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

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Fangrui Song <maskray@google.com>
Cc: bpf <bpf@vger.kernel.org>, Networking <netdev@vger.kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Nathan Chancellor <natechancellor@gmail.com>,
	clang-built-linux@googlegroups.com,
	Stanislav Fomichev <sdf@google.com>,
	"David S. Miller" <davem@davemloft.net>,
	Alexei Starovoitov <ast@kernel.org>
Subject: Re: [PATCH bpf] bpf: Support llvm-objcopy and llvm-objdump for vmlinux BTF
Date: Mon, 16 Mar 2020 20:10:31 -0700	[thread overview]
Message-ID: <CAEf4BzYTJqWU++QnQupxFBWGSMPfGt6r-5u9jbeLnEF2ipw+Mw@mail.gmail.com> (raw)
In-Reply-To: <20200317011654.zkx5r7so53skowlc@google.com>

On Mon, Mar 16, 2020 at 6:17 PM Fangrui Song <maskray@google.com> wrote:
>
> Simplify gen_btf logic to make it work with llvm-objcopy and
> llvm-objdump.  We just need to retain one section .BTF. To do so, we can
> use a simple objcopy --only-section=.BTF instead of jumping all the
> hoops via an architecture-less binary file.
>
> We use a dd comment to change the e_type field in the ELF header from
> ET_EXEC to ET_REL so that .btf.vmlinux.bin.o will be accepted by lld.
>
> Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
> Cc: Stanislav Fomichev <sdf@google.com>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> 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: Fangrui Song <maskray@google.com>
> ---
>  scripts/link-vmlinux.sh | 13 ++-----------
>  1 file changed, 2 insertions(+), 11 deletions(-)
>
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index dd484e92752e..84be8d7c361d 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/scripts/link-vmlinux.sh
> @@ -120,18 +120,9 @@ gen_btf()
>
>         info "BTF" ${2}
>         vmlinux_link ${1}
> -       LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}

Is it really tested? Seems like you just dropped .BTF generation step
completely...

>
> -       # dump .BTF section into raw binary file to link with final vmlinux
> -       bin_arch=$(LANG=C ${OBJDUMP} -f ${1} | grep architecture | \
> -               cut -d, -f1 | cut -d' ' -f2)
> -       bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
> -               awk '{print $4}')
> -       ${OBJCOPY} --change-section-address .BTF=0 \
> -               --set-section-flags .BTF=alloc -O binary \
> -               --only-section=.BTF ${1} .btf.vmlinux.bin
> -       ${OBJCOPY} -I binary -O ${bin_format} -B ${bin_arch} \
> -               --rename-section .data=.BTF .btf.vmlinux.bin ${2}
> +       # Extract .BTF section, change e_type to ET_REL, to link with final vmlinux
> +       ${OBJCOPY} --only-section=.BTF ${1} ${2} && printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16
>  }
>
>  # Create ${2} .o file with all symbols from the ${1} object file
> --
> 2.25.1.481.gfbce0eb801-goog