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, 152 insertions, 0 deletions

diff --git a/drivers/input/serio/rpckbd.c b/drivers/input/serio/rpckbd.c
new file mode 100644
index 000000000..4d817850b
--- /dev/null
+++ b/drivers/input/serio/rpckbd.c
@@ -0,0 +1,152 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ *  Copyright (c) 2000-2001 Vojtech Pavlik
+ *  Copyright (c) 2002 Russell King
+ */
+
+/*
+ * Acorn RiscPC PS/2 keyboard controller driver for Linux/ARM
+ */
+
+#include <linux/module.h>
+#include <linux/interrupt.h>
+#include <linux/serio.h>
+#include <linux/err.h>
+#include <linux/platform_device.h>
+#include <linux/io.h>
+#include <linux/slab.h>
+
+#include <mach/hardware.h>
+#include <asm/hardware/iomd.h>
+
+MODULE_AUTHOR("Vojtech Pavlik, Russell King");
+MODULE_DESCRIPTION("Acorn RiscPC PS/2 keyboard controller driver");
+MODULE_LICENSE("GPL");
+MODULE_ALIAS("platform:kart");
+
+struct rpckbd_data {
+	int tx_irq;
+	int rx_irq;
+};
+
+static int rpckbd_write(struct serio *port, unsigned char val)
+{
+	while (!(iomd_readb(IOMD_KCTRL) & (1 << 7)))
+		cpu_relax();
+
+	iomd_writeb(val, IOMD_KARTTX);
+
+	return 0;
+}
+
+static irqreturn_t rpckbd_rx(int irq, void *dev_id)
+{
+	struct serio *port = dev_id;
+	unsigned int byte;
+	int handled = IRQ_NONE;
+
+	while (iomd_readb(IOMD_KCTRL) & (1 << 5)) {
+		byte = iomd_readb(IOMD_KARTRX);
+
+		serio_interrupt(port, byte, 0);
+		handled = IRQ_HANDLED;
+	}
+	return handled;
+}
+
+static irqreturn_t rpckbd_tx(int irq, void *dev_id)
+{
+	return IRQ_HANDLED;
+}
+
+static int rpckbd_open(struct serio *port)
+{
+	struct rpckbd_data *rpckbd = port->port_data;
+
+	/* Reset the keyboard state machine. */
+	iomd_writeb(0, IOMD_KCTRL);
+	iomd_writeb(8, IOMD_KCTRL);
+	iomd_readb(IOMD_KARTRX);
+
+	if (request_irq(rpckbd->rx_irq, rpckbd_rx, 0, "rpckbd", port) != 0) {
+		printk(KERN_ERR "rpckbd.c: Could not allocate keyboard receive IRQ\n");
+		return -EBUSY;
+	}
+
+	if (request_irq(rpckbd->tx_irq, rpckbd_tx, 0, "rpckbd", port) != 0) {
+		printk(KERN_ERR "rpckbd.c: Could not allocate keyboard transmit IRQ\n");
+		free_irq(rpckbd->rx_irq, port);
+		return -EBUSY;
+	}
+
+	return 0;
+}
+
+static void rpckbd_close(struct serio *port)
+{
+	struct rpckbd_data *rpckbd = port->port_data;
+
+	free_irq(rpckbd->rx_irq, port);
+	free_irq(rpckbd->tx_irq, port);
+}
+
+/*
+ * Allocate and initialize serio structure for subsequent registration
+ * with serio core.
+ */
+static int rpckbd_probe(struct platform_device *dev)
+{
+	struct rpckbd_data *rpckbd;
+	struct serio *serio;
+	int tx_irq, rx_irq;
+
+	rx_irq = platform_get_irq(dev, 0);
+	if (rx_irq < 0)
+		return rx_irq;
+
+	tx_irq = platform_get_irq(dev, 1);
+	if (tx_irq < 0)
+		return tx_irq;
+
+	serio = kzalloc_obj(*serio);
+	rpckbd = kzalloc_obj(*rpckbd);
+	if (!serio || !rpckbd) {
+		kfree(rpckbd);
+		kfree(serio);
+		return -ENOMEM;
+	}
+
+	rpckbd->rx_irq = rx_irq;
+	rpckbd->tx_irq = tx_irq;
+
+	serio->id.type		= SERIO_8042;
+	serio->write		= rpckbd_write;
+	serio->open		= rpckbd_open;
+	serio->close		= rpckbd_close;
+	serio->dev.parent	= &dev->dev;
+	serio->port_data	= rpckbd;
+	strscpy(serio->name, "RiscPC PS/2 kbd port", sizeof(serio->name));
+	strscpy(serio->phys, "rpckbd/serio0", sizeof(serio->phys));
+
+	platform_set_drvdata(dev, serio);
+	serio_register_port(serio);
+	return 0;
+}
+
+static void rpckbd_remove(struct platform_device *dev)
+{
+	struct serio *serio = platform_get_drvdata(dev);
+	struct rpckbd_data *rpckbd = serio->port_data;
+
+	serio_unregister_port(serio);
+	kfree(rpckbd);
+}
+
+static struct platform_driver rpckbd_driver = {
+	.probe		= rpckbd_probe,
+	.remove		= rpckbd_remove,
+	.driver		= {
+		.name	= "kart",
+	},
+};
+module_platform_driver(rpckbd_driver);