net: inet: diag: expose sockets cgroup classid

This is useful for directly looking up a task based on class id rather than
having to scan through all open file descriptors.

Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/inet_diag.h

@@ -142,6 +142,7 @@ enum {
 	INET_DIAG_PAD,
 	INET_DIAG_MARK,
 	INET_DIAG_BBRINFO,
+	INET_DIAG_CLASS_ID,
 	__INET_DIAG_MAX,
 };
 

net/ipv4/inet_diag.c

@@ -274,6 +274,17 @@ int inet_sk_diag_fill(struct sock *sk, struct inet_connection_sock *icsk,
 			goto errout;
 	}
 
+	if (ext & (1 << (INET_DIAG_CLASS_ID - 1))) {
+		u32 classid = 0;
+
+#ifdef CONFIG_SOCK_CGROUP_DATA
+		classid = sock_cgroup_classid(&sk->sk_cgrp_data);
+#endif
+
+		if (nla_put_u32(skb, INET_DIAG_CLASS_ID, classid))
+			goto errout;
+	}
+
 out:
 	nlmsg_end(skb, nlh);
 	return 0;