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

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Message-ID: <CAMj1kXG8Wi+THa2SeLxiDT=+t_TKx0AL4H-azZO4DNJvyyv96g@mail.gmail.com>
Date: Sun, 28 Sep 2025 15:41:54 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>, Huacai Chen <chenhuacai@...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

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.

> 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
>