x86/boot/startup: Disable LTO for the startup code

When building with CONFIG_LTO_CLANG, there is an error in the x86 boot
startup code because it builds with a different code model than the rest
of the kernel:

  ld.lld: error: Function Import: link error: linking module flags 'Code Model': IDs have conflicting values: 'i32 2' from vmlinux.a(head64.o at 1302448), and 'i32 1' from vmlinux.a(map_kernel.o at 1314208)
  ld.lld: error: Function Import: link error: linking module flags 'Code Model': IDs have conflicting values: 'i32 2' from vmlinux.a(common.o at 1306108), and 'i32 1' from vmlinux.a(gdt_idt.o at 1314148)

As this directory is for code that only runs during early system
initialization, LTO is not very important, so filter out the LTO flags
from KBUILD_CFLAGS for arch/x86/boot/startup to resolve the build error.

Fixes: 4cecebf200ef ("x86/boot: Move the early GDT/IDT setup code into startup/")
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: llvm@lists.linux.dev
Link: https://lore.kernel.org/r/20250414-x86-boot-startup-lto-error-v1-1-7c8bed7c131c@kernel.org
Closes: https://lore.kernel.org/CA+G9fYvnun+bhYgtt425LWxzOmj+8Jf3ruKeYxQSx-F6U7aisg@mail.gmail.com/

diff --git a/arch/x86/boot/startup/Makefile b/arch/x86/boot/startup/Makefile
index ccdfc42a4d59e1b53303072868ce871b90fd4630..bb7c826e586c2d47509581ae124d897d38205a8c 100644 (file)
--- a/arch/x86/boot/startup/Makefile
+++ b/arch/x86/boot/startup/Makefile
@@ -7,8 +7,9 @@ KBUILD_CFLAGS           += -D__DISABLE_EXPORTS -mcmodel=small -fPIC \
                           -fno-stack-protector -D__NO_FORTIFY \
                           -include $(srctree)/include/linux/hidden.h
 
-# disable ftrace hooks
+# disable ftrace hooks and LTO
 KBUILD_CFLAGS  := $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS))
+KBUILD_CFLAGS  := $(filter-out $(CC_FLAGS_LTO),$(KBUILD_CFLAGS))
 KASAN_SANITIZE := n
 KCSAN_SANITIZE := n
 KMSAN_SANITIZE := n