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

Nathan Chancellor
2020-03-16T23:57:28Z

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>