[PATCH mptcp-next v3 2/3] mptcp: check the protocol in tcp_sk() with DEBUG_NET

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

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

This kind of mistake already happened a few times with MPTCP: when
wrongly using TCP-specific helpers with mptcp_sock pointers. On the
other hand, there are many 'tcp_xxx()' helpers that are taking a 'struct
sock' pointer as arguments, and some of them are only looking at fields
from 'struct sock', and nothing from 'struct tcp_sock'. It is then
tempting to use them with a 'struct mptcp_sock'.

So a new simple check is done when CONFIG_DEBUG_NET is enabled to tell
kernel devs when a non-TCP socket is being used as a TCP one. 'tcp_sk()'
macro is then re-defined to add a WARN when an unexpected socket is
being used.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Notes:
  - v2:
    - Move from include/linux/tcp.h to net/mptcp/protocol.h: specific
      to TCP (Paolo)
    - Use a macro instead of an inlined function (Paolo)
    - Adapt the commit message after the recent changes.
  - 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 | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index eefd1397106d..6b62a7f35dd9 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -348,6 +348,15 @@ static inline void msk_owned_by_me(const struct mptcp_sock *msk)
 	sock_owned_by_me((const struct sock *)msk);
 }
 
+#ifdef CONFIG_DEBUG_NET
+/* MPTCP-specific: we might (indirectly) call this helper with the wrong sk */
+#undef tcp_sk
+#define tcp_sk(ptr) ({								\
+	WARN_ON((ptr)->sk_protocol != IPPROTO_TCP);				\
+	container_of_const(ptr, struct tcp_sock, inet_conn.icsk_inet.sk);	\
+})
+#endif
+
 #define mptcp_sk(ptr) container_of_const(ptr, struct mptcp_sock, sk.icsk_inet.sk)
 
 /* the msk socket don't use the backlog, also account for the bulk

-- 
2.43.0