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, 132 insertions, 0 deletions

diff --git a/drivers/leds/led-test.c b/drivers/leds/led-test.c
new file mode 100644
index 000000000..ddf9aa967
--- /dev/null
+++ b/drivers/leds/led-test.c
@@ -0,0 +1,132 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2025 Google LLC
+ *
+ * Author: Lee Jones <lee@kernel.org>
+ */
+
+#include <kunit/device.h>
+#include <kunit/test.h>
+#include <linux/device.h>
+#include <linux/leds.h>
+
+#define LED_TEST_POST_REG_BRIGHTNESS 10
+
+struct led_test_ddata {
+	struct led_classdev cdev;
+	struct device *dev;
+};
+
+static enum led_brightness led_test_brightness_get(struct led_classdev *cdev)
+{
+	return LED_TEST_POST_REG_BRIGHTNESS;
+}
+
+static void led_test_class_register(struct kunit *test)
+{
+	struct led_test_ddata *ddata = test->priv;
+	struct led_classdev *cdev_clash, *cdev = &ddata->cdev;
+	struct device *dev = ddata->dev;
+	int ret;
+
+	/* Register a LED class device */
+	cdev->name = "led-test";
+	cdev->brightness_get = led_test_brightness_get;
+	cdev->brightness = 0;
+
+	ret = devm_led_classdev_register(dev, cdev);
+	KUNIT_ASSERT_EQ(test, ret, 0);
+
+	KUNIT_EXPECT_EQ(test, cdev->max_brightness, LED_FULL);
+	KUNIT_EXPECT_EQ(test, cdev->brightness, LED_TEST_POST_REG_BRIGHTNESS);
+	KUNIT_EXPECT_STREQ(test, cdev->dev->kobj.name, "led-test");
+
+	/* Register again with the same name - expect it to pass with the LED renamed */
+	cdev_clash = devm_kmemdup(dev, cdev, sizeof(*cdev), GFP_KERNEL);
+	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, cdev_clash);
+
+	ret = devm_led_classdev_register(dev, cdev_clash);
+	KUNIT_ASSERT_EQ(test, ret, 0);
+
+	KUNIT_EXPECT_STREQ(test, cdev_clash->dev->kobj.name, "led-test_1");
+	KUNIT_EXPECT_STREQ(test, cdev_clash->name, "led-test");
+
+	/* Enable name conflict rejection and register with the same name again - expect failure */
+	cdev_clash->flags |= LED_REJECT_NAME_CONFLICT;
+	ret = devm_led_classdev_register(dev, cdev_clash);
+	KUNIT_EXPECT_EQ(test, ret, -EEXIST);
+}
+
+static void led_test_class_add_lookup_and_get(struct kunit *test)
+{
+	struct led_test_ddata *ddata = test->priv;
+	struct led_classdev *cdev = &ddata->cdev, *cdev_get;
+	struct device *dev = ddata->dev;
+	struct led_lookup_data lookup;
+	int ret;
+
+	/* First, register a LED class device */
+	cdev->name = "led-test";
+	ret = devm_led_classdev_register(dev, cdev);
+	KUNIT_ASSERT_EQ(test, ret, 0);
+
+	/* Then make the LED available for lookup */
+	lookup.provider = cdev->name;
+	lookup.dev_id = dev_name(dev);
+	lookup.con_id = "led-test-1";
+	led_add_lookup(&lookup);
+
+	/* Finally, attempt to look it up via the API - imagine this was an orthogonal driver */
+	cdev_get = devm_led_get(dev, "led-test-1");
+	KUNIT_ASSERT_FALSE(test, IS_ERR(cdev_get));
+
+	KUNIT_EXPECT_STREQ(test, cdev_get->name, cdev->name);
+
+	led_remove_lookup(&lookup);
+}
+
+static struct kunit_case led_test_cases[] = {
+	KUNIT_CASE(led_test_class_register),
+	KUNIT_CASE(led_test_class_add_lookup_and_get),
+	{ }
+};
+
+static int led_test_init(struct kunit *test)
+{
+	struct led_test_ddata *ddata;
+	struct device *dev;
+
+	ddata = kunit_kzalloc(test, sizeof(*ddata), GFP_KERNEL);
+	if (!ddata)
+		return -ENOMEM;
+
+	test->priv = ddata;
+
+	dev = kunit_device_register(test, "led_test");
+	if (IS_ERR(dev))
+		return PTR_ERR(dev);
+
+	ddata->dev = get_device(dev);
+
+	return 0;
+}
+
+static void led_test_exit(struct kunit *test)
+{
+	struct led_test_ddata *ddata = test->priv;
+
+	if (ddata && ddata->dev)
+		put_device(ddata->dev);
+}
+
+static struct kunit_suite led_test_suite = {
+	.name = "led",
+	.init = led_test_init,
+	.exit = led_test_exit,
+	.test_cases = led_test_cases,
+};
+kunit_test_suite(led_test_suite);
+
+MODULE_AUTHOR("Lee Jones <lee@kernel.org>");
+MODULE_DESCRIPTION("KUnit tests for the LED framework");
+MODULE_LICENSE("GPL");