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, 113 insertions, 0 deletions

diff --git a/drivers/input/mouse/rpcmouse.c b/drivers/input/mouse/rpcmouse.c
new file mode 100644
index 000000000..6774029e0
--- /dev/null
+++ b/drivers/input/mouse/rpcmouse.c
@@ -0,0 +1,113 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ *  Acorn RiscPC mouse driver for Linux/ARM
+ *
+ *  Copyright (c) 2000-2002 Vojtech Pavlik
+ *  Copyright (C) 1996-2002 Russell King
+ */
+
+/*
+ *
+ * This handles the Acorn RiscPCs mouse.  We basically have a couple of
+ * hardware registers that track the sensor count for the X-Y movement and
+ * another register holding the button state.  On every VSYNC interrupt we read
+ * the complete state and then work out if something has changed.
+ */
+
+#include <linux/module.h>
+#include <linux/ptrace.h>
+#include <linux/interrupt.h>
+#include <linux/init.h>
+#include <linux/input.h>
+#include <linux/io.h>
+
+#include <mach/hardware.h>
+#include <asm/irq.h>
+#include <asm/hardware/iomd.h>
+
+MODULE_AUTHOR("Vojtech Pavlik, Russell King");
+MODULE_DESCRIPTION("Acorn RiscPC mouse driver");
+MODULE_LICENSE("GPL");
+
+static short rpcmouse_lastx, rpcmouse_lasty;
+static struct input_dev *rpcmouse_dev;
+
+static irqreturn_t rpcmouse_irq(int irq, void *dev_id)
+{
+	struct input_dev *dev = dev_id;
+	short x, y, dx, dy, b;
+
+	x = (short) iomd_readl(IOMD_MOUSEX);
+	y = (short) iomd_readl(IOMD_MOUSEY);
+	b = (short) (__raw_readl(IOMEM(0xe0310000)) ^ 0x70);
+
+	dx = x - rpcmouse_lastx;
+	dy = y - rpcmouse_lasty;
+
+	rpcmouse_lastx = x;
+	rpcmouse_lasty = y;
+
+	input_report_rel(dev, REL_X, dx);
+	input_report_rel(dev, REL_Y, -dy);
+
+	input_report_key(dev, BTN_LEFT,   b & 0x40);
+	input_report_key(dev, BTN_MIDDLE, b & 0x20);
+	input_report_key(dev, BTN_RIGHT,  b & 0x10);
+
+	input_sync(dev);
+
+	return IRQ_HANDLED;
+}
+
+
+static int __init rpcmouse_init(void)
+{
+	int err;
+
+	rpcmouse_dev = input_allocate_device();
+	if (!rpcmouse_dev)
+		return -ENOMEM;
+
+	rpcmouse_dev->name = "Acorn RiscPC Mouse";
+	rpcmouse_dev->phys = "rpcmouse/input0";
+	rpcmouse_dev->id.bustype = BUS_HOST;
+	rpcmouse_dev->id.vendor  = 0x0005;
+	rpcmouse_dev->id.product = 0x0001;
+	rpcmouse_dev->id.version = 0x0100;
+
+	rpcmouse_dev->evbit[0] = BIT_MASK(EV_KEY) | BIT_MASK(EV_REL);
+	rpcmouse_dev->keybit[BIT_WORD(BTN_LEFT)] = BIT_MASK(BTN_LEFT) |
+		BIT_MASK(BTN_MIDDLE) | BIT_MASK(BTN_RIGHT);
+	rpcmouse_dev->relbit[0]	= BIT_MASK(REL_X) | BIT_MASK(REL_Y);
+
+	rpcmouse_lastx = (short) iomd_readl(IOMD_MOUSEX);
+	rpcmouse_lasty = (short) iomd_readl(IOMD_MOUSEY);
+
+	if (request_irq(IRQ_VSYNCPULSE, rpcmouse_irq, IRQF_SHARED, "rpcmouse", rpcmouse_dev)) {
+		printk(KERN_ERR "rpcmouse: unable to allocate VSYNC interrupt\n");
+		err = -EBUSY;
+		goto err_free_dev;
+	}
+
+	err = input_register_device(rpcmouse_dev);
+	if (err)
+		goto err_free_irq;
+
+	return 0;
+
+ err_free_irq:
+	free_irq(IRQ_VSYNCPULSE, rpcmouse_dev);
+ err_free_dev:
+	input_free_device(rpcmouse_dev);
+
+	return err;
+}
+
+static void __exit rpcmouse_exit(void)
+{
+	free_irq(IRQ_VSYNCPULSE, rpcmouse_dev);
+	input_unregister_device(rpcmouse_dev);
+}
+
+module_init(rpcmouse_init);
+module_exit(rpcmouse_exit);