Adds support for calling sock_ops BPF program when there is a
retransmission. Two arguments are used; one for the sequence number and
other for the number of segments retransmitted. Does not include syn-ack
retransmissions.

New op: BPF_SOCK_OPS_RETRANS_CB.

Signed-off-by: Lawrence Brakmo <redacted>
---
 include/uapi/linux/bpf.h | 4 ++++
 include/uapi/linux/tcp.h | 3 ++-
 net/ipv4/tcp_output.c    | 3 +++
 3 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 6998032..eb26cdb 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h

@@ -1039,6 +1039,10 @@ 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
+                                        */
 };

 #define TCP_BPF_IW             1001    /* Set TCP initial congestion window */

diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
index 129032ca..ec03a2b 100644
--- a/include/uapi/linux/tcp.h
+++ b/include/uapi/linux/tcp.h

@@ -270,7 +270,8 @@ struct tcp_diag_md5sig {

 /* 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
                                                         */

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index d12f7f7..f7d34f01 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c

@@ -2908,6 +2908,9 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
        if (likely(!err)) {
                TCP_SKB_CB(skb)->sacked |= TCPCB_EVER_RETRANS;
                trace_tcp_retransmit_skb(sk, skb);
+               if (BPF_SOCK_OPS_TEST_FLAG(tp, BPF_SOCK_OPS_RETRANS_CB_FLAG))
+                       tcp_call_bpf_2arg(sk, BPF_SOCK_OPS_RETRANS_CB,
+                                         TCP_SKB_CB(skb)->seq, segs);