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, 48 insertions, 0 deletions

diff --git a/drivers/usb/common/led.c b/drivers/usb/common/led.c
new file mode 100644
index 000000000..1de18d90b
--- /dev/null
+++ b/drivers/usb/common/led.c
@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * LED Triggers for USB Activity
+ *
+ * Copyright 2014 Michal Sojka <sojka@merica.cz>
+ */
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/leds.h>
+#include <linux/usb.h>
+#include "common.h"
+
+#define BLINK_DELAY 30
+
+DEFINE_LED_TRIGGER(ledtrig_usb_gadget);
+DEFINE_LED_TRIGGER(ledtrig_usb_host);
+
+void usb_led_activity(enum usb_led_event ev)
+{
+	struct led_trigger *trig = NULL;
+
+	switch (ev) {
+	case USB_LED_EVENT_GADGET:
+		trig = ledtrig_usb_gadget;
+		break;
+	case USB_LED_EVENT_HOST:
+		trig = ledtrig_usb_host;
+		break;
+	}
+	/* led_trigger_blink_oneshot() handles trig == NULL gracefully */
+	led_trigger_blink_oneshot(trig, BLINK_DELAY, BLINK_DELAY, 0);
+}
+EXPORT_SYMBOL_GPL(usb_led_activity);
+
+
+void __init ledtrig_usb_init(void)
+{
+	led_trigger_register_simple("usb-gadget", &ledtrig_usb_gadget);
+	led_trigger_register_simple("usb-host", &ledtrig_usb_host);
+}
+
+void __exit ledtrig_usb_exit(void)
+{
+	led_trigger_unregister_simple(ledtrig_usb_gadget);
+	led_trigger_unregister_simple(ledtrig_usb_host);
+}