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, 53 insertions, 0 deletions

diff --git a/arch/xtensa/boot/Makefile b/arch/xtensa/boot/Makefile
new file mode 100644
index 000000000..d8b0fadf4
--- /dev/null
+++ b/arch/xtensa/boot/Makefile
@@ -0,0 +1,53 @@
+#
+# arch/xtensa/boot/Makefile
+#
+# 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.
+#
+#
+
+
+# KBUILD_CFLAGS used when building rest of boot (takes effect recursively)
+KBUILD_CFLAGS	+= -fno-builtin
+
+subdir-y	:= lib
+targets		+= vmlinux.bin vmlinux.bin.gz
+targets		+= uImage xipImage
+
+# Subdirs for the boot loader(s)
+
+boot-$(CONFIG_XTENSA_PLATFORM_ISS)    += Image
+boot-$(CONFIG_XTENSA_PLATFORM_XT2000) += Image zImage uImage
+boot-$(CONFIG_XTENSA_PLATFORM_XTFPGA) += Image zImage uImage
+
+all: $(boot-y)
+Image: boot-elf
+zImage: boot-redboot
+uImage: $(obj)/uImage
+xipImage: $(obj)/xipImage
+
+boot-elf boot-redboot: $(addprefix $(obj)/,$(subdir-y))
+	$(Q)$(MAKE) $(build)=$(obj)/$@ $(MAKECMDGOALS)
+
+OBJCOPYFLAGS = --strip-all -R .comment -R .notes -O binary
+
+$(obj)/vmlinux.bin: vmlinux FORCE
+	$(call if_changed,objcopy)
+
+$(obj)/vmlinux.bin.gz: $(obj)/vmlinux.bin FORCE
+	$(call if_changed,gzip)
+
+boot-elf: $(obj)/vmlinux.bin
+boot-redboot: $(obj)/vmlinux.bin.gz
+
+UIMAGE_LOADADDR = $(CONFIG_KERNEL_LOAD_ADDRESS)
+UIMAGE_COMPRESSION = gzip
+
+$(obj)/uImage: $(obj)/vmlinux.bin.gz FORCE
+	$(call if_changed,uimage)
+	$(Q)$(kecho) '  Kernel: $@ is ready'
+
+$(obj)/xipImage: vmlinux FORCE
+	$(call if_changed,objcopy)
+	$(Q)$(kecho) '  Kernel: $@ is ready'