mptcp: setsockopt support for TCP_MD5SIG

This adds setsockopt support for TCP_MD5SIG and TCP_MD5SIG_EXT options.
The implementation:
- Allows setting these options (getsockopt remains unsupported)
- Applies them only to the first subflow
- Forces fallback to TCP (since MD5 isn't compatible with MPTCP)

Setting these options triggers fallback to TCP to maintain MD5
compatibility.

Note that TCP_MD5SIG and TCP_MD5SIG_EXT are unsupported for TCP too.

Closes: multipath-tcp/mptcp_net-next#575
Signed-off-by: Geliang Tang <[email protected]>

Author: Geliang Tang

net/mptcp/sockopt.c

@@ -13,6 +13,7 @@
 #include <net/tcp.h>
 #include <net/mptcp.h>
 #include "protocol.h"
+#include "mib.h"
 
 #define MIN_INFO_OPTLEN_SIZE		16
 #define MIN_FULL_INFO_OPTLEN_SIZE	40
@@ -567,11 +568,12 @@ static bool mptcp_supported_sockopt(int level, int optname)
 		case TCP_FASTOPEN_CONNECT:
 		case TCP_FASTOPEN_KEY:
 		case TCP_FASTOPEN_NO_COOKIE:
+		/* MD5 will force a fallback to TCP: OK to set while not connected */
+		case TCP_MD5SIG:
+		case TCP_MD5SIG_EXT:
 			return true;
 		}
 
-		/* TCP_MD5SIG, TCP_MD5SIG_EXT are not supported, MD5 is not compatible with MPTCP */
-
 		/* TCP_REPAIR, TCP_REPAIR_QUEUE, TCP_QUEUE_SEQ, TCP_REPAIR_OPTIONS,
 		 * TCP_REPAIR_WINDOW are not supported, better avoid this mess
 		 */
@@ -830,6 +832,12 @@ static int mptcp_setsockopt_sol_tcp(struct mptcp_sock *msk, int optname,
 		/* See tcp.c: TCP_DEFER_ACCEPT does not fail */
 		mptcp_setsockopt_first_sf_only(msk, SOL_TCP, optname, optval, optlen);
 		return 0;
+#ifdef CONFIG_TCP_MD5SIG
+	case TCP_MD5SIG:
+	case TCP_MD5SIG_EXT:
+		__mptcp_try_fallback(msk, MPTCP_MIB_MD5SIGFALLBACK);
+		fallthrough;
+#endif
 	case TCP_FASTOPEN:
 	case TCP_FASTOPEN_CONNECT:
 	case TCP_FASTOPEN_KEY: