Fix definitions of SOCKET_MAX, TCP_MAX_CONNS

...and, given that I keep getting this wrong, add a convenience
macro, MAX_FROM_BITS().

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

passt.h

@@ -42,7 +42,7 @@ union epoll_ref;
 /**
  * union epoll_ref - Breakdown of reference for epoll socket bookkeeping
  * @proto:	IP protocol number
- * @s:		Socket number (implies 2^24 limit on number of descriptors)
+ * @s:		Socket number (implies 2^24-1 limit on number of descriptors)
  * @tcp:	TCP-specific reference part
  * @udp:	UDP-specific reference part
  * @icmp:	ICMP-specific reference part
@@ -53,7 +53,7 @@ union epoll_ref {
 	struct {
 		int32_t		proto:8,
 #define SOCKET_REF_BITS		24
-#define SOCKET_MAX		(1 << SOCKET_REF_BITS)
+#define SOCKET_MAX		MAX_FROM_BITS(SOCKET_REF_BITS)
 				s:SOCKET_REF_BITS;
 		union {
 			union tcp_epoll_ref tcp;

tcp.h

@@ -8,8 +8,8 @@
 
 #define TCP_TIMER_INTERVAL		1000	/* ms */
 
-#define TCP_CONN_INDEX_BITS		17	/* 128k */
-#define TCP_MAX_CONNS			(1 << TCP_CONN_INDEX_BITS)
+#define TCP_CONN_INDEX_BITS		17	/* 128k - 1 */
+#define TCP_MAX_CONNS			MAX_FROM_BITS(TCP_CONN_INDEX_BITS)
 
 struct ctx;
 

tcp_conn.h

@@ -54,7 +54,7 @@ struct tcp_tap_conn {
 
 #define TCP_RETRANS_BITS		3
 	unsigned int	retrans		:TCP_RETRANS_BITS;
-#define TCP_MAX_RETRANS			((1U << TCP_RETRANS_BITS) - 1)
+#define TCP_MAX_RETRANS			MAX_FROM_BITS(TCP_RETRANS_BITS)
 
 #define TCP_WS_BITS			4	/* RFC 7323 */
 #define TCP_WS_MAX			14

util.h

@@ -40,6 +40,8 @@
 #define ROUND_DOWN(x, y)	((x) & ~((y) - 1))
 #define ROUND_UP(x, y)		(((x) + (y) - 1) & ~((y) - 1))
 
+#define MAX_FROM_BITS(n)	((int)((1U << (n)) - 1))
+
 #define BIT(n)			(1UL << (n))
 #define BITMAP_BIT(n)		(BIT((n) % (sizeof(long) * 8)))
 #define BITMAP_WORD(n)		(n / (sizeof(long) * 8))