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, 74 insertions, 0 deletions

diff --git a/drivers/hid/hid-primax.c b/drivers/hid/hid-primax.c
new file mode 100644
index 000000000..8db054280
--- /dev/null
+++ b/drivers/hid/hid-primax.c
@@ -0,0 +1,74 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * HID driver for primax and similar keyboards with in-band modifiers
+ *
+ * Copyright 2011 Google Inc. All Rights Reserved
+ *
+ * Author:
+ *	Terry Lambert <tlambert@google.com>
+ */
+
+#include <linux/device.h>
+#include <linux/hid.h>
+#include <linux/module.h>
+
+#include "hid-ids.h"
+
+static int px_raw_event(struct hid_device *hid, struct hid_report *report,
+	 u8 *data, int size)
+{
+	int idx = size;
+
+	switch (report->id) {
+	case 0:		/* keyboard input */
+		/*
+		 * Convert in-band modifier key values into out of band
+		 * modifier bits and pull the key strokes from the report.
+		 * Thus a report data set which looked like:
+		 *
+		 * [00][00][E0][30][00][00][00][00]
+		 * (no modifier bits + "Left Shift" key + "1" key)
+		 *
+		 * Would be converted to:
+		 *
+		 * [01][00][00][30][00][00][00][00]
+		 * (Left Shift modifier bit + "1" key)
+		 *
+		 * As long as it's in the size range, the upper level
+		 * drivers don't particularly care if there are in-band
+		 * 0-valued keys, so they don't stop parsing.
+		 */
+		while (--idx > 1) {
+			if (data[idx] < 0xE0 || data[idx] > 0xE7)
+				continue;
+			data[0] |= (1 << (data[idx] - 0xE0));
+			data[idx] = 0;
+		}
+		hid_report_raw_event(hid, HID_INPUT_REPORT, data, size, size, 0);
+		return 1;
+
+	default:	/* unknown report */
+		/* Unknown report type; pass upstream */
+		hid_info(hid, "unknown report type %d\n", report->id);
+		break;
+	}
+
+	return 0;
+}
+
+static const struct hid_device_id px_devices[] = {
+	{ HID_USB_DEVICE(USB_VENDOR_ID_PRIMAX, USB_DEVICE_ID_PRIMAX_KEYBOARD) },
+	{ }
+};
+MODULE_DEVICE_TABLE(hid, px_devices);
+
+static struct hid_driver px_driver = {
+	.name = "primax",
+	.id_table = px_devices,
+	.raw_event = px_raw_event,
+};
+module_hid_driver(px_driver);
+
+MODULE_AUTHOR("Terry Lambert <tlambert@google.com>");
+MODULE_DESCRIPTION("HID driver for primax and similar keyboards with in-band modifiers");
+MODULE_LICENSE("GPL");