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

[Jiri Olsa <olsajiri@gmail.com>]

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@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 *"?

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

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

jirka