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/keyboard/xtkbd.c b/drivers/input/keyboard/xtkbd.c
new file mode 100644
index 000000000..c0b4978a8
--- /dev/null
+++ b/drivers/input/keyboard/xtkbd.c
@@ -0,0 +1,152 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ *  Copyright (c) 1999-2001 Vojtech Pavlik
+ */
+
+/*
+ * XT keyboard driver for Linux
+ */
+
+#include <linux/slab.h>
+#include <linux/module.h>
+#include <linux/input.h>
+#include <linux/serio.h>
+
+#define DRIVER_DESC	"XT keyboard driver"
+
+MODULE_AUTHOR("Vojtech Pavlik <vojtech@ucw.cz>");
+MODULE_DESCRIPTION(DRIVER_DESC);
+MODULE_LICENSE("GPL");
+
+#define XTKBD_EMUL0	0xe0
+#define XTKBD_EMUL1	0xe1
+#define XTKBD_KEY	0x7f
+#define XTKBD_RELEASE	0x80
+
+static unsigned char xtkbd_keycode[256] = {
+	  0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15,
+	 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31,
+	 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47,
+	 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63,
+	 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79,
+	 80, 81, 82, 83,  0,  0,  0, 87, 88,  0,  0,  0,  0,  0,  0,  0,
+	  0,  0,  0,  0,  0, 87, 88,  0,  0,  0,  0,110,111,103,108,105,
+	106
+};
+
+struct xtkbd {
+	unsigned char keycode[256];
+	struct input_dev *dev;
+	struct serio *serio;
+	char phys[32];
+};
+
+static irqreturn_t xtkbd_interrupt(struct serio *serio,
+	unsigned char data, unsigned int flags)
+{
+	struct xtkbd *xtkbd = serio_get_drvdata(serio);
+
+	switch (data) {
+		case XTKBD_EMUL0:
+		case XTKBD_EMUL1:
+			break;
+		default:
+
+			if (xtkbd->keycode[data & XTKBD_KEY]) {
+				input_report_key(xtkbd->dev, xtkbd->keycode[data & XTKBD_KEY], !(data & XTKBD_RELEASE));
+				input_sync(xtkbd->dev);
+			} else {
+				printk(KERN_WARNING "xtkbd.c: Unknown key (scancode %#x) %s.\n",
+					data & XTKBD_KEY, data & XTKBD_RELEASE ? "released" : "pressed");
+			}
+	}
+	return IRQ_HANDLED;
+}
+
+static int xtkbd_connect(struct serio *serio, struct serio_driver *drv)
+{
+	struct xtkbd *xtkbd;
+	struct input_dev *input_dev;
+	int err = -ENOMEM;
+	int i;
+
+	xtkbd = kmalloc_obj(*xtkbd);
+	input_dev = input_allocate_device();
+	if (!xtkbd || !input_dev)
+		goto fail1;
+
+	xtkbd->serio = serio;
+	xtkbd->dev = input_dev;
+	snprintf(xtkbd->phys, sizeof(xtkbd->phys), "%s/input0", serio->phys);
+	memcpy(xtkbd->keycode, xtkbd_keycode, sizeof(xtkbd->keycode));
+
+	input_dev->name = "XT Keyboard";
+	input_dev->phys = xtkbd->phys;
+	input_dev->id.bustype = BUS_XTKBD;
+	input_dev->id.vendor  = 0x0001;
+	input_dev->id.product = 0x0001;
+	input_dev->id.version = 0x0100;
+	input_dev->dev.parent = &serio->dev;
+
+	input_dev->evbit[0] = BIT_MASK(EV_KEY) | BIT_MASK(EV_REP);
+	input_dev->keycode = xtkbd->keycode;
+	input_dev->keycodesize = sizeof(unsigned char);
+	input_dev->keycodemax = ARRAY_SIZE(xtkbd_keycode);
+
+	for (i = 0; i < 255; i++)
+		set_bit(xtkbd->keycode[i], input_dev->keybit);
+	clear_bit(0, input_dev->keybit);
+
+	serio_set_drvdata(serio, xtkbd);
+
+	err = serio_open(serio, drv);
+	if (err)
+		goto fail2;
+
+	err = input_register_device(xtkbd->dev);
+	if (err)
+		goto fail3;
+
+	return 0;
+
+ fail3:	serio_close(serio);
+ fail2:	serio_set_drvdata(serio, NULL);
+ fail1:	input_free_device(input_dev);
+	kfree(xtkbd);
+	return err;
+}
+
+static void xtkbd_disconnect(struct serio *serio)
+{
+	struct xtkbd *xtkbd = serio_get_drvdata(serio);
+
+	serio_close(serio);
+	serio_set_drvdata(serio, NULL);
+	input_unregister_device(xtkbd->dev);
+	kfree(xtkbd);
+}
+
+static const struct serio_device_id xtkbd_serio_ids[] = {
+	{
+		.type	= SERIO_XT,
+		.proto	= SERIO_ANY,
+		.id	= SERIO_ANY,
+		.extra	= SERIO_ANY,
+	},
+	{ 0 }
+};
+
+MODULE_DEVICE_TABLE(serio, xtkbd_serio_ids);
+
+static struct serio_driver xtkbd_drv = {
+	.driver		= {
+		.name	= "xtkbd",
+	},
+	.description	= DRIVER_DESC,
+	.id_table	= xtkbd_serio_ids,
+	.interrupt	= xtkbd_interrupt,
+	.connect	= xtkbd_connect,
+	.disconnect	= xtkbd_disconnect,
+};
+
+module_serio_driver(xtkbd_drv);