Re: [PATCH bpf-next] mptcp: Add struct mptcp_sock definition when CONFIG_MPTCP is disabled

[Matthieu Baerts <matthieu.baerts@tessares.net>]

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@kernel.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 *"?

Cheers,
Matt
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