From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Javier Martinez Canillas <javierm@redhat.com>
Date: Mon, 24 Aug 2020 14:46:27 +0200
Subject: [PATCH] tftp: roll over block counter to prevent timeouts with data
 packets

The block number is a 16-bit counter which only allows to fetch
files no bigger than 65535 * blksize. To avoid this limit, the
counter is rolled over. This behavior isn't defined in RFC 1350
but is handled by many TFTP servers and it's what GRUB was doing
before implicitly due an overflow.

Fixing that bug led to TFTP timeouts, since GRUB wasn't acking
data packets anymore for files with size bigger than the maximum
mentioned above. Restore the old behavior to prevent this issue.

Resolves: rhbz#1869987

Suggested-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
---
 grub-core/net/tftp.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/grub-core/net/tftp.c b/grub-core/net/tftp.c
index 79c16f9b041..84162e6f1bd 100644
--- a/grub-core/net/tftp.c
+++ b/grub-core/net/tftp.c
@@ -183,8 +183,20 @@ tftp_receive (grub_net_udp_socket_t sock __attribute__ ((unused)),
 	  return GRUB_ERR_NONE;
 	}
 
-      /* Ack old/retransmitted block. */
-      if (grub_be_to_cpu16 (tftph->u.data.block) < data->block + 1)
+      /*
+       * Ack old/retransmitted block.
+       *
+       * The block number is a 16-bit counter which only allows to fetch
+       * files no bigger than 65535 * blksize. To avoid this limit, the
+       * counter is rolled over. This behavior isn't defined in RFC 1350
+       * but is handled by many TFTP servers and it's what GRUB was doing
+       * before implicitly due an overflow.
+       *
+       * Fixing that bug led to TFTP timeouts, since GRUB wasn't acking
+       * data packets anymore for files with size bigger than the maximum
+       * mentioned above. Restore the old behavior to prevent this issue.
+       */
+      if (grub_be_to_cpu16 (tftph->u.data.block) < ((data->block + 1) & 0xffffu))
 	ack (data, grub_be_to_cpu16 (tftph->u.data.block));
       /* Ignore unexpected block. */
       else if (grub_be_to_cpu16 (tftph->u.data.block) > data->block + 1)