[PATCH 4.20 18/52] tcp: do not report TCP_CM_INQ of 0 for closed connections

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

4.20-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Soheil Hassas Yeganeh <soheil@google.com>

[ Upstream commit 6466e715651f9f358e60c5ea4880e4731325827f ]

Returning 0 as inq to userspace indicates there is no more data to
read, and the application needs to wait for EPOLLIN. For a connection
that has received FIN from the remote peer, however, the application
must continue reading until getting EOF (return value of 0
from tcp_recvmsg) or an error, if edge-triggered epoll (EPOLLET) is
being used. Otherwise, the application will never receive a new
EPOLLIN, since there is no epoll edge after the FIN.

Return 1 when there is no data left on the queue but the
connection has received FIN, so that the applications continue
reading.

Fixes: b75eba76d3d72 (tcp: send in-queue bytes in cmsg upon read)
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/ipv4/tcp.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1914,6 +1914,11 @@ static int tcp_inq_hint(struct sock *sk)
 		inq = tp->rcv_nxt - tp->copied_seq;
 		release_sock(sk);
 	}
+	/* After receiving a FIN, tell the user-space to continue reading
+	 * by returning a non-zero inq.
+	 */
+	if (inq == 0 && sock_flag(sk, SOCK_DONE))
+		inq = 1;
 	return inq;
 }