Re: [PATCH] Makefile: fix GDB warning with CONFIG_RELR

From: Nathan Chancellor <nathan@kernel.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>,
	clang-built-linux@googlegroups.com,
	Fangrui Song <maskray@google.com>,
	Elliot Berman <eberman@quicinc.com>,
	Sami Tolvanen <samitolvanen@google.com>,
	Peter Collingbourne <pcc@google.com>,
	Michal Marek <michal.lkml@markovi.net>,
	linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Makefile: fix GDB warning with CONFIG_RELR
Date: Tue, 23 Mar 2021 12:05:32 -0700	[thread overview]
Message-ID: <20210323190532.eiqxmskiankf7hn3@archlinux-ax161> (raw)
In-Reply-To: <20210319000708.1694662-1-ndesaulniers@google.com>

On Thu, Mar 18, 2021 at 05:07:06PM -0700, Nick Desaulniers wrote:
> GDB produces the following warning when debugging kernels built with
> CONFIG_RELR:
> 
> BFD: /android0/linux-next/vmlinux: unknown type [0x13] section `.relr.dyn'
> 
> when loading a kernel built with CONFIG_RELR into GDB. It can also
> prevent debugging symbols using such relocations.
> 
> Peter sugguests:
>   [That flag] means that lld will use dynamic tags and section type
>   numbers in the OS-specific range rather than the generic range. The
>   kernel itself doesn't care about these numbers; it determines the
>   location of the RELR section using symbols defined by a linker script.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1057
> Suggested-by: Peter Collingbourne <pcc@google.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

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

> ---
>  Makefile                      | 2 +-
>  scripts/tools-support-relr.sh | 3 ++-
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index 5160ff8903c1..47741cb60995 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1088,7 +1088,7 @@ LDFLAGS_vmlinux	+= $(call ld-option, -X,)
>  endif
>  
>  ifeq ($(CONFIG_RELR),y)
> -LDFLAGS_vmlinux	+= --pack-dyn-relocs=relr
> +LDFLAGS_vmlinux	+= --pack-dyn-relocs=relr --use-android-relr-tags
>  endif
>  
>  # We never want expected sections to be placed heuristically by the
> diff --git a/scripts/tools-support-relr.sh b/scripts/tools-support-relr.sh
> index 45e8aa360b45..cb55878bd5b8 100755
> --- a/scripts/tools-support-relr.sh
> +++ b/scripts/tools-support-relr.sh
> @@ -7,7 +7,8 @@ trap "rm -f $tmp_file.o $tmp_file $tmp_file.bin" EXIT
>  cat << "END" | $CC -c -x c - -o $tmp_file.o >/dev/null 2>&1
>  void *p = &p;
>  END
> -$LD $tmp_file.o -shared -Bsymbolic --pack-dyn-relocs=relr -o $tmp_file
> +$LD $tmp_file.o -shared -Bsymbolic --pack-dyn-relocs=relr \
> +  --use-android-relr-tags -o $tmp_file
>  
>  # Despite printing an error message, GNU nm still exits with exit code 0 if it
>  # sees a relr section. So we need to check that nothing is printed to stderr.
> -- 
> 2.31.0.rc2.261.g7f71774620-goog
>