[PATCH mptcp-next v3 3/3] mptcp: check the protocol in mptcp_sk() with DEBUG_NET

["Matthieu Baerts (NGI0)" <matttbe@kernel.org>]

Fuzzers and static checkers might not detect when mptcp_sk() is used
with a non mptcp_sock structure.

This is similar to the parent commit, where it is easy to use mptcp_sk()
with a TCP sock, e.g. with a subflow sk.

So a new simple check is done when CONFIG_DEBUG_NET is enabled to tell
kernel devs when a non-MPTCP socket is being used as an MPTCP one.
'mptcp_sk()' macro is then defined differently: with an extra WARN to
complain when an unexpected socket is being used.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Notes:
  v2:
    - Use a macro instead of an inlined function (Paolo)
  v3:
    - add parenthesis around 'ptr' (checkpatch)
    - there is still this check from checkpatch but I guess that's fine:
      Macro argument reuse 'ptr' - possible side-effects?
---
 net/mptcp/protocol.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 6b62a7f35dd9..2b837676d1fc 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -355,9 +355,14 @@ static inline void msk_owned_by_me(const struct mptcp_sock *msk)
 	WARN_ON((ptr)->sk_protocol != IPPROTO_TCP);				\
 	container_of_const(ptr, struct tcp_sock, inet_conn.icsk_inet.sk);	\
 })
-#endif
+#define mptcp_sk(ptr) ({						\
+	WARN_ON((ptr)->sk_protocol != IPPROTO_MPTCP);			\
+	container_of_const(ptr, struct mptcp_sock, sk.icsk_inet.sk);	\
+})
 
+#else /* !CONFIG_DEBUG_NET */
 #define mptcp_sk(ptr) container_of_const(ptr, struct mptcp_sock, sk.icsk_inet.sk)
+#endif
 
 /* the msk socket don't use the backlog, also account for the bulk
  * free memory

-- 
2.43.0