netdev - Re: [PATCH bpf-next] mptcp: Add struct mptcp_sock definition when CONFIG

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Date:   Tue, 12 Jul 2022 15:06:58 +0200
From:   Matthieu Baerts <matthieu.baerts@...sares.net>
To:     Jiri Olsa <olsajiri@...il.com>
Cc:     Mat Martineau <mathew.j.martineau@...ux.intel.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Geliang Tang <geliang.tang@...e.com>, mptcp@...ts.linux.dev
Subject: Re: [PATCH bpf-next] mptcp: Add struct mptcp_sock definition when
 CONFIG_MPTCP is disabled

Hi Jiri,

On 12/07/2022 14:12, Jiri Olsa wrote:
> On Tue, Jul 12, 2022 at 11:06:38AM +0200, Matthieu Baerts wrote:
>> Hi Jiri, Mat,
>>
>> On 11/07/2022 23:21, Mat Martineau wrote:
>>> On Mon, 11 Jul 2022, Jiri Olsa wrote:
>>>
>>>> The btf_sock_ids array needs struct mptcp_sock BTF ID for
>>>> the bpf_skc_to_mptcp_sock helper.
>>>>
>>>> When CONFIG_MPTCP is disabled, the 'struct mptcp_sock' is not
>>>> defined and resolve_btfids will complain with:
>>>>
>>>>  BTFIDS  vmlinux
>>>> WARN: resolve_btfids: unresolved symbol mptcp_sock
>>>>
>>>> Adding empty difinition for struct mptcp_sock when CONFIG_MPTCP
>>>> is disabled.
>>>>
>>>> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
>>>> ---
>>>> include/net/mptcp.h | 4 ++++
>>>> 1 file changed, 4 insertions(+)
>>>>
>>>> diff --git a/include/net/mptcp.h b/include/net/mptcp.h
>>>> index ac9cf7271d46..25741a52c666 100644
>>>> --- a/include/net/mptcp.h
>>>> +++ b/include/net/mptcp.h
>>>> @@ -59,6 +59,10 @@ struct mptcp_addr_info {
>>>>     };
>>>> };
>>>>
>>>> +#if !IS_ENABLED(CONFIG_MPTCP)
>>>> +struct mptcp_sock { };
>>>> +#endif
>>>
>>> The only use of struct mptcp_sock I see with !CONFIG_MPTCP is from this
>>> stub at the end of mptcp.h:
>>>
>>> static inline struct mptcp_sock *bpf_mptcp_sock_from_subflow(struct sock
>>> *sk) { return NULL; }
>>>
>>> It's normally defined in net/mptcp/protocol.h for the MPTCP subsystem code.
>>>
>>> The conditional could be added on the line before the stub to make it
>>> clear that the empty struct is associated with that inline stub.
>>
>> If this is required only for this specific BPF function, why not
>> modifying this stub (or add a define) to return "void *" instead of
>> "struct mptcp_sock *"?
> 
> so btf_sock_ids array needs BTF ID for 'struct mptcp_sock' and if CONFIG_MPTCP
> is not enabled, then resolve_btfids (which resolves and populate all BTF IDs)
> won't find it and will complain
> 
> btf_sock_ids keeps all socket IDs regardles the state of their CONFIG options,
> and relies that sock structs are defined even if related CONFIG option is disabled

Thank you for the explanation. I didn't know about that.

Then it is fine for me to leave it in mptcp.h. If it is not directly
linked to bpf_mptcp_sock_from_subflow(), I guess it can stay there but
maybe better to wait for Mat's answer about that.

> if that is false assumption then maybe we need to make btf_sock_ids values optional
> somehow

Cheers,
Matt
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