Re: [PATCH] kbuild: Fix changing ELF file type for output of gen_btf for big endian

[Nicolas Schier <nicolas@fjasle.eu>]

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On Thu, Feb 08, 2024 at 01:21:06PM -0700 Nathan Chancellor wrote:
> Commit 90ceddcb4950 ("bpf: Support llvm-objcopy for vmlinux BTF")
> changed the ELF type of .btf.vmlinux.bin.o from ET_EXEC to ET_REL via
> dd, which works fine for little endian platforms:
> 
>    00000000  7f 45 4c 46 02 01 01 00  00 00 00 00 00 00 00 00  |.ELF............|
>   -00000010  03 00 b7 00 01 00 00 00  00 00 00 80 00 80 ff ff  |................|
>   +00000010  01 00 b7 00 01 00 00 00  00 00 00 80 00 80 ff ff  |................|
> 
> However, for big endian platforms, it changes the wrong byte, resulting
> in an invalid ELF file type, which ld.lld rejects:
> 
>    00000000  7f 45 4c 46 02 02 01 00  00 00 00 00 00 00 00 00  |.ELF............|
>   -00000010  00 03 00 16 00 00 00 01  00 00 00 00 00 10 00 00  |................|
>   +00000010  01 03 00 16 00 00 00 01  00 00 00 00 00 10 00 00  |................|
> 
>   Type:                              <unknown>: 103
> 
>   ld.lld: error: .btf.vmlinux.bin.o: unknown file type
> 
> Fix this by using a different seek value for dd when targeting big
> endian, so that the correct byte gets changed and everything works
> correctly for all linkers.
> 
>    00000000  7f 45 4c 46 02 02 01 00  00 00 00 00 00 00 00 00  |.ELF............|
>   -00000010  00 03 00 16 00 00 00 01  00 00 00 00 00 10 00 00  |................|
>   +00000010  00 01 00 16 00 00 00 01  00 00 00 00 00 10 00 00  |................|
> 
>   Type:                              REL (Relocatable file)
> 
> Cc: stable@vger.kernel.org
> Fixes: 90ceddcb4950 ("bpf: Support llvm-objcopy for vmlinux BTF")
> Link: https://github.com/llvm/llvm-project/pull/75643
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>  scripts/link-vmlinux.sh | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index a432b171be82..8a9f48b3cb32 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/scripts/link-vmlinux.sh
> @@ -135,8 +135,15 @@ gen_btf()
>  	${OBJCOPY} --only-section=.BTF --set-section-flags .BTF=alloc,readonly \
>  		--strip-all ${1} ${2} 2>/dev/null
>  	# Change e_type to ET_REL so that it can be used to link final vmlinux.
> -	# Unlike GNU ld, lld does not allow an ET_EXEC input.
> -	printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16 status=none
> +	# Unlike GNU ld, lld does not allow an ET_EXEC input. Make sure the correct
> +	# byte gets changed with big endian platforms, otherwise e_type may be an
> +	# invalid value.
> +	if is_enabled CONFIG_CPU_BIG_ENDIAN; then
> +		seek=17
> +	else
> +		seek=16
> +	fi
> +	printf '\1' | dd of=${2} conv=notrunc bs=1 seek=${seek} status=none
>  }
>  
>  # Create ${2} .S file with all symbols from the ${1} object file
> 
> ---
> base-commit: 54be6c6c5ae8e0d93a6c4641cb7528eb0b6ba478
> change-id: 20240208-fix-elf-type-btf-vmlinux-bin-o-big-endian-dbc55a1e1296
> 
> Best regards,
> -- 
> Nathan Chancellor <nathan@kernel.org>
> 

Thanks for the verbose examples!
Looks good to me.

Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

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