Merge branch 'Allow BPF TCP CCs to write app_limited'

Yixin Shen says:

====================

This series allow BPF TCP CCs to write app_limited of struct
tcp_sock. A built-in CC or one from a kernel module is already
able to write to app_limited of struct tcp_sock. Until now,
a BPF CC doesn't have write access to this member of struct
tcp_sock.

v2:
 - Merge the test of writing app_limited into the test of
   writing sk_pacing. (Martin KaFai Lau)
====================

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

net/ipv4/bpf_tcp_ca.c

@@ -113,6 +113,9 @@ static int bpf_tcp_ca_btf_struct_access(struct bpf_verifier_log *log,
 	case offsetof(struct tcp_sock, ecn_flags):
 		end = offsetofend(struct tcp_sock, ecn_flags);
 		break;
+	case offsetof(struct tcp_sock, app_limited):
+		end = offsetofend(struct tcp_sock, app_limited);
+		break;
 	default:
 		bpf_log(log, "no write support to tcp_sock at off %d\n", off);
 		return -EACCES;

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

@@ -16,6 +16,16 @@ static inline struct tcp_sock *tcp_sk(const struct sock *sk)
 	return (struct tcp_sock *)sk;
 }
 
+static inline unsigned int tcp_left_out(const struct tcp_sock *tp)
+{
+	return tp->sacked_out + tp->lost_out;
+}
+
+static inline unsigned int tcp_packets_in_flight(const struct tcp_sock *tp)
+{
+	return tp->packets_out - tcp_left_out(tp) + tp->retrans_out;
+}
+
 SEC("struct_ops/write_sk_pacing_init")
 void BPF_PROG(write_sk_pacing_init, struct sock *sk)
 {
@@ -31,11 +41,12 @@ SEC("struct_ops/write_sk_pacing_cong_control")
 void BPF_PROG(write_sk_pacing_cong_control, struct sock *sk,
 	      const struct rate_sample *rs)
 {
-	const struct tcp_sock *tp = tcp_sk(sk);
+	struct tcp_sock *tp = tcp_sk(sk);
 	unsigned long rate =
 		((tp->snd_cwnd * tp->mss_cache * USEC_PER_SEC) << 3) /
 		(tp->srtt_us ?: 1U << 3);
 	sk->sk_pacing_rate = min(rate, sk->sk_max_pacing_rate);
+	tp->app_limited = (tp->delivered + tcp_packets_in_flight(tp)) ?: 1;
 }
 
 SEC("struct_ops/write_sk_pacing_ssthresh")