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, 134 insertions, 0 deletions

diff --git a/arch/m68k/Makefile b/arch/m68k/Makefile
new file mode 100644
index 000000000..0abcf994c
--- /dev/null
+++ b/arch/m68k/Makefile
@@ -0,0 +1,134 @@
+#
+# m68k/Makefile
+#
+# This file is included by the global makefile so that you can add your own
+# architecture-specific flags and dependencies.
+#
+# 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) 1994 by Hamish Macdonald
+# Copyright (C) 2002,2011 Greg Ungerer <gerg@snapgear.com>
+#
+
+KBUILD_DEFCONFIG := multi_defconfig
+
+ifdef cross_compiling
+    ifeq ($(CROSS_COMPILE),)
+		CROSS_COMPILE := $(call cc-cross-prefix, \
+			m68k-linux-gnu- m68k-linux- m68k-unknown-linux-gnu-)
+    endif
+endif
+
+#
+#	Enable processor type. Ordering of these is important - we want to
+#	use the minimum processor type of the range we support. The logic
+#	for 680x0 will only allow use of the -m68060 or -m68040 if no other
+#	680x0 type is specified - and no option is specified for 68030 or
+#	68020. The other m68k/ColdFire types always specify some type of
+#	compiler cpu type flag.
+#
+ifndef CONFIG_M68040
+cpuflags-$(CONFIG_M68060)	= -m68060
+endif
+ifndef CONFIG_M68060
+cpuflags-$(CONFIG_M68040)	= -m68040
+endif
+cpuflags-$(CONFIG_M68030)	=
+cpuflags-$(CONFIG_M68020)	=
+cpuflags-$(CONFIG_M68000)	= -m68000
+cpuflags-$(CONFIG_M5441x)	= $(call cc-option,-mcpu=54455,-mcfv4e)
+cpuflags-$(CONFIG_M54xx)	= $(call cc-option,-mcpu=5475,-m5200)
+cpuflags-$(CONFIG_M5407)	= $(call cc-option,-mcpu=5407,-m5200)
+cpuflags-$(CONFIG_M532x)	= $(call cc-option,-mcpu=532x,-m5307)
+cpuflags-$(CONFIG_M537x)	= $(call cc-option,-mcpu=537x,-m5307)
+cpuflags-$(CONFIG_M5307)	= $(call cc-option,-mcpu=5307,-m5200)
+cpuflags-$(CONFIG_M528x)	= $(call cc-option,-mcpu=528x,-m5307)
+cpuflags-$(CONFIG_M5275)	= $(call cc-option,-mcpu=5275,-m5307)
+cpuflags-$(CONFIG_M5272)	= $(call cc-option,-mcpu=5272,-m5307)
+cpuflags-$(CONFIG_M5271)	= $(call cc-option,-mcpu=5271,-m5307)
+cpuflags-$(CONFIG_M523x)	= $(call cc-option,-mcpu=523x,-m5307)
+cpuflags-$(CONFIG_M525x)	= $(call cc-option,-mcpu=5253,-m5200)
+cpuflags-$(CONFIG_M5249)	= $(call cc-option,-mcpu=5249,-m5200)
+cpuflags-$(CONFIG_M520x)	= $(call cc-option,-mcpu=5208,-m5200)
+cpuflags-$(CONFIG_M5206e)	= $(call cc-option,-mcpu=5206e,-m5200)
+cpuflags-$(CONFIG_M5206)	= $(call cc-option,-mcpu=5206,-m5200)
+
+# Evaluate tune cc-option calls now
+cpuflags-y := $(cpuflags-y)
+
+KBUILD_AFLAGS += $(cpuflags-y)
+KBUILD_CFLAGS += $(cpuflags-y)
+
+KBUILD_CFLAGS += -pipe -ffreestanding
+
+ifdef CONFIG_MMU
+KBUILD_CFLAGS += -ffixed-a2
+else
+# we can use a m68k-linux-gcc toolchain with these in place
+KBUILD_CPPFLAGS += -DUTS_SYSNAME=\"uClinux\"
+KBUILD_CPPFLAGS += -D__uClinux__
+endif
+
+KBUILD_LDFLAGS := -m m68kelf
+
+ifdef CONFIG_SUN3
+LDFLAGS_vmlinux = -N
+endif
+
+CHECKFLAGS += -D__mc68000__
+
+
+ifdef CONFIG_KGDB
+# If configured for kgdb support, include debugging infos and keep the
+# frame pointer
+KBUILD_CFLAGS := $(subst -fomit-frame-pointer,,$(KBUILD_CFLAGS)) -g
+endif
+
+libs-y				+= arch/m68k/lib/
+
+
+all:	zImage
+
+lilo:	vmlinux
+	if [ -f $(INSTALL_PATH)/vmlinux ]; then mv -f $(INSTALL_PATH)/vmlinux $(INSTALL_PATH)/vmlinux.old; fi
+	if [ -f $(INSTALL_PATH)/System.map ]; then mv -f $(INSTALL_PATH)/System.map $(INSTALL_PATH)/System.old; fi
+	cat vmlinux > $(INSTALL_PATH)/vmlinux
+	cp System.map $(INSTALL_PATH)/System.map
+	if [ -x /sbin/lilo ]; then /sbin/lilo; else /etc/lilo/install; fi
+
+zImage compressed: vmlinux.gz
+
+vmlinux.gz: vmlinux
+
+ifndef CONFIG_KGDB
+	cp vmlinux vmlinux.tmp
+	$(STRIP) vmlinux.tmp
+	$(KGZIP) -9c vmlinux.tmp >vmlinux.gz
+	rm vmlinux.tmp
+else
+	$(KGZIP) -9c vmlinux >vmlinux.gz
+endif
+
+bzImage: vmlinux.bz2
+
+vmlinux.bz2: vmlinux
+
+ifndef CONFIG_KGDB
+	cp vmlinux vmlinux.tmp
+	$(STRIP) vmlinux.tmp
+	$(KBZIP2) -1c vmlinux.tmp >vmlinux.bz2
+	rm vmlinux.tmp
+else
+	$(KBZIP2) -1c vmlinux >vmlinux.bz2
+endif
+
+CLEAN_FILES += vmlinux.gz vmlinux.bz2
+
+archheaders:
+	$(Q)$(MAKE) $(build)=arch/m68k/kernel/syscalls all
+
+install: KBUILD_IMAGE := vmlinux.gz
+install:
+	$(call cmd,install)