336,18 @@ access, hardware makes sure that the tag of the memory that is being accessed is equal to the tag of the pointer that is used to access this memory. In case of a tag mismatch, a fault is generated, and a report is printed. -Hardware tag-based KASAN uses 0xFF as a match-all pointer tag (accesses through +Hardware Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses through pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently reserved to tag freed memory regions. -Hardware tag-based KASAN currently only supports tagging of slab, page_alloc, -and VM_ALLOC-based vmalloc memory. - -If the hardware does not support MTE (pre ARMv8.5), hardware tag-based KASAN +If the hardware does not support MTE (pre ARMv8.5), Hardware Tag-Based KASAN will not be enabled. In this case, all KASAN boot parameters are ignored. Note that enabling CONFIG_KASAN_HW_TAGS always results in in-kernel TBI being enabled. Even when ``kasan.mode=off`` is provided or when the hardware does not support MTE (but supports TBI). -Hardware tag-based KASAN only reports the first found bug. After that, MTE tag +Hardware Tag-Based KASAN only reports the first found bug. After that, MTE tag checking gets disabled. Shadow memory @@ -414,19 +445,18 @@ generic ``noinstr`` one. Note that disabling compiler instrumentation (either on a per-file or a per-function basis) makes KASAN ignore the accesses that happen directly in that code for software KASAN modes. It does not help when the accesses happen -indirectly (through calls to instrumented functions) or with the hardware -tag-based mode that does not use compiler instrumentation. +indirectly (through calls to instrumented functions) or with Hardware +Tag-Based KASAN, which does not use compiler instrumentation. For software KASAN modes, to disable KASAN reports in a part of the kernel code for the current task, annotate this part of the code with a ``kasan_disable_current()``/``kasan_enable_current()`` section. This also disables the reports for indirect accesses that happen through function calls. -For tag-based KASAN modes (include the hardware one), to disable access -checking, use ``kasan_reset_tag()`` or ``page_kasan_tag_reset()``. Note that -temporarily disabling access checking via ``page_kasan_tag_reset()`` requires -saving and restoring the per-page KASAN tag via -``page_kasan_tag``/``page_kasan_tag_set``. +For tag-based KASAN modes, to disable access checking, use +``kasan_reset_tag()`` or ``page_kasan_tag_reset()``. Note that temporarily +disabling access checking via ``page_kasan_tag_reset()`` requires saving and +restoring the per-page KASAN tag via ``page_kasan_tag``/``page_kasan_tag_set``. Tests ~~~~~ diff --git a/Documentation/dev-tools/ktap.rst b/Documentation/dev-tools/ktap.rst index 5ee735c6687f..d0a9565b0f44 100644 --- a/Documentation/dev-tools/ktap.rst +++ b/Documentation/dev-tools/ktap.rst @@ -115,34 +115,32 @@ The diagnostic data field is optional, and results which have neither a directive nor any diagnostic data do not need to include the "#" field separator. -Example result lines include: - -.. code-block:: none +Example result lines include:: ok 1 test_case_name The test "test_case_name" passed. -.. code-block:: none +:: not ok 1 test_case_name The test "test_case_name" failed. -.. code-block:: none +:: ok 1 test # SKIP necessary dependency unavailable The test "test" was SKIPPED with the diagnostic message "necessary dependency unavailable". -.. code-block:: none +:: not ok 1 test # TIMEOUT 30 seconds The test "test" timed out, with diagnostic data "30 seconds". -.. code-block:: none +:: ok 5 check return code # rcode=0 @@ -202,7 +200,7 @@ allowed to be either indented or not indented. An example of a test with two nested subtests: -.. code-block:: none +:: KTAP version 1 1..1 @@ -215,7 +213,7 @@ An example of a test with two nested subtests: An example format with multiple levels of nested testing: -.. code-block:: none +:: KTAP version 1 1..2 @@ -250,7 +248,7 @@ nested version line, uses a line of the form Example KTAP output -------------------- -.. code-block:: none +:: KTAP version 1 1..1 diff --git a/Documentation/dev-tools/kunit/api/index.rst b/Documentation/dev-tools/kunit/api/index.rst index 3006cadcf44a..45ce04823f9f 100644 --- a/Documentation/dev-tools/kunit/api/index.rst +++ b/Documentation/dev-tools/kunit/api/index.rst @@ -6,6 +6,7 @@ API Reference .. toctree:: test + resource This section documents the KUnit kernel testing API. It is divided into the following sections: @@ -13,3 +14,7 @@ following sections: Documentation/dev-tools/kunit/api/test.rst - documents all of the