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, 216 insertions, 0 deletions

diff --git a/drivers/usb/atm/xusbatm.c b/drivers/usb/atm/xusbatm.c
new file mode 100644
index 000000000..0befbf63d
--- /dev/null
+++ b/drivers/usb/atm/xusbatm.c
@@ -0,0 +1,216 @@
+// SPDX-License-Identifier: GPL-2.0+
+/******************************************************************************
+ *  xusbatm.c -	dumb usbatm-based driver for modems initialized in userspace
+ *
+ *  Copyright (C) 2005 Duncan Sands, Roman Kagan (rkagan % mail ! ru)
+ ******************************************************************************/
+
+#include <linux/module.h>
+#include <linux/etherdevice.h>		/* for eth_random_addr() */
+
+#include "usbatm.h"
+
+
+#define XUSBATM_DRIVERS_MAX	8
+
+#define XUSBATM_PARM(name, type, parmtype, desc) \
+	static type name[XUSBATM_DRIVERS_MAX]; \
+	static unsigned int num_##name; \
+	module_param_array(name, parmtype, &num_##name, 0444); \
+	MODULE_PARM_DESC(name, desc)
+
+XUSBATM_PARM(vendor, unsigned short, ushort, "USB device vendor");
+XUSBATM_PARM(product, unsigned short, ushort, "USB device product");
+
+XUSBATM_PARM(rx_endpoint, unsigned char, byte, "rx endpoint number");
+XUSBATM_PARM(tx_endpoint, unsigned char, byte, "tx endpoint number");
+XUSBATM_PARM(rx_padding, unsigned char, byte, "rx padding (default 0)");
+XUSBATM_PARM(tx_padding, unsigned char, byte, "tx padding (default 0)");
+XUSBATM_PARM(rx_altsetting, unsigned char, byte, "rx altsetting (default 0)");
+XUSBATM_PARM(tx_altsetting, unsigned char, byte, "rx altsetting (default 0)");
+
+static const char xusbatm_driver_name[] = "xusbatm";
+
+static struct usbatm_driver xusbatm_drivers[XUSBATM_DRIVERS_MAX];
+static struct usb_device_id xusbatm_usb_ids[XUSBATM_DRIVERS_MAX + 1];
+static struct usb_driver xusbatm_usb_driver;
+
+static struct usb_interface *xusbatm_find_intf(struct usb_device *usb_dev, int altsetting, u8 ep)
+{
+	struct usb_host_interface *alt;
+	struct usb_interface *intf;
+	int i, j;
+
+	for (i = 0; i < usb_dev->actconfig->desc.bNumInterfaces; i++)
+		if ((intf = usb_dev->actconfig->interface[i]) && (alt = usb_altnum_to_altsetting(intf, altsetting)))
+			for (j = 0; j < alt->desc.bNumEndpoints; j++)
+				if (alt->endpoint[j].desc.bEndpointAddress == ep)
+					return intf;
+	return NULL;
+}
+
+static int xusbatm_capture_intf(struct usbatm_data *usbatm, struct usb_device *usb_dev,
+		struct usb_interface *intf, int altsetting, int claim)
+{
+	int ifnum = intf->altsetting->desc.bInterfaceNumber;
+	int ret;
+
+	if (claim && (ret = usb_driver_claim_interface(&xusbatm_usb_driver, intf, usbatm))) {
+		usb_err(usbatm, "%s: failed to claim interface %2d (%d)!\n", __func__, ifnum, ret);
+		return ret;
+	}
+	ret = usb_set_interface(usb_dev, ifnum, altsetting);
+	if (ret) {
+		usb_err(usbatm, "%s: altsetting %2d for interface %2d failed (%d)!\n", __func__, altsetting, ifnum, ret);
+		return ret;
+	}
+	return 0;
+}
+
+static void xusbatm_release_intf(struct usb_device *usb_dev, struct usb_interface *intf, int claimed)
+{
+	if (claimed) {
+		usb_set_intfdata(intf, NULL);
+		usb_driver_release_interface(&xusbatm_usb_driver, intf);
+	}
+}
+
+static int xusbatm_bind(struct usbatm_data *usbatm,
+			struct usb_interface *intf, const struct usb_device_id *id)
+{
+	struct usb_device *usb_dev = interface_to_usbdev(intf);
+	int drv_ix = id - xusbatm_usb_ids;
+	int rx_alt = rx_altsetting[drv_ix];
+	int tx_alt = tx_altsetting[drv_ix];
+	struct usb_interface *rx_intf = xusbatm_find_intf(usb_dev, rx_alt, rx_endpoint[drv_ix]);
+	struct usb_interface *tx_intf = xusbatm_find_intf(usb_dev, tx_alt, tx_endpoint[drv_ix]);
+	int ret;
+
+	usb_dbg(usbatm, "%s: binding driver %d: vendor %04x product %04x"
+		" rx: ep %02x padd %d alt %2d tx: ep %02x padd %d alt %2d\n",
