bpf: Add BPF_FUNC_tcp_send_ack helper

Add a helper to send out a tcp-ack.  It will be used in the later
bpf_dctcp implementation that requires to send out an ack
when the CE state changed.
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200109004551.3900448-1-kafai@fb.com

include/uapi/linux/bpf.h

@@ -2837,6 +2837,14 @@ union bpf_attr {
  * 	Return
  * 		On success, the strictly positive length of the string,	including
  * 		the trailing NUL character. On error, a negative value.
+ *
+ * int bpf_tcp_send_ack(void *tp, u32 rcv_nxt)
+ *	Description
+ *		Send out a tcp-ack. *tp* is the in-kernel struct tcp_sock.
+ *		*rcv_nxt* is the ack_seq to be sent out.
+ *	Return
+ *		0 on success, or a negative error in case of failure.
+ *
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -2954,7 +2962,8 @@ union bpf_attr {
 	FN(probe_read_user),		\
 	FN(probe_read_kernel),		\
 	FN(probe_read_user_str),	\
-	FN(probe_read_kernel_str),
+	FN(probe_read_kernel_str),	\
+	FN(tcp_send_ack),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call

net/ipv4/bpf_tcp_ca.c

@@ -143,11 +143,33 @@ static int bpf_tcp_ca_btf_struct_access(struct bpf_verifier_log *log,
 	return NOT_INIT;
 }
 
+BPF_CALL_2(bpf_tcp_send_ack, struct tcp_sock *, tp, u32, rcv_nxt)
+{
+	/* bpf_tcp_ca prog cannot have NULL tp */
+	__tcp_send_ack((struct sock *)tp, rcv_nxt);
+	return 0;
+}
+
+static const struct bpf_func_proto bpf_tcp_send_ack_proto = {
+	.func		= bpf_tcp_send_ack,
+	.gpl_only	= false,
+	/* In case we want to report error later */
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_BTF_ID,
+	.arg2_type	= ARG_ANYTHING,
+	.btf_id		= &tcp_sock_id,
+};
+
 static const struct bpf_func_proto *
 bpf_tcp_ca_get_func_proto(enum bpf_func_id func_id,
 			  const struct bpf_prog *prog)
 {
-	return bpf_base_func_proto(func_id);
+	switch (func_id) {
+	case BPF_FUNC_tcp_send_ack:
+		return &bpf_tcp_send_ack_proto;
+	default:
+		return bpf_base_func_proto(func_id);
+	}
 }
 
 static const struct bpf_verifier_ops bpf_tcp_ca_verifier_ops = {