RE: [PATCH bpf v2] bpf: support pre-2.25-binutils objcopy for vmlinux BTF

From: John Fastabend <john.fastabend@gmail.com>
To: Stanislav Fomichev <sdf@google.com>,
	netdev@vger.kernel.org, bpf@vger.kernel.org
Cc: davem@davemloft.net, ast@kernel.org, daniel@iogearbox.net,
	Stanislav Fomichev <sdf@google.com>,
	Andrii Nakryiko <andriin@fb.com>,
	John Fastabend <john.fastabend@gmail.com>
Subject: RE: [PATCH bpf v2] bpf: support pre-2.25-binutils objcopy for vmlinux BTF
Date: Tue, 26 Nov 2019 15:36:37 -0800	[thread overview]
Message-ID: <5dddb7059b13e_13b82abee0d625bc2d@john-XPS-13-9370.notmuch> (raw)
In-Reply-To: <20191126232818.226454-1-sdf@google.com>

Stanislav Fomichev wrote:
> If vmlinux BTF generation fails, but CONFIG_DEBUG_INFO_BTF is set,
> .BTF section of vmlinux is empty and kernel will prohibit
> BPF loading and return "in-kernel BTF is malformed".
> 
> --dump-section argument to binutils' objcopy was added in version 2.25.
> When using pre-2.25 binutils, BTF generation silently fails. Convert
> to --only-section which is present on pre-2.25 binutils.
> 
> Documentation/process/changes.rst states that binutils 2.21+
> is supported, not sure those standards apply to BPF subsystem.
> 
> v2:
> * exit and print an error if gen_btf fails (John Fastabend)
> 
> Cc: Andrii Nakryiko <andriin@fb.com>
> Cc: John Fastabend <john.fastabend@gmail.com>
> Fixes: 341dfcf8d78ea ("btf: expose BTF info through sysfs")
> Signed-off-by: Stanislav Fomichev <sdf@google.com>
> ---
>  scripts/link-vmlinux.sh | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index 06495379fcd8..2998ddb323e3 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/scripts/link-vmlinux.sh
> @@ -127,7 +127,8 @@ gen_btf()
>  		cut -d, -f1 | cut -d' ' -f2)
>  	bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
>  		awk '{print $4}')
> -	${OBJCOPY} --dump-section .BTF=.btf.vmlinux.bin ${1} 2>/dev/null
> +	${OBJCOPY} --set-section-flags .BTF=alloc -O binary \
> +		--only-section=.BTF ${1} .btf.vmlinux.bin 2>/dev/null
>  	${OBJCOPY} -I binary -O ${bin_format} -B ${bin_arch} \
>  		--rename-section .data=.BTF .btf.vmlinux.bin ${2}
>  }
> @@ -253,6 +254,10 @@ btf_vmlinux_bin_o=""
>  if [ -n "${CONFIG_DEBUG_INFO_BTF}" ]; then
>  	if gen_btf .tmp_vmlinux.btf .btf.vmlinux.bin.o ; then
>  		btf_vmlinux_bin_o=.btf.vmlinux.bin.o
> +	else
> +		echo >&2 "Failed to generate BTF for vmlinux"
> +		echo >&2 "Try to disable CONFIG_DEBUG_INFO_BTF"

I think we should encourage upgrading binutils first? Maybe

"binutils 2.25+ required for BTF please upgrade or disable CONFIG_DEBUG_INFO_BTF"

otherwise I guess its going to be a bit mystical why it works in
cases and not others to folks unfamiliar with the details.

> +		exit 1
>  	fi
>  fi
>  
> -- 
> 2.24.0.432.g9d3f5f5b63-goog
> 