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, 93 insertions, 0 deletions

diff --git a/arch/microblaze/Makefile b/arch/microblaze/Makefile
new file mode 100644
index 000000000..02e6be9c5
--- /dev/null
+++ b/arch/microblaze/Makefile
@@ -0,0 +1,93 @@
+# SPDX-License-Identifier: GPL-2.0
+KBUILD_DEFCONFIG := mmu_defconfig
+
+UTS_SYSNAME = -DUTS_SYSNAME=\"Linux\"
+
+# What CPU version are we building for, and crack it open
+# as major.minor.rev
+CPU_VER   := $(CONFIG_XILINX_MICROBLAZE0_HW_VER)
+CPU_MAJOR := $(word 1, $(subst ., , $(CPU_VER)))
+CPU_MINOR := $(word 2, $(subst ., , $(CPU_VER)))
+CPU_REV   := $(word 3, $(subst ., , $(CPU_VER)))
+
+export CPU_VER CPU_MAJOR CPU_MINOR CPU_REV
+
+# Use cpu-related CONFIG_ vars to set compile options.
+# The various CONFIG_XILINX cpu features options are integers 0/1/2...
+# rather than bools y/n
+
+# Work out HW multipler support. This is tricky.
+# 1. Spartan2 has no HW multipliers.
+# 2. MicroBlaze v3.x always uses them, except in Spartan 2
+# 3. All other FPGa/CPU ver combos, we can trust the CONFIG_ settings
+ifeq (,$(findstring spartan2,$(CONFIG_XILINX_MICROBLAZE0_FAMILY)))
+  ifeq ($(CPU_MAJOR),3)
+    CPUFLAGS-1 += -mno-xl-soft-mul
+  else
+    # USE_HW_MUL can be 0, 1, or 2, defining a hierarchy of HW Mul support.
+    CPUFLAGS-$(subst 1,,$(CONFIG_XILINX_MICROBLAZE0_USE_HW_MUL)) += -mxl-multiply-high
+    CPUFLAGS-$(CONFIG_XILINX_MICROBLAZE0_USE_HW_MUL) += -mno-xl-soft-mul
+  endif
+endif
+CPUFLAGS-$(CONFIG_XILINX_MICROBLAZE0_USE_DIV) += -mno-xl-soft-div
+CPUFLAGS-$(CONFIG_XILINX_MICROBLAZE0_USE_BARREL) += -mxl-barrel-shift
+CPUFLAGS-$(CONFIG_XILINX_MICROBLAZE0_USE_PCMP_INSTR) += -mxl-pattern-compare
+
+ifdef CONFIG_CPU_BIG_ENDIAN
+KBUILD_CFLAGS += -mbig-endian
+KBUILD_AFLAGS += -mbig-endian
+KBUILD_LDFLAGS += -EB
+else
+KBUILD_CFLAGS += -mlittle-endian
+KBUILD_AFLAGS += -mlittle-endian
+KBUILD_LDFLAGS += -EL
+endif
+
+CPUFLAGS-1 += $(call cc-option,-mcpu=v$(CPU_VER))
+
+# r31 holds current when in kernel mode
+KBUILD_CFLAGS += -ffixed-r31 $(CPUFLAGS-y) $(CPUFLAGS-1) $(CPUFLAGS-2)
+
+libs-y += arch/microblaze/lib/
+
+boot := arch/microblaze/boot
+
+# Are we making a simpleImage.<boardname> target? If so, crack out the boardname
+DTB:=$(subst simpleImage.,,$(filter simpleImage.%, $(MAKECMDGOALS)))
+
+export DTB
+
+all: linux.bin
+
+archheaders:
+	$(Q)$(MAKE) $(build)=arch/microblaze/kernel/syscalls all
+
+PHONY += linux.bin linux.bin.gz linux.bin.ub
+linux.bin.ub linux.bin.gz: linux.bin
+linux.bin: vmlinux
+linux.bin linux.bin.gz linux.bin.ub:
+	$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
+	@echo 'Kernel: $(boot)/$@ is ready' ' (#'$(or $(KBUILD_BUILD_VERSION),`cat .version`)')'
+
+PHONY += simpleImage.$(DTB)
+simpleImage.$(DTB): vmlinux
+	$(Q)$(MAKE) $(build)=$(boot) $(addprefix $(boot)/$@., ub unstrip strip)
+	@echo 'Kernel: $(boot)/$@ is ready' ' (#'$(or $(KBUILD_BUILD_VERSION),`cat .version`)')'
+
+define archhelp
+  echo '* linux.bin    - Create raw binary'
+  echo '  linux.bin.gz - Create compressed raw binary'
+  echo '  linux.bin.ub - Create U-Boot wrapped raw binary'
+  echo '  simpleImage.<dt> - Create the following images with <dt>.dtb linked in'
+  echo '                    simpleImage.<dt>        : raw image'
+  echo '                    simpleImage.<dt>.ub     : raw image with U-Boot header'
+  echo '                    simpleImage.<dt>.unstrip: ELF (identical to vmlinux)'
+  echo '                    simpleImage.<dt>.strip  : stripped ELF'
+  echo '  Targets with <dt> embed a device tree blob inside the image'
+  echo '  These targets support board with firmware that does not'
+  echo '  support passing a device tree directly. Replace <dt> with the'
+  echo '  name of a dts file from the arch/microblaze/boot/dts/ directory'
+  echo '  (minus the .dts extension).'
+endef
+
+MRPROPER_FILES += $(boot)/simpleImage.*