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, 166 insertions, 0 deletions

diff --git a/drivers/input/serio/q40kbd.c b/drivers/input/serio/q40kbd.c
new file mode 100644
index 000000000..2f553efbe
--- /dev/null
+++ b/drivers/input/serio/q40kbd.c
@@ -0,0 +1,166 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ *  Copyright (c) 2000-2001 Vojtech Pavlik
+ *
+ *  Based on the work of:
+ *	Richard Zidlicky <Richard.Zidlicky@stud.informatik.uni-erlangen.de>
+ */
+
+/*
+ * Q40 PS/2 keyboard controller driver for Linux/m68k
+ */
+
+#include <linux/module.h>
+#include <linux/serio.h>
+#include <linux/interrupt.h>
+#include <linux/err.h>
+#include <linux/bitops.h>
+#include <linux/platform_device.h>
+#include <linux/slab.h>
+
+#include <asm/io.h>
+#include <linux/uaccess.h>
+#include <asm/q40_master.h>
+#include <asm/irq.h>
+#include <asm/q40ints.h>
+
+#define DRV_NAME	"q40kbd"
+
+MODULE_AUTHOR("Vojtech Pavlik <vojtech@ucw.cz>");
+MODULE_DESCRIPTION("Q40 PS/2 keyboard controller driver");
+MODULE_LICENSE("GPL");
+MODULE_ALIAS("platform:" DRV_NAME);
+
+struct q40kbd {
+	struct serio *port;
+	spinlock_t lock;
+};
+
+static irqreturn_t q40kbd_interrupt(int irq, void *dev_id)
+{
+	struct q40kbd *q40kbd = dev_id;
+
+	guard(spinlock_irqsave)(&q40kbd->lock);
+
+	if (Q40_IRQ_KEYB_MASK & master_inb(INTERRUPT_REG))
+		serio_interrupt(q40kbd->port, master_inb(KEYCODE_REG), 0);
+
+	master_outb(-1, KEYBOARD_UNLOCK_REG);
+
+	return IRQ_HANDLED;
+}
+
+/*
+ * q40kbd_flush() flushes all data that may be in the keyboard buffers
+ */
+
+static void q40kbd_flush(struct q40kbd *q40kbd)
+{
+	int maxread = 100;
+
+	guard(spinlock_irqsave)(&q40kbd->lock);
+
+	while (maxread-- && (Q40_IRQ_KEYB_MASK & master_inb(INTERRUPT_REG)))
+		master_inb(KEYCODE_REG);
+}
+
+static void q40kbd_stop(void)
+{
+	master_outb(0, KEY_IRQ_ENABLE_REG);
+	master_outb(-1, KEYBOARD_UNLOCK_REG);
+}
+
+/*
+ * q40kbd_open() is called when a port is open by the higher layer.
+ * It allocates the interrupt and enables in in the chip.
+ */
+
+static int q40kbd_open(struct serio *port)
+{
+	struct q40kbd *q40kbd = port->port_data;
+
+	q40kbd_flush(q40kbd);
+
+	/* off we go */
+	master_outb(-1, KEYBOARD_UNLOCK_REG);
+	master_outb(1, KEY_IRQ_ENABLE_REG);
+
+	return 0;
+}
+
+static void q40kbd_close(struct serio *port)
+{
+	struct q40kbd *q40kbd = port->port_data;
+
+	q40kbd_stop();
+	q40kbd_flush(q40kbd);
+}
+
+static int q40kbd_probe(struct platform_device *pdev)
+{
+	struct q40kbd *q40kbd;
+	struct serio *port;
+	int error;
+
+	q40kbd = kzalloc_obj(*q40kbd);
+	port = kzalloc_obj(*port);
+	if (!q40kbd || !port) {
+		error = -ENOMEM;
+		goto err_free_mem;
+	}
+
+	q40kbd->port = port;
+	spin_lock_init(&q40kbd->lock);
+
+	port->id.type = SERIO_8042;
+	port->open = q40kbd_open;
+	port->close = q40kbd_close;
+	port->port_data = q40kbd;
+	port->dev.parent = &pdev->dev;
+	strscpy(port->name, "Q40 Kbd Port", sizeof(port->name));
+	strscpy(port->phys, "Q40", sizeof(port->phys));
+
+	q40kbd_stop();
+
+	error = request_irq(Q40_IRQ_KEYBOARD, q40kbd_interrupt, 0,
+			    DRV_NAME, q40kbd);
+	if (error) {
+		dev_err(&pdev->dev, "Can't get irq %d.\n", Q40_IRQ_KEYBOARD);
+		goto err_free_mem;
+	}
+
+	serio_register_port(q40kbd->port);
+
+	platform_set_drvdata(pdev, q40kbd);
+	printk(KERN_INFO "serio: Q40 kbd registered\n");
+
+	return 0;
+
+err_free_mem:
+	kfree(port);
+	kfree(q40kbd);
+	return error;
+}
+
+static void q40kbd_remove(struct platform_device *pdev)
+{
+	struct q40kbd *q40kbd = platform_get_drvdata(pdev);
+
+	/*
+	 * q40kbd_close() will be called as part of unregistering
+	 * and will ensure that IRQ is turned off, so it is safe
+	 * to unregister port first and free IRQ later.
+	 */
+	serio_unregister_port(q40kbd->port);
+	free_irq(Q40_IRQ_KEYBOARD, q40kbd);
+	kfree(q40kbd);
+}
+
+static struct platform_driver q40kbd_driver = {
+	.driver		= {
+		.name	= "q40kbd",
+	},
+	.remove		= q40kbd_remove,
+};
+
+module_platform_driver_probe(q40kbd_driver, q40kbd_probe);