Re: [PATCH bpf] bpf: Support llvm-objcopy for vmlinux BTF

Stanislav Fomichev
2020-03-17T00:01:40Z

On 03/16, Nathan Chancellor wrote:
> On Mon, Mar 16, 2020 at 03:25:18PM -0700, Stanislav Fomichev wrote:
> > Commit da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux
> > BTF") switched from --dump-section to
> > --only-section/--change-section-address for BTF export assuming
> > those ("legacy") options should cover all objcopy versions.
> >
> > Turns out llvm-objcopy doesn't implement --change-section-address [1],
> > but it does support --dump-section. Let's partially roll back and
> > try to use --dump-section first and fall back to
> > --only-section/--change-section-address for the older binutils.
> >
> > 1. https://bugs.llvm.org/show_bug.cgi?id=45217
> >
> > Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
> > Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> > Reported-by: Nathan Chancellor <natechancellor@gmail.com>
> > Link: https://github.com/ClangBuiltLinux/linux/issues/871
> > Signed-off-by: Stanislav Fomichev <sdf@google.com>
> > ---
> > scripts/link-vmlinux.sh | 10 ++++++++++
> > 1 file changed, 10 insertions(+)
> >
> > diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> > index dd484e92752e..8ddf57cbc439 100755
> > --- a/scripts/link-vmlinux.sh
> > +++ b/scripts/link-vmlinux.sh
> > @@ -127,6 +127,16 @@ gen_btf()
> > cut -d, -f1 | cut -d' ' -f2)
> > bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
> > awk '{print $4}')
> > +
> > + # Compatibility issues:
> > + # - pre-2.25 binutils objcopy doesn't support --dump-section
> > + # - llvm-objcopy doesn't support --change-section-address, but
> > + # does support --dump-section
> > + #
> > + # Try to use --dump-section which should cover both recent
> > + # binutils and llvm-objcopy and fall back to --only-section
> > + # for pre-2.25 binutils.
> > + ${OBJCOPY} --dump-section .BTF=$bin_file ${1} 2>/dev/null || \
> > ${OBJCOPY} --change-section-address .BTF=0 \
> > --set-section-flags .BTF=alloc -O binary \
> > --only-section=.BTF ${1} .btf.vmlinux.bin
> > --
> > 2.25.1.481.gfbce0eb801-goog
> >
>
> Hi Stanislav,
>
> Thank you for the patch! This is targeting the bpf tree but it uses the
> bin_file variable that comes from commit af73d78bd384 ("kbuild: Remove
> debug info from kallsyms linking") in the bpf-next tree. In this form,
> when applied to mainline, the first command fails because $bin_file
> doesn't exist and falls back to the second command, which results in no
> net change for llvm-objcopy.
>
> When manually applied to next-20200316 or applied to mainline with
> $bin_file replaced with .btf.vmlinux.bin, x86_64 and aarch64 successfully
> generate BTF with tip of tree llvm-objcopy.
>
> Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Oops, sorry, I did it on top of bpf-next, but then blindly cherry-picked
on top of bpf tree. In this case, should we put in only in bpf-next?
Do we need a stable backport for the older versions?