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, 142 insertions, 0 deletions

diff --git a/drivers/input/misc/m68kspkr.c b/drivers/input/misc/m68kspkr.c
new file mode 100644
index 000000000..0542334df
--- /dev/null
+++ b/drivers/input/misc/m68kspkr.c
@@ -0,0 +1,142 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ *  m68k beeper driver for Linux
+ *
+ *  Copyright (c) 2002 Richard Zidlicky
+ *  Copyright (c) 2002 Vojtech Pavlik
+ *  Copyright (c) 1992 Orest Zborowski
+ */
+
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/input.h>
+#include <linux/platform_device.h>
+#include <asm/machdep.h>
+#include <asm/io.h>
+
+MODULE_AUTHOR("Richard Zidlicky <rz@linux-m68k.org>");
+MODULE_DESCRIPTION("m68k beeper driver");
+MODULE_LICENSE("GPL");
+
+static struct platform_device *m68kspkr_platform_device;
+
+static int m68kspkr_event(struct input_dev *dev, unsigned int type, unsigned int code, int value)
+{
+	unsigned int count = 0;
+
+	if (type != EV_SND)
+		return -1;
+
+	switch (code) {
+		case SND_BELL: if (value) value = 1000;
+		case SND_TONE: break;
+		default: return -1;
+	}
+
+	if (value > 20 && value < 32767)
+		count = 1193182 / value;
+
+	mach_beep(count, -1);
+
+	return 0;
+}
+
+static int m68kspkr_probe(struct platform_device *dev)
+{
+	struct input_dev *input_dev;
+	int err;
+
+	input_dev = input_allocate_device();
+	if (!input_dev)
+		return -ENOMEM;
+
+	input_dev->name = "m68k beeper";
+	input_dev->phys = "m68k/generic";
+	input_dev->id.bustype = BUS_HOST;
+	input_dev->id.vendor  = 0x001f;
+	input_dev->id.product = 0x0001;
+	input_dev->id.version = 0x0100;
+	input_dev->dev.parent = &dev->dev;
+
+	input_dev->evbit[0] = BIT_MASK(EV_SND);
+	input_dev->sndbit[0] = BIT_MASK(SND_BELL) | BIT_MASK(SND_TONE);
+	input_dev->event = m68kspkr_event;
+
+	err = input_register_device(input_dev);
+	if (err) {
+		input_free_device(input_dev);
+		return err;
+	}
+
+	platform_set_drvdata(dev, input_dev);
+
+	return 0;
+}
+
+static void m68kspkr_remove(struct platform_device *dev)
+{
+	struct input_dev *input_dev = platform_get_drvdata(dev);
+
+	input_unregister_device(input_dev);
+	/* turn off the speaker */
+	m68kspkr_event(NULL, EV_SND, SND_BELL, 0);
+}
+
+static void m68kspkr_shutdown(struct platform_device *dev)
+{
+	/* turn off the speaker */
+	m68kspkr_event(NULL, EV_SND, SND_BELL, 0);
+}
+
+static struct platform_driver m68kspkr_platform_driver = {
+	.driver		= {
+		.name	= "m68kspkr",
+	},
+	.probe		= m68kspkr_probe,
+	.remove		= m68kspkr_remove,
+	.shutdown	= m68kspkr_shutdown,
+};
+
+static int __init m68kspkr_init(void)
+{
+	int err;
+
+	if (!mach_beep) {
+		printk(KERN_INFO "m68kspkr: no lowlevel beep support\n");
+		return -ENODEV;
+        }
+
+	err = platform_driver_register(&m68kspkr_platform_driver);
+	if (err)
+		return err;
+
+	m68kspkr_platform_device = platform_device_alloc("m68kspkr", -1);
+	if (!m68kspkr_platform_device) {
+		err = -ENOMEM;
+		goto err_unregister_driver;
+	}
+
+	err = platform_device_add(m68kspkr_platform_device);
+	if (err)
+		goto err_free_device;
+
+	return 0;
+
+ err_free_device:
+	platform_device_put(m68kspkr_platform_device);
+ err_unregister_driver:
+	platform_driver_unregister(&m68kspkr_platform_driver);
+
+	return err;
+}
+
+static void __exit m68kspkr_exit(void)
+{
+	platform_device_unregister(m68kspkr_platform_device);
+	platform_driver_unregister(&m68kspkr_platform_driver);
+}
+
+module_init(m68kspkr_init);
+module_exit(m68kspkr_exit);