tcp: add TCP_CC_INFO socket option

Some Congestion Control modules can provide per flow information,
but current way to get this information is to use netlink.

Like TCP_INFO, let's add TCP_CC_INFO so that applications can
issue a getsockopt() if they have a socket file descriptor,
instead of playing complex netlink games.

Sample usage would be :

  union tcp_cc_info info;
  socklen_t len = sizeof(info);

  if (getsockopt(fd, SOL_TCP, TCP_CC_INFO, &info, &len) == -1)
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/tcp.h

@@ -112,6 +112,7 @@ enum {
 #define TCP_FASTOPEN		23	/* Enable FastOpen on listeners */
 #define TCP_TIMESTAMP		24
 #define TCP_NOTSENT_LOWAT	25	/* limit number of unsent bytes in write queue */
+#define TCP_CC_INFO		26	/* Get Congestion Control (optional) info */
 
 struct tcp_repair_opt {
 	__u32	opt_code;

net/ipv4/tcp.c

@@ -252,6 +252,7 @@
 #include <linux/types.h>
 #include <linux/fcntl.h>
 #include <linux/poll.h>
+#include <linux/inet_diag.h>
 #include <linux/init.h>
 #include <linux/fs.h>
 #include <linux/skbuff.h>
@@ -2739,6 +2740,26 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
 			return -EFAULT;
 		return 0;
 	}
+	case TCP_CC_INFO: {
+		const struct tcp_congestion_ops *ca_ops;
+		union tcp_cc_info info;
+		size_t sz = 0;
+		int attr;
+
+		if (get_user(len, optlen))
+			return -EFAULT;
+
+		ca_ops = icsk->icsk_ca_ops;
+		if (ca_ops && ca_ops->get_info)
+			sz = ca_ops->get_info(sk, ~0U, &attr, &info);
+
+		len = min_t(unsigned int, len, sz);
+		if (put_user(len, optlen))
+			return -EFAULT;
+		if (copy_to_user(optval, &info, len))
+			return -EFAULT;
+		return 0;
+	}
 	case TCP_QUICKACK:
 		val = !icsk->icsk_ack.pingpong;
 		break;