netdev - [PATCH 1/3] netlink: Add notifier when changing netlink socket membership

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Message-Id: <20240131120535.933424-2-stanislaw.gruszka@linux.intel.com>
Date: Wed, 31 Jan 2024 13:05:33 +0100
From: Stanislaw Gruszka <stanislaw.gruszka@...ux.intel.com>
To: linux-pm@...r.kernel.org
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org
Subject: [PATCH 1/3] netlink: Add notifier when changing netlink socket membership

Add notification when adding/removing multicast group to/from
client socket via setsockopt() syscall.

It can be used with conjunction with netlink_has_listeners() to check
if consumers of netlink multicast messages emerge or disappear.

A client can call netlink_register_notifier() to register a callback.
In the callback check for state NETLINK_CHANGE and NETLINK_URELEASE to
get notification for change in the netlink socket membership.

Thus, a client can now send events only when there are active consumers,
preventing unnecessary work when none exist.

Signed-off-by: Stanislaw Gruszka <stanislaw.gruszka@...ux.intel.com>
---
 include/linux/notifier.h | 1 +
 net/netlink/af_netlink.c | 6 ++++++
 2 files changed, 7 insertions(+)

diff --git a/include/linux/notifier.h b/include/linux/notifier.h
index 45702bdcbceb..8f5a5eed1e0e 100644
--- a/include/linux/notifier.h
+++ b/include/linux/notifier.h
@@ -227,6 +227,7 @@ static inline int notifier_to_errno(int ret)
 /* Virtual Terminal events are defined in include/linux/vt.h. */
 
 #define NETLINK_URELEASE	0x0001	/* Unicast netlink socket released */
+#define NETLINK_CHANGE		0x0002  /* Changed membership of netlink socket */
 
 /* Console keyboard events.
  * Note: KBD_KEYCODE is always sent before KBD_UNBOUND_KEYCODE, KBD_UNICODE and
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index eb086b06d60d..674af2cb0f12 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -1680,6 +1680,11 @@ static int netlink_setsockopt(struct socket *sock, int level, int optname,
 	case NETLINK_ADD_MEMBERSHIP:
 	case NETLINK_DROP_MEMBERSHIP: {
 		int err;
+		struct netlink_notify n = {
+			.net = sock_net(sk),
+			.protocol = sk->sk_protocol,
+			.portid = nlk->portid,
+		};
 
 		if (!netlink_allowed(sock, NL_CFG_F_NONROOT_RECV))
 			return -EPERM;
@@ -1700,6 +1705,7 @@ static int netlink_setsockopt(struct socket *sock, int level, int optname,
 		if (optname == NETLINK_DROP_MEMBERSHIP && nlk->netlink_unbind)
 			nlk->netlink_unbind(sock_net(sk), val);
 
+		blocking_notifier_call_chain(&netlink_chain, NETLINK_CHANGE, &n);
 		break;
 	}
 	case NETLINK_BROADCAST_ERROR:
-- 
2.34.1