[PATCH net-next 4/8] mptcp: check the protocol in mptcp_sk() with DEBUG_NET

[Matthieu Baerts (NGI0)]

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.

Reviewed-by: Mat Martineau <martineau@xxxxxxxxxx>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>
---
 net/mptcp/protocol.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 026ed360bd72..051b100d9403 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -356,9 +356,15 @@ 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) ({						\
+	typeof(ptr) _ptr = (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