linux-kernel - Re: [PATCH v2] Makefile: Improve compressed debug info support detection

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Date:   Thu, 11 Jun 2020 16:38:57 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Fangrui Song <maskray@...gle.com>,
        Rong Chen <rong.a.chen@...el.com>,
        kernel test robot <lkp@...el.com>, kbuild-all@...ts.01.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] Makefile: Improve compressed debug info support detection

On Thu, Jun 11, 2020 at 3:03 PM Arvind Sankar <nivedita@...m.mit.edu> wrote:
>
> Commit
>   10e68b02c861 ("Makefile: support compressed debug info")
> added support for compressed debug sections.
>
> Support is detected by checking
> - does the compiler support -gz=zlib
> - does the assembler support --compressed-debug-sections=zlib
> - does the linker support --compressed-debug-sections=zlib
>
> However, the gcc driver's support for this option is somewhat
> convoluted. The driver's builtin specs are set based on the version of
> binutils that it was configured with. It reports an error if the
> configure-time linker/assembler (i.e., not necessarily the actual
> assembler that will be run) do not support the option, but only if the
> assembler (or linker) is actually invoked when -gz=zlib is passed.
>
> The cc-option check in scripts/Kconfig.include does not invoke the
> assembler, so the gcc driver reports success even if it does not support
> the option being passed to the assembler.
>
> Because the as-option check passes the option directly to the assembler
> via -Wa,--compressed-debug-sections=zlib, the gcc driver does not see
> this option and will never report an error.

Thanks for expanding on the failure.
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

>
> Combined with an installed version of binutils that is more recent than
> the one the compiler was built with, it is possible for all three tests
> to succeed, yet an actual compilation with -gz=zlib to fail.
>
> Moreover, it is unnecessary to explicitly pass
> --compressed-debug-sections=zlib to the assembler via -Wa, since the
> driver will do that automatically when it supports -gz=zlib.
>
> Convert the as-option to just -gz=zlib, simplifying it as well as
> performing a better test of the gcc driver's capabilities.
>
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> ---
>  Makefile          | 2 +-
>  lib/Kconfig.debug | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 839f9fee22cb..cb29e56f227a 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -842,7 +842,7 @@ endif
>
>  ifdef CONFIG_DEBUG_INFO_COMPRESSED
>  DEBUG_CFLAGS   += -gz=zlib
> -KBUILD_AFLAGS  += -Wa,--compress-debug-sections=zlib
> +KBUILD_AFLAGS  += -gz=zlib
>  KBUILD_LDFLAGS += --compress-debug-sections=zlib
>  endif
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index cb98741601bd..94ce36be470c 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -229,7 +229,7 @@ config DEBUG_INFO_COMPRESSED
>         bool "Compressed debugging information"
>         depends on DEBUG_INFO
>         depends on $(cc-option,-gz=zlib)
> -       depends on $(as-option,-Wa$(comma)--compress-debug-sections=zlib)
> +       depends on $(as-option,-gz=zlib)
>         depends on $(ld-option,--compress-debug-sections=zlib)
>         help
>           Compress the debug information using zlib.  Requires GCC 5.0+ or Clang
> --
> 2.26.2
>


-- 
Thanks,
~Nick Desaulniers