On Mon, Feb 12, 2024 at 6:16 PM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
>
> On Tue, Feb 13, 2024 at 11:06 AM Nathan Chancellor <nathan@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx
> > Fixes: 90ceddcb4950 ("bpf: Support llvm-objcopy for vmlinux BTF")
> > Link: https://github.com/llvm/llvm-project/pull/75643
> > Suggested-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> > Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Thanks for updating the comment.
Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>
>
> 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@xxxxxxxxxx
> > ---
> > 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@xxxxxxxxxx>
> >
> >
>
>
> --
> Best Regards
> Masahiro Yamada
>
--
宋方睿
