netlink: add API to retrieve all group memberships

This patch adds getsockopt(SOL_NETLINK, NETLINK_LIST_MEMBERSHIPS) to
retrieve all groups a socket is a member of. Currently, we have to use
getsockname() and look at the nl.nl_groups bitmask. However, this mask is
limited to 32 groups. Hence, similar to NETLINK_ADD_MEMBERSHIP and
NETLINK_DROP_MEMBERSHIP, this adds a separate sockopt to manager higher
groups IDs than 32.

This new NETLINK_LIST_MEMBERSHIPS option takes a pointer to __u32 and the
size of the array. The array is filled with the full membership-set of the
socket, and the required array size is returned in optlen. Hence,
user-space can retry with a properly sized array in case it was too small.
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/netlink.h

@@ -109,6 +109,7 @@ struct nlmsgerr {
 #define NETLINK_RX_RING			6
 #define NETLINK_TX_RING			7
 #define NETLINK_LISTEN_ALL_NSID		8
+#define NETLINK_LIST_MEMBERSHIPS	9
 
 struct nl_pktinfo {
 	__u32	group;

net/netlink/af_netlink.c

@@ -2290,6 +2290,28 @@ static int netlink_getsockopt(struct socket *sock, int level, int optname,
 			return -EFAULT;
 		err = 0;
 		break;
+	case NETLINK_LIST_MEMBERSHIPS: {
+		int pos, idx, shift;
+
+		err = 0;
+		netlink_table_grab();
+		for (pos = 0; pos * 8 < nlk->ngroups; pos += sizeof(u32)) {
+			if (len - pos < sizeof(u32))
+				break;
+
+			idx = pos / sizeof(unsigned long);
+			shift = (pos % sizeof(unsigned long)) * 8;
+			if (put_user((u32)(nlk->groups[idx] >> shift),
+				     (u32 __user *)(optval + pos))) {
+				err = -EFAULT;
+				break;
+			}
+		}
+		if (put_user(ALIGN(nlk->ngroups / 8, sizeof(u32)), optlen))
+			err = -EFAULT;
+		netlink_table_ungrab();
+		break;
+	}
 	default:
 		err = -ENOPROTOOPT;
 	}