[PATCH net] hv_sock: Report EOF instead of -EIO for FIN

[Dexuan Cui <decui@microsoft.com>]

Commit f0c5827d07cb unluckily causes a regression for the FIN packet,
and the final read syscall gets an error rather than 0.

Ideally, we would want to fix hvs_channel_readable_payload() so that it
could return 0 in the FIN scenario, but it's not good for the hv_sock
driver to use the VMBus ringbuffer's cached priv_read_index, which is
internal data in the VMBus driver.

Fix the regression in hv_sock by returning 0 rather than -EIO.

Fixes: f0c5827d07cb ("hv_sock: Return the readable bytes in hvs_stream_has_data()")
Cc: stable@vger.kernel.org
Reported-by: Ben Hillis <Ben.Hillis@microsoft.com>
Reported-by: Mitchell Levy <levymitchell0@gmail.com>
Signed-off-by: Dexuan Cui <decui@microsoft.com>
---
 net/vmw_vsock/hyperv_transport.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/net/vmw_vsock/hyperv_transport.c b/net/vmw_vsock/hyperv_transport.c
index 069386a74557..63d3549125be 100644
--- a/net/vmw_vsock/hyperv_transport.c
+++ b/net/vmw_vsock/hyperv_transport.c
@@ -703,8 +703,22 @@ static s64 hvs_stream_has_data(struct vsock_sock *vsk)
 	switch (hvs_channel_readable_payload(hvs->chan)) {
 	case 1:
 		need_refill = !hvs->recv_desc;
-		if (!need_refill)
-			return -EIO;
+		if (!need_refill) {
+			/* Here hvs->recv_data_len is 0, so hvs->recv_desc must
+			 * be NULL unless it points to the 0-byte-payload FIN
+			 * packet: see hvs_update_recv_data().
+			 *
+			 * Here all the payload has been dequeued, but
+			 * hvs_channel_readable_payload() still returns 1,
+			 * because the VMBus ringbuffer's read_index is not
+			 * updated for the FIN packet: hvs_stream_dequeue() ->
+			 * hv_pkt_iter_next() updates the cached priv_read_index
+			 * but has no opportunity to update the read_index in
+			 * hv_pkt_iter_close() as hvs_stream_has_data() returns
+			 * 0 for the FIN packet, so it won't get dequeued.
+			 */
+			return 0;
+		}
 
 		hvs->recv_desc = hv_pkt_iter_first(hvs->chan);
 		if (!hvs->recv_desc)
-- 
2.49.0