bpf: Add BPF_SOCK_OPS_RETRANS_CB

Adds support for calling sock_ops BPF program when there is a
retransmission. Three arguments are used; one for the sequence number,
another for the number of segments retransmitted, and the last one for
the return value of tcp_transmit_skb (0 => success).
Does not include syn-ack retransmissions.

New op: BPF_SOCK_OPS_RETRANS_CB.
Signed-off-by: Lawrence Brakmo <brakmo@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/uapi/linux/bpf.h

@@ -1005,7 +1005,8 @@ struct bpf_sock_ops {
 
 /* Definitions for bpf_sock_ops_cb_flags */
 #define BPF_SOCK_OPS_RTO_CB_FLAG	(1<<0)
-#define BPF_SOCK_OPS_ALL_CB_FLAGS       0x1		/* Mask of all currently
+#define BPF_SOCK_OPS_RETRANS_CB_FLAG	(1<<1)
+#define BPF_SOCK_OPS_ALL_CB_FLAGS       0x3		/* Mask of all currently
 							 * supported cb flags
 							 */
 
@@ -1047,6 +1048,12 @@ enum {
 					 * Arg2: value of icsk_rto
 					 * Arg3: whether RTO has expired
 					 */
+	BPF_SOCK_OPS_RETRANS_CB,	/* Called when skb is retransmitted.
+					 * Arg1: sequence number of 1st byte
+					 * Arg2: # segments
+					 * Arg3: return value of
+					 *       tcp_transmit_skb (0 => success)
+					 */
 };
 
 #define TCP_BPF_IW		1001	/* Set TCP initial congestion window */

net/ipv4/tcp_output.c

@@ -2905,6 +2905,10 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 		err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
 	}
 
+	if (BPF_SOCK_OPS_TEST_FLAG(tp, BPF_SOCK_OPS_RETRANS_CB_FLAG))
+		tcp_call_bpf_3arg(sk, BPF_SOCK_OPS_RETRANS_CB,
+				  TCP_SKB_CB(skb)->seq, segs, err);
+
 	if (likely(!err)) {
 		TCP_SKB_CB(skb)->sacked |= TCPCB_EVER_RETRANS;
 		trace_tcp_retransmit_skb(sk, skb);