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

Masahiro Yamada
2024-02-13T00:55:46Z

On Fri, Feb 9, 2024 at 5:21 AM Nathan Chancellor <nathan@kernel.org> 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 |................|

I am afraid this dump is confusing.

The byte stream "03 00" is ET_DYN, as specified in ELF:

Name Value
------------------
ET_REL 1
ET_EXEC 2
ET_DYN 3

It disagrees with your commit message "from ET_EXEC to ET_REL"

The dump for the old ELF was "02 00", wasn't it?

> +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:

Fangrui pointed out this is true for inutils >= 2.35

>
> 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 |................|

- 00 02
+ 01 02

>
> 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 |................|

Ditto.

> +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

Do you want to send v2 to update the commit description?

The current code will work, but another approach might be to
update both byte 16 and byte 17 because e_type is a 16-bit field.

It works without relying on the MSB of the previous e_type being zero.
The comment does not need updating because the intention is obvious
from the code.

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

> ---
> base-commit: 54be6c6c5ae8e0d93a6c4641cb7528eb0b6ba478
> change-id: 20240208-fix-elf-type-btf-vmlinux-bin-o-big-endian-dbc55a1e1296
>
> Best regards,
> --
> Nathan Chancellor <nathan@kernel.org>
>

--
Best Regards
Masahiro Yamada