From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:41817) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ULHfk-00079R-SA for qemu-devel@nongnu.org; Thu, 28 Mar 2013 14:35:38 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ULHfj-0005vu-Lf for qemu-devel@nongnu.org; Thu, 28 Mar 2013 14:35:36 -0400 Received: from mx1.redhat.com ([209.132.183.28]:63218) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ULHfj-0005ut-DP for qemu-devel@nongnu.org; Thu, 28 Mar 2013 14:35:35 -0400 Received: from int-mx02.intmail.prod.int.phx2.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id r2SIZXhm011231 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Thu, 28 Mar 2013 14:35:33 -0400 From: Hans de Goede Date: Thu, 28 Mar 2013 19:39:01 +0100 Message-Id: <1364495941-3513-1-git-send-email-hdegoede@redhat.com> Subject: [Qemu-devel] [PATCH] spice-qemu-char: vmc_write: Don't write more bytes then we're asked too List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Gerd Hoffmann Cc: Hans de Goede , qemu-devel@nongnu.org This one took me eons to debug, but I've finally found it now, oh well. The usage of the MIN macro in this line: last_out = MIN(len, qemu_chr_be_can_write(scd->chr)); Causes qemu_chr_be_can_write to be called *twice*, since the MIN macro evaluates its arguments twice (bad MIN macro, bad!). And the result of the call can change between the 2 calls since the guest may have consumed some data from the virtio ringbuffer between the calls! When this happens it is possible for qemu_chr_be_can_write to return less then len in the call made for the comparision, and then to return more then len in the actual call for the return-value of MIN, after which we will end up writing len data + some extra garbage, not good. This patch fixes this by only calling qemu_chr_be_can_write once. Signed-off-by: Hans de Goede --- spice-qemu-char.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/spice-qemu-char.c b/spice-qemu-char.c index 7e6bd2d..fb4af9a 100644 --- a/spice-qemu-char.c +++ b/spice-qemu-char.c @@ -35,7 +35,8 @@ static int vmc_write(SpiceCharDeviceInstance *sin, const uint8_t *buf, int len) uint8_t* p = (uint8_t*)buf; while (len > 0) { - last_out = MIN(len, qemu_chr_be_can_write(scd->chr)); + int can_write = qemu_chr_be_can_write(scd->chr); + last_out = MIN(len, can_write); if (last_out <= 0) { break; } -- 1.8.1.4