[Qemu-devel] [PATCH] spice-qemu-char: vmc_write: Don't write more bytes then we're asked too

[Qemu-devel] [PATCH] spice-qemu-char: vmc_write: Don't write more bytes then we're asked too

[Hans de Goede]

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>
---
 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