Re: [PATCH v2] kbuild: Fix changing ELF file type for output of gen_btf for big endian

[Justin Stitt <justinstitt@google.com>]

Hi,

On Mon, Feb 12, 2024 at 07:05:10PM -0700, Nathan Chancellor wrote:
> Commit 90ceddcb4950 ("bpf: Support llvm-objcopy for vmlinux BTF")
> changed the ELF type of .btf.vmlinux.bin.o 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 updating the entire 16-bit e_type field rather than just a
> single byte, so that everything works correctly for all platforms and
> 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)
>
> While in the area, update the comment to mention that binutils 2.35+
> matches LLD's behavior of rejecting an ET_EXEC input, which occurred
> after the comment was added.
>
> Cc: stable@vger.kernel.org
> Fixes: 90ceddcb4950 ("bpf: Support llvm-objcopy for vmlinux BTF")
> Link: https://github.com/llvm/llvm-project/pull/75643
> Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

I prefer this version to v1 as well. This seems better than setting the
seek value.

Reviewed-by: Justin Stitt <justinstitt@google.com>

> ---
> Changes in v2:
> - Rather than change the seek value for dd, update the entire e_type
>   field (Masahiro). Due to this change, I did not carry forward the
>   tags of v1.
> - Slightly update commit message to remove mention of ET_EXEC, which
>   does not match the dump (Masahiro).
> - Update comment to mention binutils 2.35+ has the same behavior as LLD
>   (Fangrui).
> - Link to v1: https://lore.kernel.org/r/20240208-fix-elf-type-btf-vmlinux-bin-o-big-endian-v1-1-cb3112491edc@kernel.org
> ---
>  scripts/link-vmlinux.sh | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index a432b171be82..7862a8101747 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/scripts/link-vmlinux.sh
> @@ -135,8 +135,13 @@ 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
> +	# GNU ld 2.35+ and lld do not allow an ET_EXEC input.
> +	if is_enabled CONFIG_CPU_BIG_ENDIAN; then
> +		et_rel='\0\1'
> +	else
> +		et_rel='\1\0'
> +	fi
> +	printf "${et_rel}" | dd of=${2} conv=notrunc bs=1 seek=16 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
Justin