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spice-qemu-char: vmc_write: Don't write more bytes then we're asked too

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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 <hdegoede@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
This commit is contained in:
Hans de Goede 2013-04-05 11:30:24 +02:00 committed by Gerd Hoffmann
parent b010cec86b
commit 75c439bc65
1 changed files with 2 additions and 1 deletions
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spice-qemu-char.c
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@ -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;
}