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

[Justin Stitt <justinstitt@google.com>]

On Thu, Feb 8, 2024 at 12:21 PM 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  |................|
>   +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>

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

> ---
>  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>
>