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, 95 insertions, 0 deletions

diff --git a/arch/mips/dec/tc.c b/arch/mips/dec/tc.c
new file mode 100644
index 000000000..dba583976
--- /dev/null
+++ b/arch/mips/dec/tc.c
@@ -0,0 +1,95 @@
+/*
+ *	TURBOchannel architecture calls.
+ *
+ *	Copyright (c) Harald Koerfgen, 1998
+ *	Copyright (c) 2001, 2003, 2005, 2006  Maciej W. Rozycki
+ *	Copyright (c) 2005  James Simmons
+ *
+ *	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/compiler.h>
+#include <linux/errno.h>
+#include <linux/init.h>
+#include <linux/string.h>
+#include <linux/tc.h>
+#include <linux/types.h>
+
+#include <asm/addrspace.h>
+#include <asm/bootinfo.h>
+#include <asm/paccess.h>
+
+#include <asm/dec/interrupts.h>
+#include <asm/dec/prom.h>
+#include <asm/dec/system.h>
+
+/*
+ * Protected read byte from TURBOchannel slot space.
+ */
+int tc_preadb(u8 *valp, void __iomem *addr)
+{
+	return get_dbe(*valp, (u8 *)addr);
+}
+
+/*
+ * Get TURBOchannel bus information as specified by the spec, plus
+ * the slot space base address and the number of slots.
+ */
+int __init tc_bus_get_info(struct tc_bus *tbus)
+{
+	if (!dec_tc_bus)
+		return -ENXIO;
+
+	memcpy(&tbus->info, rex_gettcinfo(), sizeof(tbus->info));
+	tbus->slot_base = CPHYSADDR((long)rex_slot_address(0));
+
+	switch (mips_machtype) {
+	case MACH_DS5000_200:
+		tbus->num_tcslots = 7;
+		break;
+	case MACH_DS5000_2X0:
+	case MACH_DS5900:
+		tbus->ext_slot_base = 0x20000000;
+		tbus->ext_slot_size = 0x20000000;
+		fallthrough;
+	case MACH_DS5000_1XX:
+		tbus->num_tcslots = 3;
+		break;
+	case MACH_DS5000_XX:
+		tbus->num_tcslots = 2;
+	default:
+		break;
+	}
+	return 0;
+}
+
+/*
+ * Get the IRQ for the specified slot.
+ */
+void __init tc_device_get_irq(struct tc_dev *tdev)
+{
+	switch (tdev->slot) {
+	case 0:
+		tdev->interrupt = dec_interrupt[DEC_IRQ_TC0];
+		break;
+	case 1:
+		tdev->interrupt = dec_interrupt[DEC_IRQ_TC1];
+		break;
+	case 2:
+		tdev->interrupt = dec_interrupt[DEC_IRQ_TC2];
+		break;
+	/*
+	 * Yuck! DS5000/200 onboard devices
+	 */
+	case 5:
+		tdev->interrupt = dec_interrupt[DEC_IRQ_TC5];
+		break;
+	case 6:
+		tdev->interrupt = dec_interrupt[DEC_IRQ_TC6];
+		break;
+	default:
+		tdev->interrupt = -1;
+		break;
+	}
+}