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, 171 insertions, 0 deletions

diff --git a/drivers/usb/serial/bus.c b/drivers/usb/serial/bus.c
new file mode 100644
index 000000000..9e2a18c0b
--- /dev/null
+++ b/drivers/usb/serial/bus.c
@@ -0,0 +1,171 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * USB Serial Converter Bus specific functions
+ *
+ * Copyright (C) 2002 Greg Kroah-Hartman (greg@kroah.com)
+ */
+
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/tty.h>
+#include <linux/slab.h>
+#include <linux/module.h>
+#include <linux/usb.h>
+#include <linux/usb/serial.h>
+
+static int usb_serial_device_match(struct device *dev,
+				   const struct device_driver *drv)
+{
+	const struct usb_serial_port *port = to_usb_serial_port(dev);
+	const struct usb_serial_driver *driver = to_usb_serial_driver(drv);
+
+	/*
+	 * drivers are already assigned to ports in serial_probe so it's
+	 * a simple check here.
+	 */
+	if (driver == port->serial->type)
+		return 1;
+
+	return 0;
+}
+
+static int usb_serial_device_probe(struct device *dev)
+{
+	struct usb_serial_port *port = to_usb_serial_port(dev);
+	struct usb_serial_driver *driver;
+	struct device *tty_dev;
+	int retval = 0;
+	int minor;
+
+	/* make sure suspend/resume doesn't race against port_probe */
+	retval = usb_autopm_get_interface(port->serial->interface);
+	if (retval)
+		return retval;
+
+	driver = port->serial->type;
+	if (driver->port_probe) {
+		retval = driver->port_probe(port);
+		if (retval)
+			goto err_autopm_put;
+	}
+
+	minor = port->minor;
+	tty_dev = tty_port_register_device(&port->port, usb_serial_tty_driver,
+					   minor, dev);
+	if (IS_ERR(tty_dev)) {
+		retval = PTR_ERR(tty_dev);
+		goto err_port_remove;
+	}
+
+	usb_autopm_put_interface(port->serial->interface);
+
+	dev_info(&port->serial->dev->dev,
+		 "%s converter now attached to ttyUSB%d\n",
+		 driver->description, minor);
+
+	return 0;
+
+err_port_remove:
+	if (driver->port_remove)
+		driver->port_remove(port);
+err_autopm_put:
+	usb_autopm_put_interface(port->serial->interface);
+
+	return retval;
+}
+
+static void usb_serial_device_remove(struct device *dev)
+{
+	struct usb_serial_port *port = to_usb_serial_port(dev);
+	struct usb_serial_driver *driver;
+	int minor;
+	int autopm_err;
+
+	/*
+	 * Make sure suspend/resume doesn't race against port_remove.
+	 *
+	 * Note that no further runtime PM callbacks will be made if
+	 * autopm_get fails.
+	 */
+	autopm_err = usb_autopm_get_interface(port->serial->interface);
+
+	minor = port->minor;
+	tty_unregister_device(usb_serial_tty_driver, minor);
+
+	driver = port->serial->type;
+	if (driver->port_remove)
+		driver->port_remove(port);
+
+	dev_info(dev, "%s converter now disconnected from ttyUSB%d\n",
+		 driver->description, minor);
+
+	if (!autopm_err)
+		usb_autopm_put_interface(port->serial->interface);
+}
+
+static ssize_t new_id_store(struct device_driver *driver,
+			    const char *buf, size_t count)
+{
+	struct usb_serial_driver *usb_drv = to_usb_serial_driver(driver);
+	ssize_t retval = usb_store_new_id(&usb_drv->dynids, usb_drv->id_table,
+					 driver, buf, count);
+
+	if (retval >= 0 && usb_drv->usb_driver != NULL)
+		retval = usb_store_new_id(&usb_drv->usb_driver->dynids,
+					  usb_drv->usb_driver->id_table,
+					  &usb_drv->usb_driver->driver,
+					  buf, count);
+	return retval;
+}
+
+static ssize_t new_id_show(struct device_driver *driver, char *buf)
+{
+	struct usb_serial_driver *usb_drv = to_usb_serial_driver(driver);
+
+	return usb_show_dynids(&usb_drv->dynids, buf);
+}
+static DRIVER_ATTR_RW(new_id);
+
+static struct attribute *usb_serial_drv_attrs[] = {
+	&driver_attr_new_id.attr,
+	NULL,
+};
+ATTRIBUTE_GROUPS(usb_serial_drv);
+
+static void free_dynids(struct usb_serial_driver *drv)
+{
+	struct usb_dynid *dynid, *n;
+
+	guard(mutex)(&usb_dynids_lock);
+	list_for_each_entry_safe(dynid, n, &drv->dynids.list, node) {
+		list_del(&dynid->node);
+		kfree(dynid);
+	}
+}
+
+const struct bus_type usb_serial_bus_type = {
+	.name =		"usb-serial",
+	.match =	usb_serial_device_match,
+	.probe =	usb_serial_device_probe,
+	.remove =	usb_serial_device_remove,
+	.drv_groups = 	usb_serial_drv_groups,
+};
+
+int usb_serial_bus_register(struct usb_serial_driver *driver)
+{
+	int retval;
+
+	driver->driver.bus = &usb_serial_bus_type;
+	INIT_LIST_HEAD(&driver->dynids.list);
+
+	retval = driver_register(&driver->driver);
+
+	return retval;
+}
+
+void usb_serial_bus_deregister(struct usb_serial_driver *driver)
+{
+	free_dynids(driver);
+	driver_unregister(&driver->driver);
+}
+