[PATCH] net: tftp: Fix for DATA ACK for block count out of order

[seanedmond at linux.microsoft.com]

From: Sean Edmond <seanedmond at microsoft.com>

In rfc7440, if an ACK is not received by the server or if the
last data block in a window is dropped, the server will timeout and
retransmit the window.  In this case, the block count received will be
less than the internal block count.  In this case, the client
should not ACK.  ACK should only be sent if the received block
count is greater than the expected block count.

Signed-off-by: Sean Edmond <seanedmond at microsoft.com>
---
 net/tftp.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/tftp.c b/net/tftp.c
index c780c33f37..51e062bddf 100644
--- a/net/tftp.c
+++ b/net/tftp.c
@@ -592,6 +592,14 @@ static void tftp_handler(uchar *pkt, unsigned dest, struct in_addr sip,
 			debug("Received unexpected block: %d, expected: %d\n",
 			      ntohs(*(__be16 *)pkt),
 			      (ushort)(tftp_cur_block + 1));
+			/*
+			 * Only ACK if the block count received is greater than
+			 * the expected block count, otherwise skip ACK.
+			 * (required to properly handle the server retransmitting
+			 *  the window)
+			 */
+			if ((ushort)(tftp_cur_block + 1) - (short)(ntohs(*(__be16 *)pkt)) > 0)
+				break;
 			/*
 			 * If one packet is dropped most likely
 			 * all other buffers in the window
-- 
2.17.1