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>

1 files changed, 9 insertions, 8 deletions

diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
index 3e34b7d702f8..cf6a65cccbdf 100644
--- a/include/uapi/linux/netlink.h
+++ b/include/uapi/linux/netlink.h
@@ -101,14 +101,15 @@ struct nlmsgerr {
 	struct nlmsghdr msg;
 };
 
-#define NETLINK_ADD_MEMBERSHIP	1
-#define NETLINK_DROP_MEMBERSHIP	2
-#define NETLINK_PKTINFO		3
-#define NETLINK_BROADCAST_ERROR	4
-#define NETLINK_NO_ENOBUFS	5
-#define NETLINK_RX_RING		6
-#define NETLINK_TX_RING		7
-#define NETLINK_LISTEN_ALL_NSID	8
+#define NETLINK_ADD_MEMBERSHIP		1
+#define NETLINK_DROP_MEMBERSHIP		2
+#define NETLINK_PKTINFO			3
+#define NETLINK_BROADCAST_ERROR		4
+#define NETLINK_NO_ENOBUFS		5
+#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;