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, 198 insertions, 0 deletions

diff --git a/drivers/leds/trigger/ledtrig-transient.c b/drivers/leds/trigger/ledtrig-transient.c
new file mode 100644
index 000000000..8d928bb2a
--- /dev/null
+++ b/drivers/leds/trigger/ledtrig-transient.c
@@ -0,0 +1,198 @@
+// SPDX-License-Identifier: GPL-2.0
+//
+// LED Kernel Transient Trigger
+//
+// Transient trigger allows one shot timer activation. Please refer to
+// Documentation/leds/ledtrig-transient.rst for details
+// Copyright (C) 2012 Shuah Khan <shuahkhan@gmail.com>
+//
+// Based on Richard Purdie's ledtrig-timer.c and Atsushi Nemoto's
+// ledtrig-heartbeat.c
+// Design and use-case input from Jonas Bonn <jonas@southpole.se> and
+// Neil Brown <neilb@suse.de>
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/device.h>
+#include <linux/slab.h>
+#include <linux/timer.h>
+#include <linux/leds.h>
+#include "../leds.h"
+
+struct transient_trig_data {
+	int activate;
+	int state;
+	int restore_state;
+	unsigned long duration;
+	struct timer_list timer;
+	struct led_classdev *led_cdev;
+};
+
+static void transient_timer_function(struct timer_list *t)
+{
+	struct transient_trig_data *transient_data =
+		timer_container_of(transient_data, t, timer);
+	struct led_classdev *led_cdev = transient_data->led_cdev;
+
+	transient_data->activate = 0;
+	led_set_brightness_nosleep(led_cdev, transient_data->restore_state);
+}
+
+static ssize_t transient_activate_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	struct transient_trig_data *transient_data =
+		led_trigger_get_drvdata(dev);
+
+	return sprintf(buf, "%d\n", transient_data->activate);
+}
+
+static ssize_t transient_activate_store(struct device *dev,
+		struct device_attribute *attr, const char *buf, size_t size)
+{
+	struct led_classdev *led_cdev = led_trigger_get_led(dev);
+	struct transient_trig_data *transient_data =
+		led_trigger_get_drvdata(dev);
+	unsigned long state;
+	ssize_t ret;
+
+	ret = kstrtoul(buf, 10, &state);
+	if (ret)
+		return ret;
+
+	if (state != 1 && state != 0)
+		return -EINVAL;
+
+	/* cancel the running timer */
+	if (state == 0 && transient_data->activate == 1) {
+		timer_delete(&transient_data->timer);
+		transient_data->activate = state;
+		led_set_brightness_nosleep(led_cdev,
+					transient_data->restore_state);
+		return size;
+	}
+
+	/* start timer if there is no active timer */
+	if (state == 1 && transient_data->activate == 0 &&
+	    transient_data->duration != 0) {
+		transient_data->activate = state;
+		led_set_brightness_nosleep(led_cdev, transient_data->state);
+		transient_data->restore_state =
+		    (transient_data->state == LED_FULL) ? LED_OFF : LED_FULL;
+		mod_timer(&transient_data->timer,
+			  jiffies + msecs_to_jiffies(transient_data->duration));
+	}
+
+	/* state == 0 && transient_data->activate == 0
+		timer is not active - just return */
+	/* state == 1 && transient_data->activate == 1
+		timer is already active - just return */
+
+	return size;
+}
+
+static ssize_t transient_duration_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	struct transient_trig_data *transient_data = led_trigger_get_drvdata(dev);
+
+	return sprintf(buf, "%lu\n", transient_data->duration);
+}
+
+static ssize_t transient_duration_store(struct device *dev,
+		struct device_attribute *attr, const char *buf, size_t size)
+{
+	struct transient_trig_data *transient_data =
+		led_trigger_get_drvdata(dev);
+	unsigned long state;
+	ssize_t ret;
+
+	ret = kstrtoul(buf, 10, &state);
+	if (ret)
+		return ret;
+
+	transient_data->duration = state;
+	return size;
+}
+
+static ssize_t transient_state_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	struct transient_trig_data *transient_data =
+		led_trigger_get_drvdata(dev);
+	int state;
+
+	state = (transient_data->state == LED_FULL) ? 1 : 0;
+	return sprintf(buf, "%d\n", state);
+}
+
+static ssize_t transient_state_store(struct device *dev,
+		struct device_attribute *attr, const char *buf, size_t size)
+{
+	struct transient_trig_data *transient_data =
+		led_trigger_get_drvdata(dev);
+	unsigned long state;
+	ssize_t ret;
+
+	ret = kstrtoul(buf, 10, &state);
+	if (ret)
+		return ret;
+
+	if (state != 1 && state != 0)
+		return -EINVAL;
+
+	transient_data->state = (state == 1) ? LED_FULL : LED_OFF;
+	return size;
+}
+
+static DEVICE_ATTR(activate, 0644, transient_activate_show,
+		   transient_activate_store);
+static DEVICE_ATTR(duration, 0644, transient_duration_show,
+		   transient_duration_store);
+static DEVICE_ATTR(state, 0644, transient_state_show, transient_state_store);
+
+static struct attribute *transient_trig_attrs[] = {
+	&dev_attr_activate.attr,
+	&dev_attr_duration.attr,
+	&dev_attr_state.attr,
+	NULL
+};
+ATTRIBUTE_GROUPS(transient_trig);
+
+static int transient_trig_activate(struct led_classdev *led_cdev)
+{
+	struct transient_trig_data *tdata;
+
+	tdata = kzalloc_obj(struct transient_trig_data);
+	if (!tdata)
+		return -ENOMEM;
+
+	led_set_trigger_data(led_cdev, tdata);
+	tdata->led_cdev = led_cdev;
+
+	timer_setup(&tdata->timer, transient_timer_function, 0);
+
+	return 0;
+}
+
+static void transient_trig_deactivate(struct led_classdev *led_cdev)
+{
+	struct transient_trig_data *transient_data = led_get_trigger_data(led_cdev);
+
+	timer_shutdown_sync(&transient_data->timer);
+	led_set_brightness_nosleep(led_cdev, transient_data->restore_state);
+	kfree(transient_data);
+}
+
+static struct led_trigger transient_trigger = {
+	.name     = "transient",
+	.activate = transient_trig_activate,
+	.deactivate = transient_trig_deactivate,
+	.groups = transient_trig_groups,
+};
+module_led_trigger(transient_trigger);
+
+MODULE_AUTHOR("Shuah Khan <shuahkhan@gmail.com>");
+MODULE_DESCRIPTION("Transient LED trigger");
+MODULE_LICENSE("GPL v2");