kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 214 insertions, 0 deletions

diff --git a/Documentation/dev-tools/kunit/style.rst b/Documentation/dev-tools/kunit/style.rst
new file mode 100644
index 000000000..449f9f816
--- /dev/null
+++ b/Documentation/dev-tools/kunit/style.rst
@@ -0,0 +1,214 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===========================
+Test Style and Nomenclature
+===========================
+
+To make finding, writing, and using KUnit tests as simple as possible, it is
+strongly encouraged that they are named and written according to the guidelines
+below. While it is possible to write KUnit tests which do not follow these rules,
+they may break some tooling, may conflict with other tests, and may not be run
+automatically by testing systems.
+
+It is recommended that you only deviate from these guidelines when:
+
+1. Porting tests to KUnit which are already known with an existing name.
+2. Writing tests which would cause serious problems if automatically run. For
+   example, non-deterministically producing false positives or negatives, or
+   taking a long time to run.
+
+Subsystems, Suites, and Tests
+=============================
+
+To make tests easy to find, they are grouped into suites and subsystems. A test
+suite is a group of tests which test a related area of the kernel. A subsystem
+is a set of test suites which test different parts of a kernel subsystem
+or a driver.
+
+Subsystems
+----------
+
+Every test suite must belong to a subsystem. A subsystem is a collection of one
+or more KUnit test suites which test the same driver or part of the kernel. A
+test subsystem should match a single kernel module. If the code being tested
+cannot be compiled as a module, in many cases the subsystem should correspond to
+a directory in the source tree or an entry in the ``MAINTAINERS`` file. If
+unsure, follow the conventions set by tests in similar areas.
+
+Test subsystems should be named after the code being tested, either after the
+module (wherever possible), or after the directory or files being tested. Test
+subsystems should be named to avoid ambiguity where necessary.
+
+If a test subsystem name has multiple components, they should be separated by
+underscores. *Do not* include "test" or "kunit" directly in the subsystem name
+unless we are actually testing other tests or the kunit framework itself. For
+example, subsystems could be called:
+
+``ext4``
+  Matches the module and filesystem name.
+``apparmor``
+  Matches the module name and LSM name.
+``kasan``
+  Common name for the tool, prominent part of the path ``mm/kasan``
+``snd_hda_codec_hdmi``
+  Has several components (``snd``, ``hda``, ``codec``, ``hdmi``) separated by
+  underscores. Matches the module name.
+
+Avoid names as shown in examples below:
+
+``linear-ranges``
+  Names should use underscores, not dashes, to separate words. Prefer
+  ``linear_ranges``.
+``qos-kunit-test``
+  This name should use underscores, and not have "kunit-test" as a
+  suffix. ``qos`` is also ambiguous as a subsystem name, because several parts
+  of the kernel have a ``qos`` subsystem. ``power_qos`` would be a better name.
+``pc_parallel_port``
+  The corresponding module name is ``parport_pc``, so this subsystem should also
+  be named ``parport_pc``.
+
+.. note::
+        The KUnit API and tools do not explicitly know about subsystems. They are
+        a way of categorizing test suites and naming modules which provides a
+        simple, consistent way for humans to find and run tests. This may change
+        in the future.
+
+Suites
+------
+
+KUnit tests are grouped into test suites, which cover a specific area of
+functionality being tested. Test suites can have shared initialization and
+shutdown code which is run for all tests in the suite. Not all subsystems need
+to be split into multiple test suites (for example, simple drivers).
+
+Test suites are named after the subsystem they are part of. If a subsystem
+contains several suites, the specific area under test should be appended to the
+subsystem name, separated by an underscore.
+
+In the event that there are multiple types of test using KUnit within a
+subsystem (for example, both unit tests and integration tests), they should be
+put into separate suites, with the type of test as the last element in the suite
+name. Unless these tests are actually present, avoid using ``_test``, ``_unittest``
+or similar in the suite name.
+
+The full test suite name (including the subsystem name) should be specified as
+the ``.name`` member of the ``kunit_suite`` struct, and forms the base for the
+module name. For example, test suites could include:
+
+``ext4_inode``
+  Part of the ``ext4`` subsystem, testing the ``inode`` area.
+``kunit_try_catch``
+  Part of the ``kunit`` implementation itself, testing the ``try_catch`` area.
+``apparmor_property_entry``
+  Part of the ``apparmor`` subsystem, testing the ``property_entry`` area.
