linux-kernel - Re: [PATCH 3/7] x86: remove always-defined CONFIG_AS_CFI_SIGNAL

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Message-ID: <CAKwvOd=V=HF3RBP5bMwgnAZsPg7nVewZiMQ7F3bh=D6_5ejBaQ@mail.gmail.com>
Date:   Mon, 23 Mar 2020 13:45:17 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Jason A . Donenfeld" <Jason@...c4.com>,
        Ingo Molnar <mingo@...hat.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH 3/7] x86: remove always-defined CONFIG_AS_CFI_SIGNAL_FRAME

On Sun, Mar 22, 2020 at 7:09 PM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> CONFIG_AS_CFI_SIGNAL_FRAME was introduced by commit adf1423698f0
> ("[PATCH] i386/x86-64: Work around gcc bug with noreturn functions
> in unwinder").
>
> We raise the minimal supported binutils version from time to time.
> The last bump was commit 1fb12b35e5ff ("kbuild: Raise the minimum
> required binutils version to 2.21").
>
> I confirmed the code in $(call as-instr,...) can be assembled by the
> binutils 2.21 assembler and also by LLVM integrated assembler.
>
> Remove CONFIG_AS_CFI_SIGNAL_FRAME, which is always defined.

grepping for CONFIG_AS_CFI_SIGNAL_FRAME, I see another use in
arch/arc/kernel/unwind.c.  This change will cause inclusion of
additional code there, whereas for binutils produced within the past
ten years, there was not.

>
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> ---
>
>  arch/x86/Makefile             | 6 ++----
>  arch/x86/include/asm/dwarf2.h | 5 -----
>  2 files changed, 2 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 72f8f744ebd7..dd275008fc59 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -177,8 +177,6 @@ ifeq ($(ACCUMULATE_OUTGOING_ARGS), 1)
>         KBUILD_CFLAGS += $(call cc-option,-maccumulate-outgoing-args,)
>  endif
>
> -# is .cfi_signal_frame supported too?
> -cfi-sigframe := $(call as-instr,.cfi_startproc\n.cfi_signal_frame\n.cfi_endproc,-DCONFIG_AS_CFI_SIGNAL_FRAME=1)
>  cfi-sections := $(call as-instr,.cfi_sections .debug_frame,-DCONFIG_AS_CFI_SECTIONS=1)
>
>  # does binutils support specific instructions?
> @@ -190,8 +188,8 @@ sha1_ni_instr :=$(call as-instr,sha1msg1 %xmm0$(comma)%xmm1,-DCONFIG_AS_SHA1_NI=
>  sha256_ni_instr :=$(call as-instr,sha256msg1 %xmm0$(comma)%xmm1,-DCONFIG_AS_SHA256_NI=1)
>  adx_instr := $(call as-instr,adox %r10$(comma)%r10,-DCONFIG_AS_ADX=1)
>
> -KBUILD_AFLAGS += $(cfi-sigframe) $(cfi-sections) $(asinstr) $(avx_instr) $(avx2_instr) $(avx512_instr) $(sha1_ni_instr) $(sha256_ni_instr) $(adx_instr)
> -KBUILD_CFLAGS += $(cfi-sigframe) $(cfi-sections) $(asinstr) $(avx_instr) $(avx2_instr) $(avx512_instr) $(sha1_ni_instr) $(sha256_ni_instr) $(adx_instr)
> +KBUILD_AFLAGS += $(cfi-sections) $(asinstr) $(avx_instr) $(avx2_instr) $(avx512_instr) $(sha1_ni_instr) $(sha256_ni_instr) $(adx_instr)
> +KBUILD_CFLAGS += $(cfi-sections) $(asinstr) $(avx_instr) $(avx2_instr) $(avx512_instr) $(sha1_ni_instr) $(sha256_ni_instr) $(adx_instr)
>
>  KBUILD_LDFLAGS := -m elf_$(UTS_MACHINE)
>
> diff --git a/arch/x86/include/asm/dwarf2.h b/arch/x86/include/asm/dwarf2.h
> index 90807583cad7..d6697aab5706 100644
> --- a/arch/x86/include/asm/dwarf2.h
> +++ b/arch/x86/include/asm/dwarf2.h
> @@ -20,12 +20,7 @@
>  #define CFI_RESTORE_STATE      .cfi_restore_state
>  #define CFI_UNDEFINED          .cfi_undefined
>  #define CFI_ESCAPE             .cfi_escape
> -
> -#ifdef CONFIG_AS_CFI_SIGNAL_FRAME
>  #define CFI_SIGNAL_FRAME       .cfi_signal_frame

Has no uses in the kernel.

> -#else
> -#define CFI_SIGNAL_FRAME
> -#endif
>
>  #if defined(CONFIG_AS_CFI_SECTIONS)
>  #ifndef BUILD_VDSO
> --
> 2.17.1
>
> --

-- 
Thanks,
~Nick Desaulniers