[PATCH v1 bpf-next 5/8] bpf: tcp: Add kfunc to adjust sk->sk_rcvlowat.

[Kuniyuki Iwashima <kuniyu@google.com>]

We will invoke BPF SOCK_OPS prog with BPF_SOCK_OPS_RCVLOWAT_CB
to adjust sk->sk_rcvlowat when

  1. TCP stack enqueues skb to sk->sk_receive_queue
  2. TCP recvmsg() completes

Let's provide a kfunc to set sk->sk_rcvlowat.

Negative values are clamped to INT_MAX, consistent with SO_RCVLOWAT.

The wakeup flag is determined based on bpf_sock_ops_kern.skb:

  * For the enqueue hook, skb is always non-NULL, and wakeup is
    set to false because

    * tcp_data_ready() is always called after the hooks in
       tcp_queue_rcv() and tcp_ofo_queue().

    * when tcp_fastopen_add_skb() is called for TFO SYN,
       the socket is not yet accept()ed, and when called
       for TFO SYN+ACK, the socket is woken up by
       sk->sk_state_change() anyway.

  * For the recvmsg() hook, skb is always NULL, and wakeup is set
    to true because tcp_data_ready() is not called in the path.

An alternative would be to support bpf_setsockopt() by adding
BPF_SOCK_OPS_RCVLOWAT_CB to is_locked_tcp_sock_ops().

However, that approach involves excessive conditionals and an
unnecessary memcpy(), costs we do not want to pay for every skb
in the TCP fast path.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/core/filter.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index 94d07a15b2ab..9c4cd27c6d4e 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -12346,6 +12346,22 @@ __bpf_kfunc int bpf_sk_assign_tcp_reqsk(struct __sk_buff *s, struct sock *sk,
 #endif
 }
 
+__bpf_kfunc int bpf_sock_ops_tcp_set_rcvlowat(struct bpf_sock_ops_kern *skops,
+					      int rcvlowat)
+{
+#ifdef CONFIG_INET
+	if (skops->op != BPF_SOCK_OPS_RCVLOWAT_CB)
+		return -EOPNOTSUPP;
+
+	if (rcvlowat < 0)
+		rcvlowat = INT_MAX;
+
+	return __tcp_set_rcvlowat(skops->sk, rcvlowat, !skops->skb);
+#else
+	return -EOPNOTSUPP;
+#endif
+}
+
 __bpf_kfunc int bpf_sock_ops_enable_tx_tstamp(struct bpf_sock_ops_kern *skops,
 					      u64 flags)
 {
@@ -12497,6 +12513,7 @@ BTF_KFUNCS_END(bpf_kfunc_check_set_tcp_reqsk)
 
 BTF_KFUNCS_START(bpf_kfunc_check_set_sock_ops)
 BTF_ID_FLAGS(func, bpf_sock_ops_enable_tx_tstamp)
+BTF_ID_FLAGS(func, bpf_sock_ops_tcp_set_rcvlowat)
 BTF_KFUNCS_END(bpf_kfunc_check_set_sock_ops)
 
 static const struct btf_kfunc_id_set bpf_kfunc_set_skb = {
-- 
2.54.0.563.g4f69b47b94-goog