Commit 75c439bc authored by Hans de Goede's avatar Hans de Goede Committed by Gerd Hoffmann
Browse files

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



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: default avatarHans de Goede <hdegoede@redhat.com>
Signed-off-by: default avatarGerd Hoffmann <kraxel@redhat.com>
parent b010cec8

spice-qemu-char.c

+2 −1
Original line number Diff line number Diff line
@@ -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;

        }