tcp: fix SO_RCVLOWAT and RCVBUF autotuning

Applications might use SO_RCVLOWAT on TCP socket hoping to receive
one [E]POLLIN event only when a given amount of bytes are ready in socket
receive queue.

Problem is that receive autotuning is not aware of this constraint,
meaning sk_rcvbuf might be too small to allow all bytes to be stored.

Add a new (struct proto_ops)->set_rcvlowat method so that a protocol
can override the default setsockopt(SO_RCVLOWAT) behavior.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 21 insertions, 0 deletions

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index bccc4c270087..0abd8d1d3d1d 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1701,6 +1701,27 @@ int tcp_peek_len(struct socket *sock)
 }
 EXPORT_SYMBOL(tcp_peek_len);
 
+/* Make sure sk_rcvbuf is big enough to satisfy SO_RCVLOWAT hint */
+int tcp_set_rcvlowat(struct sock *sk, int val)
+{
+        sk->sk_rcvlowat = val ? : 1;
+        if (sk->sk_userlocks & SOCK_RCVBUF_LOCK)
+                return 0;
+
+        /* val comes from user space and might be close to INT_MAX */
+        val <<= 1;
+        if (val < 0)
+                val = INT_MAX;
+
+        val = min(val, sock_net(sk)->ipv4.sysctl_tcp_rmem[2]);
+        if (val > sk->sk_rcvbuf) {
+                sk->sk_rcvbuf = val;
+                tcp_sk(sk)->window_clamp = tcp_win_from_space(sk, val);
+        }
+        return 0;
+}
+EXPORT_SYMBOL(tcp_set_rcvlowat);
+
 static void tcp_update_recv_tstamps(struct sk_buff *skb,
                                     struct scm_timestamping *tss)
 {