linux-kernel - Re: [PATCH v1 1/3] LoongArch: Only link libstub to final vmlinux

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Message-ID: <CAAhV-H4fdfB3fH0DnfgxNimPBsjiOGQZV-w0g63d7TVDOK5-zw@mail.gmail.com>
Date: Thu, 25 Sep 2025 10:43:44 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc: 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 v1 1/3] LoongArch: Only link libstub to final vmlinux

Hi, Tiezhu,

On Fri, Sep 19, 2025 at 5:36 PM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
>
> When compiling with LLVM and CONFIG_LTO_CLANG is set, there exists
> the following objtool warning:
>
>   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.
>
> 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/loongarch/Makefile | 5 ++++-
>  scripts/link-vmlinux.sh | 5 ++---
>  3 files changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 9771619ac596..b8e7c63ae3d1 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
Why does the LoongArch patch modify the common parts? Since the whole
series is small enough, I suggest modifying everything in a single
patch. And the single patch can be merged via the efi tree.

Huacai

> diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
> index a3a9759414f4..e9c61f76a045 100644
> --- a/arch/loongarch/Makefile
> +++ b/arch/loongarch/Makefile
> @@ -164,7 +164,10 @@ 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 += $(objtree)/drivers/firmware/efi/libstub/lib.a
> +endif
>
>  drivers-y              += arch/loongarch/crypto/
>
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index 51367c2bfc21..b3cbff31d8a9 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/scripts/link-vmlinux.sh
> @@ -61,12 +61,11 @@ vmlinux_link()
>         shift
>
>         if is_enabled CONFIG_LTO_CLANG || is_enabled CONFIG_X86_KERNEL_IBT; then
> -               # Use vmlinux.o instead of performing the slow LTO link again.
>                 objs=vmlinux.o
> -               libs=
> +               libs="${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
>