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Date:   Wed, 31 Aug 2022 13:22:05 -0700
From:   Nathan Chancellor <nathan@kernel.org>
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Tom Rix <trix@redhat.com>, linux-kbuild@vger.kernel.org,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev, x86@kernel.org,
        Dmitrii Bundin <dmitrii.bundin.a@gmail.com>,
        Fangrui Song <maskray@google.com>,
        Alexey Alexandrov <aalexand@google.com>,
        Bill Wendling <morbo@google.com>,
        Greg Thelen <gthelen@google.com>
Subject: Re: [PATCH v2 4/5] Makefile.debug: re-enable debug info for .S files
Message-ID: <Yw/C7fYWP+KkVKk2@dev-arch.thelio-3990X>
References: <20220831184408.2778264-1-ndesaulniers@google.com>
 <20220831184408.2778264-5-ndesaulniers@google.com>
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On Wed, Aug 31, 2022 at 11:44:07AM -0700, Nick Desaulniers wrote:
> Alexey reported that the fraction of unknown filename instances in
> kallsyms grew from ~0.3% to ~10% recently; Bill and Greg tracked it down
> to assembler defined symbols, which regressed as a result of:
> 
> commit b8a9092330da ("Kbuild: do not emit debug info for assembly with LLVM_IAS=1")
> 
> In that commit, I allude to restoring debug info for assembler defined
> symbols in a follow up patch, but it seems I forgot to do so in
> 
> commit a66049e2cf0e ("Kbuild: make DWARF version a choice")
> 
> This patch does a few things:
> 1. Add -g to KBUILD_AFLAGS. This will instruct the compiler to instruct
>    the assembler to emit debug info. But this can cause an issue for
>    folks using a newer compiler but older assembler, because the
>    implicit default DWARF version changed from v4 to v5 in gcc-11 and
>    clang-14.
> 2. If the user is using CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT, use a
>    version check to explicitly set -Wa,-gdwarf-<version> for the
>    assembler. There's another problem with this; GAS only gained support
>    for explicit DWARF versions 3-5 in the 2.36 GNU binutils release.
> 3. Wrap -Wa,-gdwarf-<version> in as-option call to test whether the
>    assembler supports that explicit DWARF version.
> 
> Link: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=31bf18645d98b4d3d7357353be840e320649a67d
> Fixes: b8a9092330da ("Kbuild: do not emit debug info for assembly with LLVM_IAS=1")
> Reported-by: Alexey Alexandrov <aalexand@google.com>
> Reported-by: Bill Wendling <morbo@google.com>
> Reported-by: Greg Thelen <gthelen@google.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

> ---
> Changes v1 -> v2:
> * Use newly added compiler-specific macros, as per Bill.
> 
>  scripts/Makefile.debug | 22 ++++++++++++++++++----
>  1 file changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/scripts/Makefile.debug b/scripts/Makefile.debug
> index 9f39b0130551..46e88f0ca998 100644
> --- a/scripts/Makefile.debug
> +++ b/scripts/Makefile.debug
> @@ -4,18 +4,32 @@ ifdef CONFIG_DEBUG_INFO_SPLIT
>  DEBUG_CFLAGS	+= -gsplit-dwarf
>  else
>  DEBUG_CFLAGS	+= -g
> +KBUILD_AFLAGS	+= -g
>  endif
>  
> -ifndef CONFIG_AS_IS_LLVM
> -KBUILD_AFLAGS	+= -Wa,-gdwarf-2
> +ifdef CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
> +# gcc-11+, clang-14+
> +ifeq ($(call cc-min-version, 110000, 140000),y)
> +dwarf-version-y := 5
> +else
> +dwarf-version-y := 4
>  endif
> -
> -ifndef CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
> +else # !CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
>  dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
>  dwarf-version-$(CONFIG_DEBUG_INFO_DWARF5) := 5
>  DEBUG_CFLAGS	+= -gdwarf-$(dwarf-version-y)
>  endif
>  
> +# Binutils 2.35+ (or clang) required for -gdwarf-{4|5}.
> +# https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=31bf18645d98b4d3d7357353be840e320649a67d
> +ifneq ($(call as-option,-Wa$(comma)-gdwarf-$(dwarf-version-y)),)
> +KBUILD_AFLAGS	+= -Wa,-gdwarf-$(dwarf-version-y)
> +else
> +ifndef CONFIG_AS_IS_LLVM
> +KBUILD_AFLAGS	+= -Wa,-gdwarf-2
> +endif
> +endif
> +
>  ifdef CONFIG_DEBUG_INFO_REDUCED
>  DEBUG_CFLAGS	+= -fno-var-tracking
>  ifdef CONFIG_CC_IS_GCC
> -- 
> 2.37.2.672.g94769d06f0-goog
>