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, 92 insertions, 0 deletions

diff --git a/arch/mips/dec/wbflush.c b/arch/mips/dec/wbflush.c
new file mode 100644
index 000000000..dad64d178
--- /dev/null
+++ b/arch/mips/dec/wbflush.c
@@ -0,0 +1,92 @@
+/*
+ * Setup the right wbflush routine for the different DECstations.
+ *
+ * Created with information from:
+ *	DECstation 3100 Desktop Workstation Functional Specification
+ *	DECstation 5000/200 KN02 System Module Functional Specification
+ *	mipsel-linux-objdump --disassemble vmunix | grep "wbflush" :-)
+ *
+ * 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.
+ *
+ * Copyright (C) 1998 Harald Koerfgen
+ * Copyright (C) 2002 Maciej W. Rozycki
+ */
+
+#include <linux/export.h>
+#include <linux/init.h>
+
+#include <asm/bootinfo.h>
+#include <asm/wbflush.h>
+#include <asm/barrier.h>
+
+static void wbflush_kn01(void);
+static void wbflush_kn210(void);
+static void wbflush_mips(void);
+
+void (*__wbflush) (void);
+
+void __init wbflush_setup(void)
+{
+	switch (mips_machtype) {
+	case MACH_DS23100:
+	case MACH_DS5000_200:	/* DS5000 3max */
+		__wbflush = wbflush_kn01;
+		break;
+	case MACH_DS5100:	/* DS5100 MIPSMATE */
+		__wbflush = wbflush_kn210;
+		break;
+	case MACH_DS5000_1XX:	/* DS5000/100 3min */
+	case MACH_DS5000_XX:	/* Personal DS5000/2x */
+	case MACH_DS5000_2X0:	/* DS5000/240 3max+ */
+	case MACH_DS5900:	/* DS5900 bigmax */
+	default:
+		__wbflush = wbflush_mips;
+		break;
+	}
+}
+
+/*
+ * For the DS3100 and DS5000/200 the R2020/R3220 writeback buffer functions
+ * as part of Coprocessor 0.
+ */
+static void wbflush_kn01(void)
+{
+    asm(".set\tpush\n\t"
+	".set\tnoreorder\n\t"
+	"1:\tbc0f\t1b\n\t"
+	"nop\n\t"
+	".set\tpop");
+}
+
+/*
+ * For the DS5100 the writeback buffer seems to be a part of Coprocessor 3.
+ * But CP3 has to enabled first.
+ */
+static void wbflush_kn210(void)
+{
+    asm(".set\tpush\n\t"
+	".set\tnoreorder\n\t"
+	"mfc0\t$2,$12\n\t"
+	"lui\t$3,0x8000\n\t"
+	"or\t$3,$2,$3\n\t"
+	"mtc0\t$3,$12\n\t"
+	"nop\n"
+	"1:\tbc3f\t1b\n\t"
+	"nop\n\t"
+	"mtc0\t$2,$12\n\t"
+	"nop\n\t"
+	".set\tpop"
+	: : : "$2", "$3");
+}
+
+/*
+ * I/O ASIC systems use a standard writeback buffer that gets flushed
+ * upon an uncached read.
+ */
+static void wbflush_mips(void)
+{
+	__fast_iob();
+}
+EXPORT_SYMBOL(__wbflush);