linux-kernel - Re: [PATCH bpf-next] selftests/bpf: Fix compile error when MPTCP not support

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Message-ID: <9b2b3c98-503b-45ae-bcdd-ac2fcc62e14c@kernel.org>
Date: Wed, 30 Oct 2024 17:31:15 +0100
From: Matthieu Baerts <matttbe@...nel.org>
To: Tao Chen <chen.dylane@...il.com>, Mat Martineau <martineau@...nel.org>,
 Geliang Tang <geliang@...nel.org>, Andrii Nakryiko <andrii@...nel.org>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Martin KaFai Lau <martin.lau@...ux.dev>,
 Yonghong Song <yonghong.song@...ux.dev>, Jiri Olsa <jolsa@...nel.org>
Cc: bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-kselftest@...r.kernel.org, mptcp@...ts.linux.dev
Subject: Re: [PATCH bpf-next] selftests/bpf: Fix compile error when MPTCP not
 support

Hi Tao, BPF maintainers,

On 30/10/2024 12:12, Tao Chen wrote:
> 在 2024/10/30 18:49, Matthieu Baerts 写道:
>> Hi Tao Chen,
>>
>> Thank you for having shared this patch.
>>
>> On 30/10/2024 11:01, Tao Chen wrote:
>>> Fix compile error when MPTCP feature not support, though eBPF core check
>>> already done which seems invalid in this situation, the error info like:
>>> progs/mptcp_sock.c:49:40: error: no member named 'is_mptcp' in 'struct
>>> tcp_sock'
>>>     49 |         is_mptcp = bpf_core_field_exists(tsk->is_mptcp) ?
>>>
>>> The filed created in new definitions with eBPF core feature to solve
>>> this build problem, and test case result still ok in MPTCP kernel.
>>>
>>> 176/1   mptcp/base:OK
>>> 176/2   mptcp/mptcpify:OK
>>> 176     mptcp:OK
>>> Summary: 1/2 PASSED, 0 SKIPPED, 0 FAILED
>>>
>>> Fixes: 8039d353217c ("selftests/bpf: Add MPTCP test base")
>>
>> The commit you mentioned here is more than 2 years old, and as far as I
>> can see, nobody else reported this compilation issue. I guess that's
>> because people used tools/testing/selftests/bpf/config file as expected
>> to populate the kernel config, and I suppose you didn't, right?
>>
> 
> Hi Matt, thank you for your reply, as you said, i did not use tools/
> testing/selftests/bpf/config to compile kernel, i will use this helpful
> feature.
> 
>> I don't think other BPF selftests check for missing kernel config if
>> they are specified in the 'config' file, but even if it is the case, I
>> think it would be better to skip all the MPTCP tests, and not try to
>> have them checking something that doesn't exist: no need to validate
>> these tests if the expected kernel config has not been enabled.
>>
> 
> If i use the kernel not support MPTCP, the compile error still exists,
> and i can not build the bpf test successfully. Maybe skill the test case
> seems better when kernel not support. Now that bpf_core_field_exists
> check already used in the code, i think it is better to use new
> definition mode.

I understand it would be better, but it means more code to maintain to
handle that (and remembering that in future test cases). If that's not
necessary, then no need to do the effort.

@BPF maintainers: do we need to support kernels not respecting the
tools/testing/selftests/bpf/config file? Should we detect when a
required kernel config is not set and skip some tests?

>> But again, please correct me if I'm wrong, but I don't think there is
>> anything to change here to fix your compilation issue: simply make sure
>> to use this tools/testing/selftests/bpf/config file to generate your
>> kernel config, no?

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.