[PATCH] vhost: svq: fix uninitialized variable

[Laurent Vivier <lvivier@redhat.com>]

The problem has been reported by gcc with CFLAGS=-O3:

.../hw/virtio/vhost-shadow-virtqueue.c: In function ‘vhost_svq_poll’:
.../hw/virtio/vhost-shadow-virtqueue.c:538:12:
error: ‘len’ may be used uninitialized [-Werror=maybe-uninitialized]
  538 |     return len;
      |            ^~~

vhost_svq_get_buf() returns NULL if SVQ is empty but doesn't set len to 0,
and vhost_svq_poll() returns len without checking the return of
vhost_svq_get_buf(). So if the SVQ is empty vhost_svq_poll() can return
an random value.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 hw/virtio/vhost-shadow-virtqueue.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c
index 430729635815..31cf642db267 100644
--- a/hw/virtio/vhost-shadow-virtqueue.c
+++ b/hw/virtio/vhost-shadow-virtqueue.c
@@ -420,6 +420,7 @@ static VirtQueueElement *vhost_svq_get_buf(VhostShadowVirtqueue *svq,
     vring_used_elem_t used_elem;
     uint16_t last_used, last_used_chain, num;
 
+    *len = 0;
     if (!vhost_svq_more_used(svq)) {
         return NULL;
     }
-- 
2.39.1