standard testing API + +Documentation/dev-tools/kunit/api/resource.rst + + - documents the KUnit resource API diff --git a/Documentation/dev-tools/kunit/api/resource.rst b/Documentation/dev-tools/kunit/api/resource.rst new file mode 100644 index 000000000000..0a94f831259e --- /dev/null +++ b/Documentation/dev-tools/kunit/api/resource.rst @@ -0,0 +1,13 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============ +Resource API +============ + +This file documents the KUnit resource API. + +Most users won't need to use this API directly, power users can use it to store +state on a per-test basis, register custom cleanup actions, and more. + +.. kernel-doc:: include/kunit/resource.h + :internal: diff --git a/Documentation/dev-tools/kunit/architecture.rst b/Documentation/dev-tools/kunit/architecture.rst index ff9c85a0bff2..cf9e6e3eeae4 100644 --- a/Documentation/dev-tools/kunit/architecture.rst +++ b/Documentation/dev-tools/kunit/architecture.rst @@ -125,7 +125,7 @@ All expectations/assertions are formatted as: ``void __noreturn kunit_try_catch_throw(struct kunit_try_catch *try_catch)``. - ``kunit_try_catch_throw`` calls function: - ``void complete_and_exit(struct completion *, long) __noreturn;`` + ``void kthread_complete_and_exit(struct completion *, long) __noreturn;`` and terminates the special thread context. - ``<op>`` denotes a check with options: ``TRUE`` (supplied property diff --git a/Documentation/dev-tools/kunit/running_tips.rst b/Documentation/dev-tools/kunit/running_tips.rst index 7b6d26a25959..c36f6760087d 100644 --- a/Documentation/dev-tools/kunit/running_tips.rst +++ b/Documentation/dev-tools/kunit/running_tips.rst @@ -114,6 +114,7 @@ Instead of enabling ``CONFIG_GCOV_KERNEL=y``, we can set these options: CONFIG_DEBUG_KERNEL=y CONFIG_DEBUG_INFO=y + CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y CONFIG_GCOV=y @@ -122,7 +123,7 @@ Putting it together into a copy-pastable sequence of commands: .. code-block:: bash # Append coverage options to the current config - $ echo -e "CONFIG_DEBUG_KERNEL=y\nCONFIG_DEBUG_INFO=y\nCONFIG_GCOV=y" >> .kunit/.kunitconfig + $ echo -e "CONFIG_DEBUG_KERNEL=y\nCONFIG_DEBUG_INFO=y\nCONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y\nCONFIG_GCOV=y" >> .kunit/.kunitconfig $ ./tools/testing/kunit/kunit.py run # Extract the coverage information from the build dir (.kunit/) $ lcov -t "my_kunit_tests" -o coverage.info -c -d .kunit/ diff --git a/Documentation/dev-tools/kunit/start.rst b/Documentation/dev-tools/kunit/start.rst index ad168d16968f..867a4bba6bf6 100644 --- a/Documentation/dev-tools/kunit/start.rst +++ b/Documentation/dev-tools/kunit/start.rst @@ -41,13 +41,18 @@ or ``VFAT_FS``. To run ``FAT_KUNIT_TEST``, the ``.kunitconfig`` has: CONFIG_MSDOS_FS=y CONFIG_FAT_KUNIT_TEST=y -1. A good starting point for the ``.kunitconfig``, is the KUnit default - config. Run the command: +1. A good starting point for the ``.kunitconfig`` is the KUnit default config. + You can generate it by running: .. code-block:: bash cd $PATH_TO_LINUX_REPO - cp tools/testing/kunit/configs/default.config .kunitconfig + tools/testing/kunit/kunit.py config + cat .kunit/.kunitconfig + +.. note :: + ``.kunitconfig`` lives in the ``--build_dir`` used by kunit.py, which is + ``.kunit`` by default. .. note :: You may want to remove CONFIG_KUNIT_ALL_TESTS from the ``.kunitconfig`` as diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst index 1c83e7d60a8a..d62a04255c2e 100644 --- a/Documentation/dev-tools/kunit/usage.rst +++ b/Documentation/dev-tools/kunit/usage.rst @@ -125,8 +125,8 @@ We need many test cases covering all the unit's behaviors. It is common to have many similar tests. In order to reduce duplication in these closely related tests, most unit testing frameworks (including KUnit) provide the concept of a *test suite*. A test suite is a collection of test cases for a unit of code -with a setup function that gets invoked before every test case and then a tear -down function that gets invoked after every test case completes. For example: +with optional setup and teardown functions that run before/after the whole +suite and/or every test case. For example: .. code-block:: c @@ -141,16 +141,19 @@ down function that gets invoked after every test case completes. For example: .name = "example", .init = example_test_init, .exit = example_test_exit, + .suite_init = example_suite_init, + .suite_exit = example_suite_exit, .test_cases = example_test_cases, }; kunit_test_suite(example_test_suite); -In the above example, the test suite ``example_test_suite`` would run the test -cases ``example_test_foo``, ``example_test_bar``, and ``example_test_baz``. Each -would have ``example_test_init`` called immediately before it and -``example_test_exit`` called immediately after it. -``kunit_test_suite(example_test_suite)`` registers the test suite with the -KUnit test framework. +In the above example, the test suite ``example_test_suite`` would first run +``example_suite_init``, then run the test cases ``example_test_foo``, +``example_test_bar``, and ``example_test_baz``. Each would have +``example_test_init`` called immediately before it and ``example_test_exit`` +called immediately after it. Finally, ``example_suite_exit`` would be called +after everything else. ``kunit_test_suite(example_test_suite)`` registers the +test suite with the KUnit test framework. .. note:: A test case will only run if it is associated with a test suite. diff --git a/Documentation/dev-tools/testing-overview.rst b/Documentation/dev-tools/testing-overview.rst index 65feb81edb14..0aaf6ea53608 100644 --- a/Documentation/dev-tools/testing-overview.rst +++ b/Documentation/dev-tools/testing-overview.rst @@ -115,3 +115,66 @@ that none of these errors are occurring during the test. Some of these tools integrate with KUnit or kselftest and will automatically fail tests if an issue is detected. +Static Analysis Tools +===================== + +In addition to testing a running kernel, one can also analyze kernel source code +directly (**at compile time**) using **static analysis** tools. The tools +commonly used in the kernel allow one to inspect the whole source tree or just +specific files within it. They make it easier to detect and fix problems during +the development process. + +Sparse can help test the kernel by performing type-checking, lock checking, +value range checking, in addition to reporting various errors and warnings while +examining the code. See the Documentation/dev-tools/sparse.rst documentation +page for details on how to use it. + +Smatch extends Sparse and provides additional checks for programming logic +mistakes such as missing breaks in switch statements, unused return values on +error checking, forgetting to set an error code in the return of an error path, +etc. Smatch also has tests against more serious issues such as integer +overflows, null pointer dereferences, and memory leaks. See the project page at +http://smatch.sourceforge.net/. + +Coccinelle is another static analyzer at our disposal. Coccinelle is often used +to aid refactoring and collateral evolution of source code, but it can also help +to avoid certain bugs that occur in common code patterns. The types of tests +available include API tests, tests for correct usage of kernel iterators, checks +for the soundness of free operations, analysis of locking behavior, and further +tests known to help keep consistent kernel usage. See the +Documentation/dev-tools/coccinelle.rst documentation page for details. + +Beware, though, that static analysis tools suffer from **false positives**. +Errors and warns need to be evaluated carefully before attempting to fix them. + +When to use Sparse and Smatch +----------------------------- + +Sparse does type checking, such as verifying that annotated variables do not +cause endianness bugs, detecting places that use ``__user`` pointers improperly, +and analyzing the compatibility of symbol initializers. + +Smatch does flow analysis and, if allowed to build the function database, it +also does cross function analysis. Smatch tries to answer questions like where +is this buffer allocated? How big is it? Can this index be controlled by the +user? Is this variable larger than that variable? + +It's generally easier to write checks in Smatch than it is to write checks in +Sparse. Nevertheless, there are some overlaps between Sparse and Smatch checks. + +Strong points of Smatch and Coccinelle +-------------------------------------- + +Coccinelle is probably the easiest for writing checks. It works before the +pre-processor so it's easier to check for bugs in macros using Coccinelle. +Coccinelle also creates patches for you, which no other tool does. + +For example, with Coccinelle you can do a mass conversion from +``kmalloc(x * size, GFP_KERNEL)`` to ``kmalloc_array(x, size, GFP_KERNEL)``, and +that's really useful. If you just created a Smatch warning and try to push the +work of converting on to the maintainers they would be annoyed. You'd have to +argue about each warning if can really overflow or not. + +Coccinelle does no analysis of variable values, which is the strong point of +Smatch. On the other hand, Coccinelle allows you to do simple things in a simple +way. |