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, 293 insertions, 0 deletions

diff --git a/drivers/input/serio/ps2mult.c b/drivers/input/serio/ps2mult.c
new file mode 100644
index 000000000..27d554b59
--- /dev/null
+++ b/drivers/input/serio/ps2mult.c
@@ -0,0 +1,293 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * TQC PS/2 Multiplexer driver
+ *
+ * Copyright (C) 2010 Dmitry Eremin-Solenikov
+ */
+
+
+#include <linux/kernel.h>
+#include <linux/slab.h>
+#include <linux/module.h>
+#include <linux/serio.h>
+
+MODULE_AUTHOR("Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>");
+MODULE_DESCRIPTION("TQC PS/2 Multiplexer driver");
+MODULE_LICENSE("GPL");
+
+#define PS2MULT_KB_SELECTOR		0xA0
+#define PS2MULT_MS_SELECTOR		0xA1
+#define PS2MULT_ESCAPE			0x7D
+#define PS2MULT_BSYNC			0x7E
+#define PS2MULT_SESSION_START		0x55
+#define PS2MULT_SESSION_END		0x56
+
+struct ps2mult_port {
+	struct serio *serio;
+	unsigned char sel;
+	bool registered;
+};
+
+#define PS2MULT_NUM_PORTS	2
+#define PS2MULT_KBD_PORT	0
+#define PS2MULT_MOUSE_PORT	1
+
+struct ps2mult {
+	struct serio *mx_serio;
+	struct ps2mult_port ports[PS2MULT_NUM_PORTS];
+
+	spinlock_t lock;
+	struct ps2mult_port *in_port;
+	struct ps2mult_port *out_port;
+	bool escape;
+};
+
+/* First MUST come PS2MULT_NUM_PORTS selectors */
+static const unsigned char ps2mult_controls[] = {
+	PS2MULT_KB_SELECTOR, PS2MULT_MS_SELECTOR,
+	PS2MULT_ESCAPE, PS2MULT_BSYNC,
+	PS2MULT_SESSION_START, PS2MULT_SESSION_END,
+};
+
+static const struct serio_device_id ps2mult_serio_ids[] = {
+	{
+		.type	= SERIO_RS232,
+		.proto	= SERIO_PS2MULT,
+		.id	= SERIO_ANY,
+		.extra	= SERIO_ANY,
+	},
+	{ 0 }
+};
+
+MODULE_DEVICE_TABLE(serio, ps2mult_serio_ids);
+
+static void ps2mult_select_port(struct ps2mult *psm, struct ps2mult_port *port)
+{
+	struct serio *mx_serio = psm->mx_serio;
+
+	serio_write(mx_serio, port->sel);
+	psm->out_port = port;
+	dev_dbg(&mx_serio->dev, "switched to sel %02x\n", port->sel);
+}
+
+static int ps2mult_serio_write(struct serio *serio, unsigned char data)
+{
+	struct serio *mx_port = serio->parent;
+	struct ps2mult *psm = serio_get_drvdata(mx_port);
+	struct ps2mult_port *port = serio->port_data;
+	bool need_escape;
+
+	guard(spinlock_irqsave)(&psm->lock);
+
+	if (psm->out_port != port)
+		ps2mult_select_port(psm, port);
+
+	need_escape = memchr(ps2mult_controls, data, sizeof(ps2mult_controls));
+
+	dev_dbg(&serio->dev,
+		"write: %s%02x\n", need_escape ? "ESC " : "", data);
+
+	if (need_escape)
+		serio_write(mx_port, PS2MULT_ESCAPE);
+
+	serio_write(mx_port, data);
+
+	return 0;
+}
+
+static int ps2mult_serio_start(struct serio *serio)
+{
+	struct ps2mult *psm = serio_get_drvdata(serio->parent);
+	struct ps2mult_port *port = serio->port_data;
+
+	guard(spinlock_irqsave)(&psm->lock);
+
+	port->registered = true;
+
+	return 0;
+}
+
+static void ps2mult_serio_stop(struct serio *serio)
+{
+	struct ps2mult *psm = serio_get_drvdata(serio->parent);
+	struct ps2mult_port *port = serio->port_data;
+
+	guard(spinlock_irqsave)(&psm->lock);
+
+	port->registered = false;
+}
+
+static int ps2mult_create_port(struct ps2mult *psm, int i)
+{
+	struct serio *mx_serio = psm->mx_serio;
+	struct serio *serio;
+
+	serio = kzalloc_obj(*serio);
+	if (!serio)
+		return -ENOMEM;
+
+	strscpy(serio->name, "TQC PS/2 Multiplexer", sizeof(serio->name));
+	snprintf(serio->phys, sizeof(serio->phys),
+		 "%s/port%d", mx_serio->phys, i);
+	serio->id.type = SERIO_8042;
