mptcp: fix length of ADD_ADDR with port sub-option

in current Linux, MPTCP peers advertising endpoints with port numbers use
a sub-option length that wrongly accounts for the trailing TCP NOP. Also,
receivers will only process incoming ADD_ADDR with port having such wrong
sub-option length. Fix this, making ADD_ADDR compliant to RFC8684 §3.4.1.

this can be verified running tcpdump on the kselftests artifacts:

unpatched kernel:
[root@bottarga mptcp]# tcpdump -tnnr unpatched.pcap | grep add-addr
reading from file unpatched.pcap, link-type LINUX_SLL (Linux cooked v1), snapshot length 65535
IP 10.0.1.1.10000 > 10.0.1.2.53078: Flags [.], ack 101, win 509, options [nop,nop,TS val 214459678 ecr 521312851,mptcp add-addr v1 id 1 a00:201:2774:2d88:7436:85c3:17fd:101], length 0
IP 10.0.1.2.53078 > 10.0.1.1.10000: Flags [.], ack 101, win 502, options [nop,nop,TS val 521312852 ecr 214459678,mptcp add-addr[bad opt]]

patched kernel:
[root@bottarga mptcp]# tcpdump -tnnr patched.pcap | grep add-addr
reading from file patched.pcap, link-type LINUX_SLL (Linux cooked v1), snapshot length 65535
IP 10.0.1.1.10000 > 10.0.1.2.38178: Flags [.], ack 101, win 509, options [nop,nop,TS val 3728873902 ecr 2732713192,mptcp add-addr v1 id 1 10.0.2.1:10100 hmac 0xbccdfcbe59292a1f,nop,nop], length 0
IP 10.0.1.2.38178 > 10.0.1.1.10000: Flags [.], ack 101, win 502, options [nop,nop,TS val 2732713195 ecr 3728873902,mptcp add-addr v1-echo id 1 10.0.2.1:10100,nop,nop], length 0

Fixes: 22fb85ffaefb ("mptcp: add port support for ADD_ADDR suboption writing")
CC: [e-mail] # 5.11+
Reviewed-by: Mat Martineau <[e-mail]>
Acked-and-tested-by: Geliang Tang <[e-mail]>
Signed-off-by: Davide Caratti <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/mptcp/protocol.h

···
 #define TCPOLEN_MPTCP_DSS_MAP64		14
 #define TCPOLEN_MPTCP_DSS_CHECKSUM	2
 #define TCPOLEN_MPTCP_ADD_ADDR		16
-#define TCPOLEN_MPTCP_ADD_ADDR_PORT	20
+#define TCPOLEN_MPTCP_ADD_ADDR_PORT	18
 #define TCPOLEN_MPTCP_ADD_ADDR_BASE	8
-#define TCPOLEN_MPTCP_ADD_ADDR_BASE_PORT	12
+#define TCPOLEN_MPTCP_ADD_ADDR_BASE_PORT	10
 #define TCPOLEN_MPTCP_ADD_ADDR6		28
-#define TCPOLEN_MPTCP_ADD_ADDR6_PORT	32
+#define TCPOLEN_MPTCP_ADD_ADDR6_PORT	30
 #define TCPOLEN_MPTCP_ADD_ADDR6_BASE	20
-#define TCPOLEN_MPTCP_ADD_ADDR6_BASE_PORT	24
-#define TCPOLEN_MPTCP_PORT_LEN		4
+#define TCPOLEN_MPTCP_ADD_ADDR6_BASE_PORT	22
+#define TCPOLEN_MPTCP_PORT_LEN		2
+#define TCPOLEN_MPTCP_PORT_ALIGN	2
 #define TCPOLEN_MPTCP_RM_ADDR_BASE	4
 #define TCPOLEN_MPTCP_PRIO		3
 #define TCPOLEN_MPTCP_PRIO_ALIGN	4
···
 		len = TCPOLEN_MPTCP_ADD_ADDR6_BASE;
 	if (!echo)
 		len += MPTCPOPT_THMAC_LEN;
+	/* account for 2 trailing 'nop' options */
 	if (port)
-		len += TCPOLEN_MPTCP_PORT_LEN;
+		len += TCPOLEN_MPTCP_PORT_LEN + TCPOLEN_MPTCP_PORT_ALIGN;
 
 	return len;
 }