Re: [PATCH 07/14] powerpc/vdso: Improve linker flags

From: Nick Desaulniers <ndesaulniers@google.com>
To: Nathan Chancellor <nathan@kernel.org>
Cc: masahiroy@kernel.org, nicolas@fjasle.eu, trix@redhat.com,
	 linux-kbuild@vger.kernel.org, llvm@lists.linux.dev,
	 kernel test robot <lkp@intel.com>,
	mpe@ellerman.id.au, npiggin@gmail.com,
	 christophe.leroy@csgroup.eu, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH 07/14] powerpc/vdso: Improve linker flags
Date: Mon, 9 Jan 2023 14:08:41 -0800	[thread overview]
Message-ID: <CAKwvOdk6wzoPWoAoShtaeCDZXEZ2b7OAE05pHZy-ry=Eu8UnAg@mail.gmail.com> (raw)
In-Reply-To: <20221228-drop-qunused-arguments-v1-7-658cbc8fc592@kernel.org>

On Wed, Jan 4, 2023 at 11:55 AM Nathan Chancellor <nathan@kernel.org> wrote:
>
> When clang's -Qunused-arguments is dropped from KBUILD_CPPFLAGS, there
> are several warnings in the PowerPC vDSO:
>
>   clang-16: error: -Wl,-soname=linux-vdso32.so.1: 'linker' input unused [-Werror,-Wunused-command-line-argument]
>   clang-16: error: -Wl,--hash-style=both: 'linker' input unused [-Werror,-Wunused-command-line-argument]
>   clang-16: error: argument unused during compilation: '-shared' [-Werror,-Wunused-command-line-argument]
>
>   clang-16: error: argument unused during compilation: '-nostdinc' [-Werror,-Wunused-command-line-argument]
>   clang-16: error: argument unused during compilation: '-Wa,-maltivec' [-Werror,-Wunused-command-line-argument]
>
> The first group of warnings point out that linker flags were being added
> to all invocations of $(CC), even though they will only be used during
> the final vDSO link. Move those flags to ldflags-y.
>
> The second group of warnings are compiler or assembler flags that will
> be unused during linking. Filter them out from KBUILD_CFLAGS so that
> they are not used during linking.
>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
> Cc: mpe@ellerman.id.au
> Cc: npiggin@gmail.com
> Cc: christophe.leroy@csgroup.eu
> Cc: linuxppc-dev@lists.ozlabs.org
> ---
>  arch/powerpc/kernel/vdso/Makefile | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
>
> diff --git a/arch/powerpc/kernel/vdso/Makefile b/arch/powerpc/kernel/vdso/Makefile
> index 45c0cc5d34b6..769b62832b38 100644
> --- a/arch/powerpc/kernel/vdso/Makefile
> +++ b/arch/powerpc/kernel/vdso/Makefile
> @@ -47,13 +47,17 @@ KCOV_INSTRUMENT := n
>  UBSAN_SANITIZE := n
>  KASAN_SANITIZE := n
>
> -ccflags-y := -shared -fno-common -fno-builtin -nostdlib -Wl,--hash-style=both
> -ccflags-$(CONFIG_LD_IS_LLD) += $(call cc-option,--ld-path=$(LD),-fuse-ld=lld)
> -
> -CC32FLAGS := -Wl,-soname=linux-vdso32.so.1 -m32
> +ccflags-y := -fno-common -fno-builtin
> +ldflags-y := -Wl,--hash-style=both -nostdlib -shared
> +ldflags-$(CONFIG_LD_IS_LLD) += $(call cc-option,--ld-path=$(LD),-fuse-ld=lld)
> +# Filter flags that clang will warn are unused for linking
> +ldflags-y += $(filter-out $(CC_FLAGS_FTRACE) -Wa$(comma)%, $(KBUILD_CFLAGS))
> +
> +CC32FLAGS := -m32
> +LD32FLAGS := -Wl,-soname=linux-vdso32.so.1
>  AS32FLAGS := -D__VDSO32__
>
> -CC64FLAGS := -Wl,-soname=linux-vdso64.so.1
> +LD64FLAGS := -Wl,-soname=linux-vdso64.so.1
>  AS64FLAGS := -D__VDSO64__
>
>  targets += vdso32.lds
> @@ -92,14 +96,14 @@ include/generated/vdso64-offsets.h: $(obj)/vdso64.so.dbg FORCE
>
>  # actual build commands
>  quiet_cmd_vdso32ld_and_check = VDSO32L $@
> -      cmd_vdso32ld_and_check = $(VDSOCC) $(c_flags) $(CC32FLAGS) -o $@ -Wl,-T$(filter %.lds,$^) $(filter %.o,$^) -z noexecstack ; $(cmd_vdso_check)
> +      cmd_vdso32ld_and_check = $(VDSOCC) $(ldflags-y) $(CC32FLAGS) $(LD32FLAGS) -o $@ -Wl,-T$(filter %.lds,$^) $(filter %.o,$^) -z noexecstack ; $(cmd_vdso_check)
>  quiet_cmd_vdso32as = VDSO32A $@
>        cmd_vdso32as = $(VDSOCC) $(a_flags) $(CC32FLAGS) $(AS32FLAGS) -c -o $@ $<
>  quiet_cmd_vdso32cc = VDSO32C $@
>        cmd_vdso32cc = $(VDSOCC) $(c_flags) $(CC32FLAGS) -c -o $@ $<
>
>  quiet_cmd_vdso64ld_and_check = VDSO64L $@
> -      cmd_vdso64ld_and_check = $(VDSOCC) $(c_flags) $(CC64FLAGS) -o $@ -Wl,-T$(filter %.lds,$^) $(filter %.o,$^) -z noexecstack ; $(cmd_vdso_check)
> +      cmd_vdso64ld_and_check = $(VDSOCC) $(ldflags-y) $(CC64FLAGS) $(LD64FLAGS) -o $@ -Wl,-T$(filter %.lds,$^) $(filter %.o,$^) -z noexecstack ; $(cmd_vdso_check)

Let's move `-z noexecstack` up into ldflags-y? (you may add my RB with
that modification)

>  quiet_cmd_vdso64as = VDSO64A $@
>        cmd_vdso64as = $(VDSOCC) $(a_flags) $(CC64FLAGS) $(AS64FLAGS) -c -o $@ $<
>
>
> --
> 2.39.0

-- 
Thanks,
~Nick Desaulniers