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

[Fangrui Song <maskray@google.com>]

On Mon, Feb 12, 2024 at 6:16 PM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> On Tue, Feb 13, 2024 at 11:06 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 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>

Thanks for updating the comment.

Reviewed-by: Fangrui Song <maskray@google.com>

>
> Thanks.
>
> I will wait for a few days until
> the reviewers come back to give Reviewed-by again.
>
>
>
>
>
> > ---
> > 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>
> >
> >
>
>
> --
> Best Regards
> Masahiro Yamada
>


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