+	serio->write = ps2mult_serio_write;
+	serio->start = ps2mult_serio_start;
+	serio->stop = ps2mult_serio_stop;
+	serio->parent = psm->mx_serio;
+	serio->port_data = &psm->ports[i];
+
+	psm->ports[i].serio = serio;
+
+	return 0;
+}
+
+static void ps2mult_reset(struct ps2mult *psm)
+{
+	guard(spinlock_irqsave)(&psm->lock);
+
+	serio_write(psm->mx_serio, PS2MULT_SESSION_END);
+	serio_write(psm->mx_serio, PS2MULT_SESSION_START);
+
+	ps2mult_select_port(psm, &psm->ports[PS2MULT_KBD_PORT]);
+}
+
+static int ps2mult_connect(struct serio *serio, struct serio_driver *drv)
+{
+	struct ps2mult *psm;
+	int i;
+	int error;
+
+	if (!serio->write)
+		return -EINVAL;
+
+	psm = kzalloc_obj(*psm);
+	if (!psm)
+		return -ENOMEM;
+
+	spin_lock_init(&psm->lock);
+	psm->mx_serio = serio;
+
+	for (i = 0; i < PS2MULT_NUM_PORTS; i++) {
+		psm->ports[i].sel = ps2mult_controls[i];
+		error = ps2mult_create_port(psm, i);
+		if (error)
+			goto err_out;
+	}
+
+	psm->in_port = psm->out_port = &psm->ports[PS2MULT_KBD_PORT];
+
+	serio_set_drvdata(serio, psm);
+	error = serio_open(serio, drv);
+	if (error)
+		goto err_out;
+
+	ps2mult_reset(psm);
+
+	for (i = 0; i <  PS2MULT_NUM_PORTS; i++) {
+		struct serio *s = psm->ports[i].serio;
+
+		dev_info(&serio->dev, "%s port at %s\n", s->name, serio->phys);
+		serio_register_port(s);
+	}
+
+	return 0;
+
+err_out:
+	while (--i >= 0)
+		kfree(psm->ports[i].serio);
+	kfree(psm);
+	return error;
+}
+
+static void ps2mult_disconnect(struct serio *serio)
+{
+	struct ps2mult *psm = serio_get_drvdata(serio);
+
+	/* Note that serio core already take care of children ports */
+	serio_write(serio, PS2MULT_SESSION_END);
+	serio_close(serio);
+	kfree(psm);
+
+	serio_set_drvdata(serio, NULL);
+}
+
+static int ps2mult_reconnect(struct serio *serio)
+{
+	struct ps2mult *psm = serio_get_drvdata(serio);
+
+	ps2mult_reset(psm);
+
+	return 0;
+}
+
+static irqreturn_t ps2mult_interrupt(struct serio *serio,
+				     unsigned char data, unsigned int dfl)
+{
+	struct ps2mult *psm = serio_get_drvdata(serio);
+	struct ps2mult_port *in_port;
+
+	dev_dbg(&serio->dev, "Received %02x flags %02x\n", data, dfl);
+
+	guard(spinlock_irqsave)(&psm->lock);
+
+	if (psm->escape) {
+		psm->escape = false;
+		in_port = psm->in_port;
+		if (in_port->registered)
+			serio_interrupt(in_port->serio, data, dfl);
+		goto out;
+	}
+
+	switch (data) {
+	case PS2MULT_ESCAPE:
+		dev_dbg(&serio->dev, "ESCAPE\n");
+		psm->escape = true;
+		break;
+
+	case PS2MULT_BSYNC:
+		dev_dbg(&serio->dev, "BSYNC\n");
+		psm->in_port = psm->out_port;
+		break;
+
+	case PS2MULT_SESSION_START:
+		dev_dbg(&serio->dev, "SS\n");
+		break;
+
+	case PS2MULT_SESSION_END:
+		dev_dbg(&serio->dev, "SE\n");
+		break;
+
+	case PS2MULT_KB_SELECTOR:
+		dev_dbg(&serio->dev, "KB\n");
+		psm->in_port = &psm->ports[PS2MULT_KBD_PORT];
+		break;
+
+	case PS2MULT_MS_SELECTOR:
+		dev_dbg(&serio->dev, "MS\n");
+		psm->in_port = &psm->ports[PS2MULT_MOUSE_PORT];
+		break;
+
+	default:
+		in_port = psm->in_port;
+		if (in_port->registered)
+			serio_interrupt(in_port->serio, data, dfl);
+		break;
+	}
+
+ out:
+	return IRQ_HANDLED;
+}
+
+static struct serio_driver ps2mult_drv = {
+	.driver		= {
+		.name	= "ps2mult",
+	},
+	.description	= "TQC PS/2 Multiplexer driver",
+	.id_table	= ps2mult_serio_ids,
+	.interrupt	= ps2mult_interrupt,
+	.connect	= ps2mult_connect,
+	.disconnect	= ps2mult_disconnect,
+	.reconnect	= ps2mult_reconnect,
+};
+
+module_serio_driver(ps2mult_drv);