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Subject: Re: [PATCH v2] powerpc: Do not consider weak unresolved symbol
 relocations as bad
To:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Alexei Starovoitov <ast@kernel.org>,
        linux-next@vger.kernel.org, Zong Li <zong.li@sifive.com>,
        Palmer Dabbelt <palmerdabbelt@google.com>
References: <20200118170335.21440-1-alex@ghiti.fr>
From:   Alex Ghiti <alex@ghiti.fr>
Message-ID: <8a8d45c6-4ad2-c682-abfb-3d97188d0d45@ghiti.fr>
Date:   Thu, 30 Jan 2020 15:07:10 -0500
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On 1/18/20 12:03 PM, Alexandre Ghiti wrote:
> Commit 8580ac9404f6 ("bpf: Process in-kernel BTF") introduced two weak
> symbols that may be unresolved at link time which result in an absolute
> relocation to 0. relocs_check.sh emits the following warning:
>
> "WARNING: 2 bad relocations
> c000000001a41478 R_PPC64_ADDR64    _binary__btf_vmlinux_bin_start
> c000000001a41480 R_PPC64_ADDR64    _binary__btf_vmlinux_bin_end"
>
> whereas those relocations are legitimate even for a relocatable kernel
> compiled with -pie option.
>
> relocs_check.sh already excluded some weak unresolved symbols explicitly:
> remove those hardcoded symbols and add some logic that parses the symbols
> using nm, retrieves all the weak unresolved symbols and excludes those from
> the list of the potential bad relocations.
>
> Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
> ---
>
> Changes in v2:
> - Follow Stephen advice of using grep -F instead of looping over weak symbols
>    using read, patch is way smaller and cleaner.
> - Add missing nm in comment
>
>   arch/powerpc/Makefile.postlink     |  4 ++--
>   arch/powerpc/tools/relocs_check.sh | 20 ++++++++++++--------
>   2 files changed, 14 insertions(+), 10 deletions(-)
>
> diff --git a/arch/powerpc/Makefile.postlink b/arch/powerpc/Makefile.postlink
> index 134f12f89b92..2268396ff4bb 100644
> --- a/arch/powerpc/Makefile.postlink
> +++ b/arch/powerpc/Makefile.postlink
> @@ -17,11 +17,11 @@ quiet_cmd_head_check = CHKHEAD $@
>   quiet_cmd_relocs_check = CHKREL  $@
>   ifdef CONFIG_PPC_BOOK3S_64
>         cmd_relocs_check =						\
> -	$(CONFIG_SHELL) $(srctree)/arch/powerpc/tools/relocs_check.sh "$(OBJDUMP)" "$@" ; \
> +	$(CONFIG_SHELL) $(srctree)/arch/powerpc/tools/relocs_check.sh "$(OBJDUMP)" "$(NM)" "$@" ; \
>   	$(BASH) $(srctree)/arch/powerpc/tools/unrel_branch_check.sh "$(OBJDUMP)" "$@"
>   else
>         cmd_relocs_check =						\
> -	$(CONFIG_SHELL) $(srctree)/arch/powerpc/tools/relocs_check.sh "$(OBJDUMP)" "$@"
> +	$(CONFIG_SHELL) $(srctree)/arch/powerpc/tools/relocs_check.sh "$(OBJDUMP)" "$(NM)" "$@"
>   endif
>   
>   # `@true` prevents complaint when there is nothing to be done
> diff --git a/arch/powerpc/tools/relocs_check.sh b/arch/powerpc/tools/relocs_check.sh
> index 7b9fe0a567cf..014e00e74d2b 100755
> --- a/arch/powerpc/tools/relocs_check.sh
> +++ b/arch/powerpc/tools/relocs_check.sh
> @@ -10,14 +10,21 @@
>   # based on relocs_check.pl
>   # Copyright © 2009 IBM Corporation
>   
> -if [ $# -lt 2 ]; then
> -	echo "$0 [path to objdump] [path to vmlinux]" 1>&2
> +if [ $# -lt 3 ]; then
> +	echo "$0 [path to objdump] [path to nm] [path to vmlinux]" 1>&2
>   	exit 1
>   fi
>   
> -# Have Kbuild supply the path to objdump so we handle cross compilation.
> +# Have Kbuild supply the path to objdump and nm so we handle cross compilation.
>   objdump="$1"
> -vmlinux="$2"
> +nm="$2"
> +vmlinux="$3"
> +
> +# Remove from the bad relocations those that match an undefined weak symbol
> +# which will result in an absolute relocation to 0.
> +# Weak unresolved symbols are of that form in nm output:
> +# "                  w _binary__btf_vmlinux_bin_end"
> +undef_weak_symbols=$($nm "$vmlinux" | awk '$1 ~ /w/ { print $2 }')
>   
>   bad_relocs=$(
>   $objdump -R "$vmlinux" |
> @@ -26,8 +33,6 @@ $objdump -R "$vmlinux" |
>   	# These relocations are okay
>   	# On PPC64:
>   	#	R_PPC64_RELATIVE, R_PPC64_NONE
> -	#	R_PPC64_ADDR64 mach_<name>
> -	#	R_PPC64_ADDR64 __crc_<name>
>   	# On PPC:
>   	#	R_PPC_RELATIVE, R_PPC_ADDR16_HI,
>   	#	R_PPC_ADDR16_HA,R_PPC_ADDR16_LO,
> @@ -39,8 +44,7 @@ R_PPC_ADDR16_HI
>   R_PPC_ADDR16_HA
>   R_PPC_RELATIVE
>   R_PPC_NONE' |
> -	grep -E -v '\<R_PPC64_ADDR64[[:space:]]+mach_' |
> -	grep -E -v '\<R_PPC64_ADDR64[[:space:]]+__crc_'
> +	([ "$undef_weak_symbols" ] && grep -F -w -v "$undef_weak_symbols" || cat)
>   )
>   
>   if [ -z "$bad_relocs" ]; then


Hi guys,


Any thought about that ?

I do think this patch makes the whole check about absolute relocations 
clearer.
And in the future, it will avoid anyone to spend some time on those 
"bad" relocations
which actually aren't.

Thanks,

Alex