linux-kernel - [tip: x86/boot] x86/boot/startup: Disable LTO for the startup code

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Message-ID: <174488515226.31282.12005558142806434294.tip-bot2@tip-bot2>
Date: Thu, 17 Apr 2025 10:19:11 -0000
From: "tip-bot2 for Nathan Chancellor" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Linux Kernel Functional Testing <lkft@...aro.org>,
 Nathan Chancellor <nathan@...nel.org>, Ingo Molnar <mingo@...nel.org>,
 Ard Biesheuvel <ardb@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
 Kees Cook <keescook@...omium.org>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 David Woodhouse <dwmw@...zon.co.uk>, llvm@...ts.linux.dev, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: x86/boot] x86/boot/startup: Disable LTO for the startup code

The following commit has been merged into the x86/boot branch of tip:

Commit-ID:     498cb872a111e25021ca5e2d91af7b7a2e62630f
Gitweb:        https://git.kernel.org/tip/498cb872a111e25021ca5e2d91af7b7a2e62630f
Author:        Nathan Chancellor <nathan@...nel.org>
AuthorDate:    Mon, 14 Apr 2025 12:26:07 -07:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Thu, 17 Apr 2025 12:09:30 +02:00

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@...aro.org>
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Acked-by: Ard Biesheuvel <ardb@...nel.org>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: David Woodhouse <dwmw@...zon.co.uk>
Cc: llvm@...ts.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/
---
 arch/x86/boot/startup/Makefile | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/boot/startup/Makefile b/arch/x86/boot/startup/Makefile
index ccdfc42..bb7c826 100644
--- 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