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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        Hannes Frederic Sowa <hannes@stressinduktion.org>,
        Eric Dumazet <edumazet@google.com>,
        Jason Wang <jasowang@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Tommi Rantala <tommi.t.rantala@nokia.com>
Subject: [PATCH 4.9 45/88] tuntap: correctly set SOCKWQ_ASYNC_NOSPACE
Date:   Tue, 10 Mar 2020 13:38:53 +0100
Message-Id: <20200310123617.162923966@linuxfoundation.org>
In-Reply-To: <20200310123606.543939933@linuxfoundation.org>
References: <20200310123606.543939933@linuxfoundation.org>
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From: Jason Wang <jasowang@redhat.com>

commit 2f3ab6221e4c87960347d65c7cab9bd917d1f637 upstream.

When link is down, writes to the device might fail with
-EIO. Userspace needs an indication when the status is resolved.  As a
fix, tun_net_open() attempts to wake up writers - but that is only
effective if SOCKWQ_ASYNC_NOSPACE has been set in the past. This is
not the case of vhost_net which only poll for EPOLLOUT after it meets
errors during sendmsg().

This patch fixes this by making sure SOCKWQ_ASYNC_NOSPACE is set when
socket is not writable or device is down to guarantee EPOLLOUT will be
raised in either tun_chr_poll() or tun_sock_write_space() after device
is up.

Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Eric Dumazet <edumazet@google.com>
Fixes: 1bd4978a88ac2 ("tun: honor IFF_UP in tun_get_user()")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tommi Rantala <tommi.t.rantala@nokia.com>

---
 drivers/net/tun.c |   19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1106,6 +1106,13 @@ static void tun_net_init(struct net_devi
 	}
 }
 
+static bool tun_sock_writeable(struct tun_struct *tun, struct tun_file *tfile)
+{
+	struct sock *sk = tfile->socket.sk;
+
+	return (tun->dev->flags & IFF_UP) && sock_writeable(sk);
+}
+
 /* Character device part */
 
 /* Poll */
@@ -1128,10 +1135,14 @@ static unsigned int tun_chr_poll(struct
 	if (!skb_array_empty(&tfile->tx_array))
 		mask |= POLLIN | POLLRDNORM;
 
-	if (tun->dev->flags & IFF_UP &&
-	    (sock_writeable(sk) ||
-	     (!test_and_set_bit(SOCKWQ_ASYNC_NOSPACE, &sk->sk_socket->flags) &&
-	      sock_writeable(sk))))
+	/* Make sure SOCKWQ_ASYNC_NOSPACE is set if not writable to
+	 * guarantee EPOLLOUT to be raised by either here or
+	 * tun_sock_write_space(). Then process could get notification
+	 * after it writes to a down device and meets -EIO.
+	 */
+	if (tun_sock_writeable(tun, tfile) ||
+	    (!test_and_set_bit(SOCKWQ_ASYNC_NOSPACE, &sk->sk_socket->flags) &&
+	     tun_sock_writeable(tun, tfile)))
 		mask |= POLLOUT | POLLWRNORM;
 
 	if (tun->dev->reg_state != NETREG_REGISTERED)