Re: [RFC PATCH 1/1] libstub: zboot: Use -ffunction-sections + --gc-sections

[Alexandre Ghiti]

Hi Ard,

On Wed, Jun 21, 2023 at 1:45 PM Alexandre Ghiti <alexghiti@xxxxxxxxxxxx> wrote:
>
> Static linking is done at an object file (actually section) granularity,
> meaning that if a function in an object has an undefined reference, the
> link will fail, even though this function is not used in the resulting
> ELF.
>
> To avoid such failures, allow the linker to garbage collect unused sections
> (which actually are functions).
>
> Signed-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>
> ---
>  drivers/firmware/efi/libstub/Makefile       | 1 +
>  drivers/firmware/efi/libstub/Makefile.zboot | 2 +-
>  2 files changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> index 3abb2b357482..6182366d00c0 100644
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -40,6 +40,7 @@ KBUILD_CFLAGS                 := $(subst $(CC_FLAGS_FTRACE),,$(cflags-y)) \
>                                    -ffreestanding \
>                                    -fno-stack-protector \
>                                    $(call cc-option,-fno-addrsig) \
> +                                  -ffunction-sections \
>                                    -D__DISABLE_EXPORTS
>
>  #
> diff --git a/drivers/firmware/efi/libstub/Makefile.zboot b/drivers/firmware/efi/libstub/Makefile.zboot
> index 89ef820f3b34..5bb52d262133 100644
> --- a/drivers/firmware/efi/libstub/Makefile.zboot
> +++ b/drivers/firmware/efi/libstub/Makefile.zboot
> @@ -51,7 +51,7 @@ $(obj)/zboot-header.o: $(srctree)/drivers/firmware/efi/libstub/zboot-header.S FO
>
>  ZBOOT_DEPS := $(obj)/zboot-header.o $(objtree)/drivers/firmware/efi/libstub/lib.a
>
> -LDFLAGS_vmlinuz.efi.elf := -T $(srctree)/drivers/firmware/efi/libstub/zboot.lds
> +LDFLAGS_vmlinuz.efi.elf := -T $(srctree)/drivers/firmware/efi/libstub/zboot.lds --gc-sections
>  $(obj)/vmlinuz.efi.elf: $(obj)/vmlinuz.o $(ZBOOT_DEPS) FORCE
>         $(call if_changed,ld)
>
> --
> 2.39.2
>

Sorry to bother you, do you have any opinion about this patch? Does
that make sense? If not, I should find another solution for my riscv
KASLR series!

Thanks,

Alex