linux-kernel - Re: [PATCH v2 4/4] Kbuild: implement support for DWARF v5

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Date:   Tue, 24 Nov 2020 12:28:36 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Jakub Jelinek <jakub@...hat.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-toolchains@...r.kernel.org,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Fangrui Song <maskray@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Dmitry Golovin <dima@...ovin.in>,
        Alistair Delva <adelva@...gle.com>
Subject: Re: [PATCH v2 4/4] Kbuild: implement support for DWARF v5

On Tue, Nov 03, 2020 at 04:53:43PM -0800, Nick Desaulniers wrote:
> DWARF v5 is the latest standard of the DWARF debug info format.
> 
> Feature detection of DWARF5 is onerous, especially given that we've
> removed $(AS), so we must query $(CC) for DWARF5 assembler directive
> support.  GNU `as` only recently gained support for specifying
> -gdwarf-5.

With gcc, using -gdwarf-5 even without -Wa,--gdwarf-5 results in
considerably smaller debug info. gcc does not seem to generate the .file 0
directive that causes older GNU as to barf.

Should the assembler support check be restricted to CC_IS_CLANG?

>  /* Stabs debugging sections. */
>  #define STABS_DEBUG							\
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 03c494eefabd..c5b54ba51060 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -274,6 +274,14 @@ config DEBUG_INFO_DWARF4
>  	  It makes the debug information larger, but it significantly
>  	  improves the success of resolving variables in gdb on optimized code.
>  
> +config DEBUG_INFO_DWARF5
> +	bool "Generate DWARF5 debuginfo"
> +	depends on $(cc-option,-gdwarf-5)
> +	depends on $(success,$(srctree)/scripts/test_dwarf5_support.sh $(CC) $(CLANG_FLAGS))
> +	help
> +	  Genereate dwarf5 debug info. Requires binutils 2.35+, gcc 5.1+, and
> +	  gdb 8.0+.
> +
>  endchoice # "DWARF version"

Perhaps this can be expanded with some description of the advantages of
dwarf5 over dwarf4?

>  
>  config DEBUG_INFO_BTF
> diff --git a/scripts/test_dwarf5_support.sh b/scripts/test_dwarf5_support.sh
> new file mode 100755
> index 000000000000..156ad5ec4274
> --- /dev/null
> +++ b/scripts/test_dwarf5_support.sh
> @@ -0,0 +1,9 @@
> +#!/bin/sh
> +# SPDX-License-Identifier: GPL-2.0
> +
> +# Test that assembler accepts -gdwarf-5 and .file 0 directives, which were bugs
> +# in binutils < 2.35.
> +# https://sourceware.org/bugzilla/show_bug.cgi?id=25612
> +# https://sourceware.org/bugzilla/show_bug.cgi?id=25614
> +set -e
> +echo '.file 0 "filename"' | $* -Wa,-gdwarf-5 -c -x assembler -o /dev/null -

This also actually needs --gdwarf-5 to really check the support for the
option, but older versions should error on the .file 0 in any case.