netdev - [PATCH net-next 2/4] net: tcp: consistently account for overhead for SO

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Date:   Mon, 11 Jan 2021 17:24:09 -0500
From:   Heath Caldwell <hcaldwel@...mai.com>
To:     <netdev@...r.kernel.org>
CC:     Eric Dumazet <edumazet@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Josh Hunt <johunt@...mai.com>, Ji Li <jli@...mai.com>,
        Heath Caldwell <hcaldwel@...mai.com>
Subject: [PATCH net-next 2/4] net: tcp: consistently account for overhead for SO_RCVBUF for TCP

When setting SO_RCVBUF for TCP sockets, account for overhead in accord with
sysctl_tcp_adv_win_scale.  This makes the receive buffer overhead
accounting for SO_RCVBUF consistent with how it is accounted elsewhere for
TCP sockets.

Signed-off-by: Heath Caldwell <hcaldwel@...mai.com>
---
 include/net/tcp.h | 17 +++++++++++++++++
 net/core/sock.c   |  6 ++++++
 2 files changed, 23 insertions(+)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 78d13c88720f..9961de3fbf09 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1408,6 +1408,23 @@ static inline int tcp_win_from_space(const struct sock *sk, int space)
 		space - (space>>tcp_adv_win_scale);
 }
 
+/* Calculate the amount of buffer space which would allow for the advertisement
+ * of a window size of win, accounting for overhead.
+ *
+ * This is the inverse of tcp_win_from_space().
+ */
+static inline int tcp_space_from_win(const struct sock *sk, int win)
+{
+	int tcp_adv_win_scale = sock_net(sk)->ipv4.sysctl_tcp_adv_win_scale;
+
+	return tcp_adv_win_scale <= 0 ?
+		win<<(-tcp_adv_win_scale) :
+		/* Division by zero is avoided because the above expression is
+		 * used when tcp_adv_win_scale == 0.
+		 */
+		(win<<tcp_adv_win_scale) / ((1<<tcp_adv_win_scale) - 1);
+}
+
 /* Note: caller must be prepared to deal with negative returns */
 static inline int tcp_space(const struct sock *sk)
 {
diff --git a/net/core/sock.c b/net/core/sock.c
index 0a9c19f52989..e919f62d5d34 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -783,6 +783,9 @@ EXPORT_SYMBOL(sock_set_keepalive);
  */
 static inline int sock_buf_size_to_available(struct sock *sk, int buf_size)
 {
+	if (sk->sk_protocol == IPPROTO_TCP)
+		return tcp_win_from_space(sk, buf_size);
+
 	return buf_size / 2;
 }
 
@@ -792,6 +795,9 @@ static inline int sock_buf_size_to_available(struct sock *sk, int buf_size)
  */
 static inline int sock_available_to_buf_size(struct sock *sk, int available)
 {
+	if (sk->sk_protocol == IPPROTO_TCP)
+		return tcp_space_from_win(sk, available);
+
 	return available * 2;
 }
 
-- 
2.28.0