bpf: Add sock_ops RTO callback

Adds an optional call to sock_ops BPF program based on whether the
BPF_SOCK_OPS_RTO_CB_FLAG is set in bpf_sock_ops_flags.
The BPF program is passed 2 arguments: icsk_retransmits and whether the
RTO has expired.
Signed-off-by: Lawrence Brakmo <brakmo@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/uapi/linux/bpf.h

@@ -982,7 +982,8 @@ struct bpf_sock_ops {
 };
 
 /* Definitions for bpf_sock_ops_cb_flags */
-#define BPF_SOCK_OPS_ALL_CB_FLAGS       0		/* Mask of all currently
+#define BPF_SOCK_OPS_RTO_CB_FLAG	(1<<0)
+#define BPF_SOCK_OPS_ALL_CB_FLAGS       0x1		/* Mask of all currently
 							 * supported cb flags
 							 */
 
@@ -1019,6 +1020,11 @@ enum {
 					 * a congestion threshold. RTTs above
 					 * this indicate congestion
 					 */
+	BPF_SOCK_OPS_RTO_CB,		/* Called when an RTO has triggered.
+					 * Arg1: value of icsk_retransmits
+					 * Arg2: value of icsk_rto
+					 * Arg3: whether RTO has expired
+					 */
 };
 
 #define TCP_BPF_IW		1001	/* Set TCP initial congestion window */

net/ipv4/tcp_timer.c

@@ -213,11 +213,18 @@ static int tcp_write_timeout(struct sock *sk)
 						icsk->icsk_user_timeout);
 	}
 	tcp_fastopen_active_detect_blackhole(sk, expired);
+
+	if (BPF_SOCK_OPS_TEST_FLAG(tp, BPF_SOCK_OPS_RTO_CB_FLAG))
+		tcp_call_bpf_3arg(sk, BPF_SOCK_OPS_RTO_CB,
+				  icsk->icsk_retransmits,
+				  icsk->icsk_rto, (int)expired);
+
 	if (expired) {
 		/* Has it gone just too far? */
 		tcp_write_err(sk);
 		return 1;
 	}
+
 	return 0;
 }