[Qemu-devel] [PATCH 4/4] usb-ohci: td.cbp incorrectly updated near page end

[Gerd Hoffmann <kraxel@redhat.com>]

From: Andriy Gapon <avg@FreeBSD.org>

The current code that updates the cbp value after a transfer looks like this:
td.cbp += ret;
if ((td.cbp & 0xfff) + ret > 0xfff) {
	<handle page overflow>
because the 'ret' value is effectively added twice the check may fire too early
when the overflow hasn't happened yet.

Below is one of the possible changes that correct the behavior:

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 hw/usb-ohci.c |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/hw/usb-ohci.c b/hw/usb-ohci.c
index e68be70..81488c4 100644
--- a/hw/usb-ohci.c
+++ b/hw/usb-ohci.c
@@ -1025,10 +1025,10 @@ static int ohci_service_td(OHCIState *ohci, struct ohci_ed *ed)
         if (ret == len) {
             td.cbp = 0;
         } else {
-            td.cbp += ret;
             if ((td.cbp & 0xfff) + ret > 0xfff) {
-                td.cbp &= 0xfff;
-                td.cbp |= td.be & ~0xfff;
+                td.cbp = (td.be & ~0xfff) + ((td.cbp + ret) & 0xfff);
+            } else {
+                td.cbp += ret;
             }
         }
         td.flags |= OHCI_TD_T1;
-- 
1.7.1