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, 40 insertions, 0 deletions

diff --git a/drivers/char/nwbutton.h b/drivers/char/nwbutton.h
new file mode 100644
index 000000000..f2b9fdc1f
--- /dev/null
+++ b/drivers/char/nwbutton.h
@@ -0,0 +1,40 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __NWBUTTON_H
+#define __NWBUTTON_H
+
+/*
+ * 	NetWinder Button Driver-
+ *	Copyright (C) Alex Holden <alex@linuxhacker.org> 1998, 1999.
+ */
+
+#ifdef __NWBUTTON_C	/* Actually compiling the driver itself */
+
+/* Various defines: */
+
+#define NUM_PRESSES_REBOOT 2	/* How many presses to activate shutdown */
+#define BUTTON_DELAY 30 	/* How many jiffies for sequence to end */
+#define VERSION "0.3"		/* Driver version number */
+
+/* Structure definitions: */
+
+struct button_callback {
+	void (*callback) (void);
+	int count;
+};
+
+/* Function prototypes: */
+
+static void button_sequence_finished(struct timer_list *unused);
+static irqreturn_t button_handler (int irq, void *dev_id);
+int button_init (void);
+int button_add_callback (void (*callback) (void), int count);
+int button_del_callback (void (*callback) (void));
+static void button_consume_callbacks (int bpcount);
+
+#else /* Not compiling the driver itself */
+
+extern int button_add_callback (void (*callback) (void), int count);
+extern int button_del_callback (void (*callback) (void));
+
+#endif /* __NWBUTTON_C */
+#endif /* __NWBUTTON_H */