bpf: Add tcp_notsent_lowat bpf setsockopt

Adding support for TCP_NOTSENT_LOWAT sockoption (https://lwn.net/Articles/560082/)
in tcp bpf programs.
Signed-off-by: Nikita V. Shirokov <tehnerd@tehnerd.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20201009070325.226855-1-tehnerd@tehnerd.com

include/uapi/linux/bpf.h

@@ -1698,7 +1698,7 @@ union bpf_attr {
  * 		  **TCP_CONGESTION**, **TCP_BPF_IW**,
  * 		  **TCP_BPF_SNDCWND_CLAMP**, **TCP_SAVE_SYN**,
  * 		  **TCP_KEEPIDLE**, **TCP_KEEPINTVL**, **TCP_KEEPCNT**,
- * 		  **TCP_SYNCNT**, **TCP_USER_TIMEOUT**.
+ *		  **TCP_SYNCNT**, **TCP_USER_TIMEOUT**, **TCP_NOTSENT_LOWAT**.
  * 		* **IPPROTO_IP**, which supports *optname* **IP_TOS**.
  * 		* **IPPROTO_IPV6**, which supports *optname* **IPV6_TCLASS**.
  * 	Return

net/core/filter.c

@@ -4827,6 +4827,10 @@ static int _bpf_setsockopt(struct sock *sk, int level, int optname,
 				else
 					icsk->icsk_user_timeout = val;
 				break;
+			case TCP_NOTSENT_LOWAT:
+				tp->notsent_lowat = val;
+				sk->sk_write_space(sk);
+				break;
 			default:
 				ret = -EINVAL;
 			}

tools/include/uapi/linux/bpf.h

@@ -1698,7 +1698,7 @@ union bpf_attr {
  * 		  **TCP_CONGESTION**, **TCP_BPF_IW**,
  * 		  **TCP_BPF_SNDCWND_CLAMP**, **TCP_SAVE_SYN**,
  * 		  **TCP_KEEPIDLE**, **TCP_KEEPINTVL**, **TCP_KEEPCNT**,
- * 		  **TCP_SYNCNT**, **TCP_USER_TIMEOUT**.
+ *		  **TCP_SYNCNT**, **TCP_USER_TIMEOUT**, **TCP_NOTSENT_LOWAT**.
  * 		* **IPPROTO_IP**, which supports *optname* **IP_TOS**.
  * 		* **IPPROTO_IPV6**, which supports *optname* **IPV6_TCLASS**.
  * 	Return

tools/testing/selftests/bpf/progs/connect4_prog.c

@@ -23,6 +23,10 @@
 #define TCP_CA_NAME_MAX 16
 #endif
 
+#ifndef TCP_NOTSENT_LOWAT
+#define TCP_NOTSENT_LOWAT 25
+#endif
+
 #ifndef IFNAMSIZ
 #define IFNAMSIZ 16
 #endif
@@ -128,6 +132,18 @@ static __inline int set_keepalive(struct bpf_sock_addr *ctx)
 	return 0;
 }
 
+static __inline int set_notsent_lowat(struct bpf_sock_addr *ctx)
+{
+	int lowat = 65535;
+
+	if (ctx->type == SOCK_STREAM) {
+		if (bpf_setsockopt(ctx, SOL_TCP, TCP_NOTSENT_LOWAT, &lowat, sizeof(lowat)))
+			return 1;
+	}
+
+	return 0;
+}
+
 SEC("cgroup/connect4")
 int connect_v4_prog(struct bpf_sock_addr *ctx)
 {
@@ -148,6 +164,9 @@ int connect_v4_prog(struct bpf_sock_addr *ctx)
 	if (set_keepalive(ctx))
 		return 0;
 
+	if (set_notsent_lowat(ctx))
+		return 0;
+
 	if (ctx->type != SOCK_STREAM && ctx->type != SOCK_DGRAM)
 		return 0;
 	else if (ctx->type == SOCK_STREAM)