+``kasan``
+  The ``kasan`` subsystem has only one suite, so the suite name is the same as
+  the subsystem name.
+
+Avoid names, for example:
+
+``ext4_ext4_inode``
+  There is no reason to state the subsystem twice.
+``property_entry``
+  The suite name is ambiguous without the subsystem name.
+``kasan_integration_test``
+  Because there is only one suite in the ``kasan`` subsystem, the suite should
+  just be called as ``kasan``. Do not redundantly add
+  ``integration_test``. It should be a separate test suite. For example, if the
+  unit tests are added, then that suite could be named as ``kasan_unittest`` or
+  similar.
+
+Test Cases
+----------
+
+Individual tests consist of a single function which tests a constrained
+codepath, property, or function. In the test output, an individual test's
+results will show up as subtests of the suite's results.
+
+Tests should be named after what they are testing. This is often the name of the
+function being tested, with a description of the input or codepath being tested.
+As tests are C functions, they should be named and written in accordance with
+the kernel coding style.
+
+.. note::
+        As tests are themselves functions, their names cannot conflict with
+        other C identifiers in the kernel. This may require some creative
+        naming. It is a good idea to make your test functions `static` to avoid
+        polluting the global namespace.
+
+Example test names include:
+
+``unpack_u32_with_null_name``
+  Tests the ``unpack_u32`` function when a NULL name is passed in.
+``test_list_splice``
+  Tests the ``list_splice`` macro. It has the prefix ``test_`` to avoid a
+  name conflict with the macro itself.
+
+
+Should it be necessary to refer to a test outside the context of its test suite,
+the *fully-qualified* name of a test should be the suite name followed by the
+test name, separated by a colon (i.e. ``suite:test``).
+
+Test Kconfig Entries
+====================
+
+Every test suite should be tied to a Kconfig entry.
+
+This Kconfig entry must:
+
+* be named ``CONFIG_<name>_KUNIT_TEST``: where <name> is the name of the test
+  suite.
+* be listed either alongside the config entries for the driver/subsystem being
+  tested, or be under [Kernel Hacking]->[Kernel Testing and Coverage]
+* depend on ``CONFIG_KUNIT``.
+* be visible only if ``CONFIG_KUNIT_ALL_TESTS`` is not enabled.
+* have a default value of ``CONFIG_KUNIT_ALL_TESTS``.
+* have a brief description of KUnit in the help text.
+* depend on the feature being tested, rather than selecting it (so that
+  enabling ``CONFIG_KUNIT_ALL_TESTS`` does not enable unrelated functionality).
+* be ``tristate``, unless there is a specific reason that the test cannot be
+  built as a module.
+
+For example, a Kconfig entry might look like:
+
+.. code-block:: none
+
+	config FOO_KUNIT_TEST
+		tristate "KUnit test for foo" if !KUNIT_ALL_TESTS
+		depends on KUNIT
+		default KUNIT_ALL_TESTS
+		help
+		  This builds unit tests for foo.
+
+		  For more information on KUnit and unit tests in general,
+		  please refer to the KUnit documentation in Documentation/dev-tools/kunit/.
+
+		  If unsure, say N.
+
+
+Test File and Module Names
+==========================
+
+KUnit tests are often compiled as a separate module. To avoid conflicting
+with regular modules, KUnit modules should be named after the test suite,
+followed by ``_kunit`` (e.g. if "foobar" is the core module, then
+"foobar_kunit" is the KUnit test module).
+
+Test source files, whether compiled as a separate module or an
+``#include`` in another source file, are best kept in a ``tests/``
+subdirectory to not conflict with other source files (e.g. for
+tab-completion).
+
+Note that the ``_test`` suffix has also been used in some existing
+tests. The ``_kunit`` suffix is preferred, as it makes the distinction
+between KUnit and non-KUnit tests clearer.
+
+So for the common case, name the file containing the test suite
+``tests/<suite>_kunit.c``. The ``tests`` directory should be placed at
+the same level as the code under test. For example, tests for
+``lib/string.c`` live in ``lib/tests/string_kunit.c``.
+
+If the suite name contains some or all of the name of the test's parent
+directory, it may make sense to modify the source filename to reduce
+redundancy. For example, a ``foo_firmware`` suite could be in the
+``foo/tests/firmware_kunit.c`` file.