+		__func__, drv_ix, vendor[drv_ix], product[drv_ix],
+		rx_endpoint[drv_ix], rx_padding[drv_ix], rx_alt,
+		tx_endpoint[drv_ix], tx_padding[drv_ix], tx_alt);
+
+	if (!rx_intf || !tx_intf) {
+		if (!rx_intf)
+			usb_dbg(usbatm, "%s: no interface contains endpoint %02x in altsetting %2d\n",
+				__func__, rx_endpoint[drv_ix], rx_alt);
+		if (!tx_intf)
+			usb_dbg(usbatm, "%s: no interface contains endpoint %02x in altsetting %2d\n",
+				__func__, tx_endpoint[drv_ix], tx_alt);
+		return -ENODEV;
+	}
+
+	if ((rx_intf != intf) && (tx_intf != intf))
+		return -ENODEV;
+
+	if ((rx_intf == tx_intf) && (rx_alt != tx_alt)) {
+		usb_err(usbatm, "%s: altsettings clash on interface %2d (%2d vs %2d)!\n", __func__,
+				rx_intf->altsetting->desc.bInterfaceNumber, rx_alt, tx_alt);
+		return -EINVAL;
+	}
+
+	usb_dbg(usbatm, "%s: rx If#=%2d; tx If#=%2d\n", __func__,
+			rx_intf->altsetting->desc.bInterfaceNumber,
+			tx_intf->altsetting->desc.bInterfaceNumber);
+
+	ret = xusbatm_capture_intf(usbatm, usb_dev, rx_intf, rx_alt, rx_intf != intf);
+	if (ret)
+		return ret;
+
+	if ((tx_intf != rx_intf) && (ret = xusbatm_capture_intf(usbatm, usb_dev, tx_intf, tx_alt, tx_intf != intf))) {
+		xusbatm_release_intf(usb_dev, rx_intf, rx_intf != intf);
+		return ret;
+	}
+
+	return 0;
+}
+
+static void xusbatm_unbind(struct usbatm_data *usbatm,
+			   struct usb_interface *intf)
+{
+	struct usb_device *usb_dev = interface_to_usbdev(intf);
+	int i;
+
+	usb_dbg(usbatm, "%s entered\n", __func__);
+
+	for (i = 0; i < usb_dev->actconfig->desc.bNumInterfaces; i++) {
+		struct usb_interface *cur_intf = usb_dev->actconfig->interface[i];
+
+		if (cur_intf && (usb_get_intfdata(cur_intf) == usbatm)) {
+			usb_set_intfdata(cur_intf, NULL);
+			usb_driver_release_interface(&xusbatm_usb_driver, cur_intf);
+		}
+	}
+}
+
+static int xusbatm_atm_start(struct usbatm_data *usbatm,
+			     struct atm_dev *atm_dev)
+{
+	atm_dbg(usbatm, "%s entered\n", __func__);
+
+	/* use random MAC as we've no way to get it from the device */
+	eth_random_addr(atm_dev->esi);
+
+	return 0;
+}
+
+
+static int xusbatm_usb_probe(struct usb_interface *intf,
+			     const struct usb_device_id *id)
+{
+	return usbatm_usb_probe(intf, id,
+				xusbatm_drivers + (id - xusbatm_usb_ids));
+}
+
+static struct usb_driver xusbatm_usb_driver = {
+	.name		= xusbatm_driver_name,
+	.probe		= xusbatm_usb_probe,
+	.disconnect	= usbatm_usb_disconnect,
+	.id_table	= xusbatm_usb_ids
+};
+
+static int __init xusbatm_init(void)
+{
+	int i;
+
+	if (!num_vendor ||
+	    num_vendor != num_product ||
+	    num_vendor != num_rx_endpoint ||
+	    num_vendor != num_tx_endpoint) {
+		pr_warn("xusbatm: malformed module parameters\n");
+		return -EINVAL;
+	}
+
+	for (i = 0; i < num_vendor; i++) {
+		rx_endpoint[i] |= USB_DIR_IN;
+		tx_endpoint[i] &= USB_ENDPOINT_NUMBER_MASK;
+
+		xusbatm_usb_ids[i].match_flags	= USB_DEVICE_ID_MATCH_DEVICE;
+		xusbatm_usb_ids[i].idVendor	= vendor[i];
+		xusbatm_usb_ids[i].idProduct	= product[i];
+
+		xusbatm_drivers[i].driver_name	= xusbatm_driver_name;
+		xusbatm_drivers[i].bind		= xusbatm_bind;
+		xusbatm_drivers[i].unbind	= xusbatm_unbind;
+		xusbatm_drivers[i].atm_start	= xusbatm_atm_start;
+		xusbatm_drivers[i].bulk_in	= rx_endpoint[i];
+		xusbatm_drivers[i].bulk_out	= tx_endpoint[i];
+		xusbatm_drivers[i].rx_padding	= rx_padding[i];
+		xusbatm_drivers[i].tx_padding	= tx_padding[i];
+	}
+
+	return usb_register(&xusbatm_usb_driver);
+}
+module_init(xusbatm_init);
+
+static void __exit xusbatm_exit(void)
+{
+	usb_deregister(&xusbatm_usb_driver);
+}
+module_exit(xusbatm_exit);
+
+MODULE_AUTHOR("Roman Kagan, Duncan Sands");
+MODULE_DESCRIPTION("Driver for USB ADSL modems initialized in userspace");
+MODULE_LICENSE("GPL");