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

Fangrui Song
2024-02-08T21:22:36Z

On Thu, Feb 8, 2024 at 1:11 PM Nicolas Schier <nicolas@fjasle.eu> wrote:
>
> On Thu, Feb 08, 2024 at 01:21:06PM -0700 Nathan Chancellor 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>
> > ---
> > 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.

From binutils 2.35 onwards, GNU ld also rejects ET_EXEC input. So the
comment can be adjusted.

% file a2
a2: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically
linked, not stripped
% ld.bfd a2 -o a3
ld.bfd: cannot use executable file 'a2' as input to a link

> > - 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>
> >
>
> Thanks for the verbose examples!
> Looks good to me.
>
> Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

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

--
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