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

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Message-ID: <02a27e2f-d269-0b04-a4ef-ebb347e3c918@fb.com>
Date:   Tue, 26 Nov 2019 23:47:06 +0000
From:   Andrii Nakryiko <andriin@...com>
To:     Stanislav Fomichev <sdf@...ichev.me>,
        John Fastabend <john.fastabend@...il.com>
CC:     Stanislav Fomichev <sdf@...gle.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>
Subject: Re: [PATCH bpf v2] bpf: support pre-2.25-binutils objcopy for vmlinux
 BTF

On 11/26/19 3:45 PM, Stanislav Fomichev wrote:
> On 11/26, John Fastabend wrote:
>> 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@...com>
>>> Cc: John Fastabend <john.fastabend@...il.com>
>>> Fixes: 341dfcf8d78ea ("btf: expose BTF info through sysfs")
>>> Signed-off-by: Stanislav Fomichev <sdf@...gle.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.
> With the conversion from --dump-section to --only-section that I
> did in this patch, binutils 2.25+ is no longer a requirement.
> 2.21 (minimal version from Documentation/process/changes.rst) should work
> just fine.

Yeah, instead it's better to mention that pahole v1.13+ is required.

>