rxrpc: Fix rxkad token xdr encoding

The session key should be encoded with just the 8 data bytes and
no length; ENCODE_DATA precedes it with a 4 byte length, which
confuses some existing tools that try to parse this format.

Add an ENCODE_BYTES macro that does not include a length, and use
it for the key.  Also adjust the expected length.

Note that commit 774521f ("rxrpc: Fix an assertion in
rxrpc_read()") had fixed a BUG by changing the length rather than
fixing the encoding.  The original length was correct.

Fixes: 9945515 ("RxRPC: Parse security index 5 keys (Kerberos 5)")
Signed-off-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>

Author: Marc Dionne

net/rxrpc/key.c

@@ -1072,7 +1072,7 @@ static long rxrpc_read(const struct key *key,
 
 		switch (token->security_index) {
 		case RXRPC_SECURITY_RXKAD:
-			toksize += 9 * 4;	/* viceid, kvno, key*2 + len, begin,
+			toksize += 8 * 4;	/* viceid, kvno, key*2, begin,
 						 * end, primary, tktlen */
 			toksize += RND(token->kad->ticket_len);
 			break;
@@ -1138,6 +1138,14 @@ static long rxrpc_read(const struct key *key,
 			memcpy((u8 *)xdr + _l, &zero, 4 - (_l & 3));	\
 		xdr += (_l + 3) >> 2;					\
 	} while(0)
+#define ENCODE_BYTES(l, s)						\
+	do {								\
+		u32 _l = (l);						\
+		memcpy(xdr, (s), _l);					\
+		if (_l & 3)						\
+			memcpy((u8 *)xdr + _l, &zero, 4 - (_l & 3));	\
+		xdr += (_l + 3) >> 2;					\
+	} while(0)
 #define ENCODE64(x)					\
 	do {						\
 		__be64 y = cpu_to_be64(x);		\
@@ -1165,7 +1173,7 @@ static long rxrpc_read(const struct key *key,
 		case RXRPC_SECURITY_RXKAD:
 			ENCODE(token->kad->vice_id);
 			ENCODE(token->kad->kvno);
-			ENCODE_DATA(8, token->kad->session_key);
+			ENCODE_BYTES(8, token->kad->session_key);
 			ENCODE(token->kad->start);
 			ENCODE(token->kad->expiry);
 			ENCODE(token->kad->primary_flag);