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, 214 insertions, 0 deletions

diff --git a/drivers/gnss/usb.c b/drivers/gnss/usb.c
new file mode 100644
index 000000000..0d9dde023
--- /dev/null
+++ b/drivers/gnss/usb.c
@@ -0,0 +1,214 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Generic USB GNSS receiver driver
+ *
+ * Copyright (C) 2021 Johan Hovold <johan@kernel.org>
+ */
+
+#include <linux/errno.h>
+#include <linux/gnss.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/usb.h>
+
+#define GNSS_USB_READ_BUF_LEN	512
+#define GNSS_USB_WRITE_TIMEOUT	1000
+
+static const struct usb_device_id gnss_usb_id_table[] = {
+	{ USB_DEVICE(0x1199, 0xb000) },		/* Sierra Wireless XM1210 */
+	{ }
+};
+MODULE_DEVICE_TABLE(usb, gnss_usb_id_table);
+
+struct gnss_usb {
+	struct usb_device *udev;
+	struct usb_interface *intf;
+	struct gnss_device *gdev;
+	struct urb *read_urb;
+	unsigned int write_pipe;
+};
+
+static void gnss_usb_rx_complete(struct urb *urb)
+{
+	struct gnss_usb *gusb = urb->context;
+	struct gnss_device *gdev = gusb->gdev;
+	int status = urb->status;
+	int len;
+	int ret;
+
+	switch (status) {
+	case 0:
+		break;
+	case -ENOENT:
+	case -ECONNRESET:
+	case -ESHUTDOWN:
+		dev_dbg(&gdev->dev, "urb stopped: %d\n", status);
+		return;
+	case -EPIPE:
+		dev_err(&gdev->dev, "urb stopped: %d\n", status);
+		return;
+	default:
+		dev_dbg(&gdev->dev, "nonzero urb status: %d\n", status);
+		goto resubmit;
+	}
+
+	len = urb->actual_length;
+	if (len == 0)
+		goto resubmit;
+
+	ret = gnss_insert_raw(gdev, urb->transfer_buffer, len);
+	if (ret < len)
+		dev_dbg(&gdev->dev, "dropped %d bytes\n", len - ret);
+resubmit:
+	ret = usb_submit_urb(urb, GFP_ATOMIC);
+	if (ret && ret != -EPERM && ret != -ENODEV)
+		dev_err(&gdev->dev, "failed to resubmit urb: %d\n", ret);
+}
+
+static int gnss_usb_open(struct gnss_device *gdev)
+{
+	struct gnss_usb *gusb = gnss_get_drvdata(gdev);
+	int ret;
+
+	ret = usb_submit_urb(gusb->read_urb, GFP_KERNEL);
+	if (ret) {
+		if (ret != -EPERM && ret != -ENODEV)
+			dev_err(&gdev->dev, "failed to submit urb: %d\n", ret);
+		return ret;
+	}
+
+	return 0;
+}
+
+static void gnss_usb_close(struct gnss_device *gdev)
+{
+	struct gnss_usb *gusb = gnss_get_drvdata(gdev);
+
+	usb_kill_urb(gusb->read_urb);
+}
+
+static int gnss_usb_write_raw(struct gnss_device *gdev,
+		const unsigned char *buf, size_t count)
+{
+	struct gnss_usb *gusb = gnss_get_drvdata(gdev);
+	void *tbuf;
+	int ret;
+
+	tbuf = kmemdup(buf, count, GFP_KERNEL);
+	if (!tbuf)
+		return -ENOMEM;
+
+	ret = usb_bulk_msg(gusb->udev, gusb->write_pipe, tbuf, count, NULL,
+			GNSS_USB_WRITE_TIMEOUT);
+	kfree(tbuf);
+	if (ret)
+		return ret;
+
+	return count;
+}
+
+static const struct gnss_operations gnss_usb_gnss_ops = {
+	.open		= gnss_usb_open,
+	.close		= gnss_usb_close,
+	.write_raw	= gnss_usb_write_raw,
+};
+
+static int gnss_usb_probe(struct usb_interface *intf, const struct usb_device_id *id)
+{
+	struct usb_device *udev = interface_to_usbdev(intf);
+	struct usb_endpoint_descriptor *in, *out;
+	struct gnss_device *gdev;
+	struct gnss_usb *gusb;
+	struct urb *urb;
+	size_t buf_len;
+	void *buf;
+	int ret;
+
+	ret = usb_find_common_endpoints(intf->cur_altsetting, &in, &out, NULL,
+			NULL);
+	if (ret)
+		return ret;
+
+	gusb = kzalloc_obj(*gusb);
+	if (!gusb)
+		return -ENOMEM;
+
+	gdev = gnss_allocate_device(&intf->dev);
+	if (!gdev) {
+		ret = -ENOMEM;
+		goto err_free_gusb;
+	}
+
+	gdev->ops = &gnss_usb_gnss_ops;
+	gdev->type = GNSS_TYPE_NMEA;
+	gnss_set_drvdata(gdev, gusb);
+
+	urb = usb_alloc_urb(0, GFP_KERNEL);
+	if (!urb) {
+		ret = -ENOMEM;
+		goto err_put_gdev;
+	}
+
+	buf_len = max(usb_endpoint_maxp(in), GNSS_USB_READ_BUF_LEN);
+
+	buf = kzalloc(buf_len, GFP_KERNEL);
+	if (!buf) {
+		ret = -ENOMEM;
+		goto err_free_urb;
+	}
+
+	usb_fill_bulk_urb(urb, udev,
+			usb_rcvbulkpipe(udev, usb_endpoint_num(in)),
+			buf, buf_len, gnss_usb_rx_complete, gusb);
+
+	gusb->intf = intf;
+	gusb->udev = udev;
+	gusb->gdev = gdev;
+	gusb->read_urb = urb;
+	gusb->write_pipe = usb_sndbulkpipe(udev, usb_endpoint_num(out));
+
+	ret = gnss_register_device(gdev);
+	if (ret)
+		goto err_free_buf;
+
+	usb_set_intfdata(intf, gusb);
+
+	return 0;
+
+err_free_buf:
+	kfree(buf);
+err_free_urb:
+	usb_free_urb(urb);
+err_put_gdev:
+	gnss_put_device(gdev);
+err_free_gusb:
+	kfree(gusb);
+
+	return ret;
+}
+
+static void gnss_usb_disconnect(struct usb_interface *intf)
+{
+	struct gnss_usb *gusb = usb_get_intfdata(intf);
+
+	gnss_deregister_device(gusb->gdev);
+
+	kfree(gusb->read_urb->transfer_buffer);
+	usb_free_urb(gusb->read_urb);
+	gnss_put_device(gusb->gdev);
+	kfree(gusb);
+}
+
+static struct usb_driver gnss_usb_driver = {
+	.name		= "gnss-usb",
+	.probe		= gnss_usb_probe,
+	.disconnect	= gnss_usb_disconnect,
+	.id_table	= gnss_usb_id_table,
+};
+module_usb_driver(gnss_usb_driver);
+
+MODULE_AUTHOR("Johan Hovold <johan@kernel.org>");
+MODULE_DESCRIPTION("Generic USB GNSS receiver driver");
+MODULE_LICENSE("GPL v2");