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/hid/hid-gembird.c b/drivers/hid/hid-gembird.c
new file mode 100644
index 000000000..20a8de766
--- /dev/null
+++ b/drivers/hid/hid-gembird.c
@@ -0,0 +1,113 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ *  HID driver for Gembird Joypad, "PC Game Controller"
+ *
+ *  Copyright (c) 2015 Red Hat, Inc
+ *  Copyright (c) 2015 Benjamin Tissoires
+ */
+
+/*
+ */
+
+#include <linux/device.h>
+#include <linux/hid.h>
+#include <linux/module.h>
+
+#include "hid-ids.h"
+
+#define GEMBIRD_START_FAULTY_RDESC	8
+
+static const __u8 gembird_jpd_faulty_rdesc[] = {
+	0x75, 0x08,			/*   Report Size (8)		*/
+	0x95, 0x05,			/*   Report Count (5)		*/
+	0x15, 0x00,			/*   Logical Minimum (0)	*/
+	0x26, 0xff, 0x00,		/*   Logical Maximum (255)	*/
+	0x35, 0x00,			/*   Physical Minimum (0)	*/
+	0x46, 0xff, 0x00,		/*   Physical Maximum (255)	*/
+	0x09, 0x30,			/*   Usage (X)			*/
+	0x09, 0x31,			/*   Usage (Y)			*/
+	0x09, 0x32,			/*   Usage (Z)			*/
+	0x09, 0x32,			/*   Usage (Z)			*/
+	0x09, 0x35,			/*   Usage (Rz)			*/
+	0x81, 0x02,			/*   Input (Data,Var,Abs)	*/
+};
+
+/*
+ * we fix the report descriptor by:
+ * - marking the first Z axis as constant (so it is ignored by HID)
+ * - assign the original second Z to Rx
+ * - assign the original Rz to Ry
+ */
+static const __u8 gembird_jpd_fixed_rdesc[] = {
+	0x75, 0x08,			/*   Report Size (8)		*/
+	0x95, 0x02,			/*   Report Count (2)		*/
+	0x15, 0x00,			/*   Logical Minimum (0)	*/
+	0x26, 0xff, 0x00,		/*   Logical Maximum (255)	*/
+	0x35, 0x00,			/*   Physical Minimum (0)	*/
+	0x46, 0xff, 0x00,		/*   Physical Maximum (255)	*/
+	0x09, 0x30,			/*   Usage (X)			*/
+	0x09, 0x31,			/*   Usage (Y)			*/
+	0x81, 0x02,			/*   Input (Data,Var,Abs)	*/
+	0x95, 0x01,			/*   Report Count (1)		*/
+	0x09, 0x32,			/*   Usage (Z)			*/
+	0x81, 0x01,			/*   Input (Cnst,Arr,Abs)	*/
+	0x95, 0x02,			/*   Report Count (2)		*/
+	0x09, 0x33,			/*   Usage (Rx)			*/
+	0x09, 0x34,			/*   Usage (Ry)			*/
+	0x81, 0x02,			/*   Input (Data,Var,Abs)	*/
+};
+
+static const __u8 *gembird_report_fixup(struct hid_device *hdev, __u8 *rdesc,
+		unsigned int *rsize)
+{
+	__u8 *new_rdesc;
+	/* delta_size is > 0 */
+	size_t delta_size = sizeof(gembird_jpd_fixed_rdesc) -
+			    sizeof(gembird_jpd_faulty_rdesc);
+	size_t new_size = *rsize + delta_size;
+
+	if (*rsize >= 31 && !memcmp(&rdesc[GEMBIRD_START_FAULTY_RDESC],
+				    gembird_jpd_faulty_rdesc,
+				    sizeof(gembird_jpd_faulty_rdesc))) {
+		new_rdesc = devm_kzalloc(&hdev->dev, new_size, GFP_KERNEL);
+		if (new_rdesc == NULL)
+			return rdesc;
+
+		dev_info(&hdev->dev,
+			 "fixing Gembird JPD-DualForce 2 report descriptor.\n");
+
+		/* start by copying the end of the rdesc */
+		memcpy(new_rdesc + delta_size, rdesc, *rsize);
+
+		/* add the correct beginning */
+		memcpy(new_rdesc, rdesc, GEMBIRD_START_FAULTY_RDESC);
+
+		/* replace the faulty part with the fixed one */
+		memcpy(new_rdesc + GEMBIRD_START_FAULTY_RDESC,
+		       gembird_jpd_fixed_rdesc,
+		       sizeof(gembird_jpd_fixed_rdesc));
+
+		*rsize = new_size;
+		rdesc = new_rdesc;
+	}
+
+	return rdesc;
+}
+
+static const struct hid_device_id gembird_devices[] = {
+	{ HID_USB_DEVICE(USB_VENDOR_ID_GEMBIRD,
+			 USB_DEVICE_ID_GEMBIRD_JPD_DUALFORCE2) },
+	{ }
+};
+MODULE_DEVICE_TABLE(hid, gembird_devices);
+
+static struct hid_driver gembird_driver = {
+	.name = "gembird",
+	.id_table = gembird_devices,
+	.report_fixup = gembird_report_fixup,
+};
+module_hid_driver(gembird_driver);
+
+MODULE_AUTHOR("Benjamin Tissoires <benjamin.tissoires@gmail.com>");
+MODULE_DESCRIPTION("HID Gembird joypad driver");
+MODULE_LICENSE("GPL");