linux-kernel - Re: [PATCH v2] efistub: Only link libstub to final vmlinux

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Message-ID: <CAAhV-H7xOf8DEwOrNh+GQGHktOT4Ljp+7SqutGvvDZp6GLXJrA@mail.gmail.com>
Date: Sun, 28 Sep 2025 21:52:03 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Tiezhu Yang <yangtiezhu@...ngson.cn>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	loongarch@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org, 
	linux-riscv@...ts.infradead.org, linux-efi@...r.kernel.org, 
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] efistub: Only link libstub to final vmlinux

Hi, Ard,

On Sun, Sep 28, 2025 at 9:42 PM Ard Biesheuvel <ardb@...nel.org> wrote:
>
> On Sun, 28 Sept 2025 at 10:55, Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
> >
> > When compiling with LLVM and CONFIG_LTO_CLANG is set, there exists
> > the following objtool warning on LoongArch:
> >
> >   vmlinux.o: warning: objtool: __efistub_efi_boot_kernel()
> >   falls through to next function __efistub_exit_boot_func()
> >
> > This is because efi_boot_kernel() doesn't end with a return instruction
> > or an unconditional jump, then objtool has determined that the function
> > can fall through into the next function.
> >
> > At the beginning, try to do something to make efi_boot_kernel() ends with
> > an unconditional jump instruction, but this modification seems not proper.
> >
> > Since the efistub functions are useless for stack unwinder, they can be
> > ignored by objtool. After many discussions, no need to link libstub to
> > the vmlinux.o, only link libstub to the final vmlinux.
> >
>
> Please try keeping these changes confined to arch/loongarch. This
> problem does not exist on other architectures, and changing the way
> vmlinux is constructed might create other issues down the road.
ARM, RISC-V and LoongArch do things exactly in the same way. Now
LoongArch is the first of the three to enable objtool, so we meet the
problem first.

But yes, I also don't want to change the way of constructing vmlinux.
So I prefer the earliest way to fix this problem.
https://lore.kernel.org/loongarch/CAAhV-H7fRHGFVKV8HitRgmuoDPt5ODt--iSuV0EmeeUb9d5FNw@mail.gmail.com/T/#meef7411abd14f4c28c85e686614aa9211fccdca0

Huacai

>
> > Do the similar things for arm64 and riscv, otherwise there may be objtool
> > warnings when arm64 and riscv support objtool, this is to make consistent
> > with the archs that use libstub.
> >
> > Link: https://lore.kernel.org/lkml/pq4h7jgndnt6p45lj4kgubxjd5gidfetugcuf5rcxzxxanzetd@6rrlpjnjsmuy/
> > Suggested-by: Josh Poimboeuf <jpoimboe@...nel.org>
> > Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
> > ---
> >  Makefile                | 1 +
> >  arch/arm64/Makefile     | 5 ++++-
> >  arch/loongarch/Makefile | 5 ++++-
> >  arch/riscv/Makefile     | 5 ++++-
>
> >  scripts/link-vmlinux.sh | 5 ++---
> >  5 files changed, 15 insertions(+), 6 deletions(-)
> >
> > diff --git a/Makefile b/Makefile
> > index 10355ecf32cb..8ba2e28ef3d1 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -1201,6 +1201,7 @@ KBUILD_VMLINUX_OBJS := built-in.a $(patsubst %/, %/lib.a, $(filter %/, $(libs-y)
> >  KBUILD_VMLINUX_LIBS := $(filter-out %/, $(libs-y))
> >
> >  export KBUILD_VMLINUX_LIBS
> > +export KBUILD_VMLINUX_LIBS_PRELINK
> >  export KBUILD_LDS          := arch/$(SRCARCH)/kernel/vmlinux.lds
> >
> >  ifdef CONFIG_TRIM_UNUSED_KSYMS
> > diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> > index 73a10f65ce8b..038f37ef2143 100644
> > --- a/arch/arm64/Makefile
> > +++ b/arch/arm64/Makefile
> > @@ -156,7 +156,10 @@ KBUILD_CPPFLAGS += -DKASAN_SHADOW_SCALE_SHIFT=$(KASAN_SHADOW_SCALE_SHIFT)
> >  KBUILD_AFLAGS += -DKASAN_SHADOW_SCALE_SHIFT=$(KASAN_SHADOW_SCALE_SHIFT)
> >
> >  libs-y         := arch/arm64/lib/ $(libs-y)
> > -libs-$(CONFIG_EFI_STUB) += $(objtree)/drivers/firmware/efi/libstub/lib.a
> > +
> > +ifdef CONFIG_EFI_STUB
> > +KBUILD_VMLINUX_LIBS_PRELINK += $(objtree)/drivers/firmware/efi/libstub/lib.a
> > +endif
> >
> >  # Default target when executing plain make
> >  boot           := arch/arm64/boot
> > diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
> > index ae419e32f22e..4eb904c20718 100644
> > --- a/arch/loongarch/Makefile
> > +++ b/arch/loongarch/Makefile
> > @@ -169,7 +169,10 @@ CHECKFLAGS += $(shell $(CC) $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) -dM -E -x c /dev
> >  endif
> >
> >  libs-y += arch/loongarch/lib/
> > -libs-$(CONFIG_EFI_STUB) += $(objtree)/drivers/firmware/efi/libstub/lib.a
> > +
> > +ifdef CONFIG_EFI_STUB
> > +KBUILD_VMLINUX_LIBS_PRELINK += $(objtree)/drivers/firmware/efi/libstub/lib.a
> > +endif
> >
> >  drivers-y              += arch/loongarch/crypto/
> >
> > diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> > index df57654a615e..cfd82b2c1bbf 100644
> > --- a/arch/riscv/Makefile
> > +++ b/arch/riscv/Makefile
> > @@ -173,7 +173,10 @@ boot-image-$(CONFIG_XIP_KERNEL)            := xipImage
> >  KBUILD_IMAGE                           := $(boot)/$(boot-image-y)
> >
> >  libs-y += arch/riscv/lib/
> > -libs-$(CONFIG_EFI_STUB) += $(objtree)/drivers/firmware/efi/libstub/lib.a
> > +
> > +ifdef CONFIG_EFI_STUB
> > +KBUILD_VMLINUX_LIBS_PRELINK += $(objtree)/drivers/firmware/efi/libstub/lib.a
> > +endif
> >
> >  ifeq ($(KBUILD_EXTMOD),)
> >  ifeq ($(CONFIG_MMU),y)
> > diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> > index 51367c2bfc21..b3cbff31d8a9 100755
> > --- a/scripts/link-vmlinux.sh
> > +++ b/scripts/link-vmlinux.sh
> > @@ -61,12 +61,11 @@ vmlinux_link()
> >         shift
> >
> >         if is_enabled CONFIG_LTO_CLANG || is_enabled CONFIG_X86_KERNEL_IBT; then
> > -               # Use vmlinux.o instead of performing the slow LTO link again.
> >                 objs=vmlinux.o
> > -               libs=
> > +               libs="${KBUILD_VMLINUX_LIBS_PRELINK}"
> >         else
> >                 objs=vmlinux.a
> > -               libs="${KBUILD_VMLINUX_LIBS}"
> > +               libs="${KBUILD_VMLINUX_LIBS} ${KBUILD_VMLINUX_LIBS_PRELINK}"
> >         fi
> >
> >         if is_enabled CONFIG_GENERIC_BUILTIN_DTB; then
> > --
> > 2.42.0
> >