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, 50 insertions, 0 deletions

diff --git a/arch/m68k/coldfire/reset.c b/arch/m68k/coldfire/reset.c
new file mode 100644
index 000000000..6e5f8ab39
--- /dev/null
+++ b/arch/m68k/coldfire/reset.c
@@ -0,0 +1,50 @@
+/*
+ * reset.c  -- common ColdFire SoC reset support
+ *
+ * (C) Copyright 2012, Greg Ungerer <gerg@uclinux.org>
+ *
+ * This file is subject to the terms and conditions of the GNU General Public
+ * License.  See the file COPYING in the main directory of this archive
+ * for more details.
+ */
+
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/io.h>
+#include <asm/machdep.h>
+#include <asm/coldfire.h>
+#include <asm/mcfsim.h>
+
+/*
+ *	There are 2 common methods amongst the ColdFure parts for reseting
+ *	the CPU. But there are couple of exceptions, the 5272 and the 547x
+ *	have something completely special to them, and we let their specific
+ *	subarch code handle them.
+ */
+
+#ifdef MCFSIM_SYPCR
+static void mcf_cpu_reset(void)
+{
+	local_irq_disable();
+	/* Set watchdog to soft reset, and enabled */
+	mcf_write8(0xc0, MCFSIM_SYPCR);
+	for (;;)
+		/* wait for watchdog to timeout */;
+}
+#endif
+
+#ifdef MCF_RCR
+static void mcf_cpu_reset(void)
+{
+	local_irq_disable();
+	mcf_write8(MCF_RCR_SWRESET, MCF_RCR);
+}
+#endif
+
+static int __init mcf_setup_reset(void)
+{
+	mach_reset = mcf_cpu_reset;
+	return 0;
+}
+
+arch_initcall(mcf_setup_reset);