libvhost-user: fix crash when rings aren't ready

Calling libvhost-user functions like vu_queue_get_avail_bytes() when the
queue doesn't yet have addresses will result in the crashes like the
following:

Program received signal SIGSEGV, Segmentation fault.
0x000055c414112ce4 in vring_avail_idx (vq=0x55c41582fd68, vq=0x55c41582fd68)
    at /home/dgilbert/git/qemu/contrib/libvhost-user/libvhost-user.c:940
940            vq->shadow_avail_idx = vq->vring.avail->idx;
(gdb) p vq
$1 = (VuVirtq *) 0x55c41582fd68
(gdb) p vq->vring
$2 = {num = 0, desc = 0x0, avail = 0x0, used = 0x0, log_guest_addr = 0, flags = 0}

    at /home/dgilbert/git/qemu/contrib/libvhost-user/libvhost-user.c:940
No locals.
    at /home/dgilbert/git/qemu/contrib/libvhost-user/libvhost-user.c:960
        num_heads = <optimized out>
    out_bytes=out_bytes@entry=0x7fffd035d7c4, max_in_bytes=max_in_bytes@entry=0,
    max_out_bytes=max_out_bytes@entry=0) at /home/dgilbert/git/qemu/contrib/libvhost-user/libvhost-user.c:1034

Add a pre-condition checks on vring.avail before accessing it.

Fix documentation and return type of vu_queue_empty() while at it.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

contrib/libvhost-user/libvhost-user.c

@@ -1031,6 +1031,11 @@ vu_queue_get_avail_bytes(VuDev *dev, VuVirtq *vq, unsigned int *in_bytes,
     idx = vq->last_avail_idx;
 
     total_bufs = in_total = out_total = 0;
+    if (unlikely(dev->broken) ||
+        unlikely(!vq->vring.avail)) {
+        goto done;
+    }
+
     while ((rc = virtqueue_num_heads(dev, vq, idx)) > 0) {
         unsigned int max, num_bufs, indirect = 0;
         struct vring_desc *desc;
@@ -1121,11 +1126,16 @@ vu_queue_avail_bytes(VuDev *dev, VuVirtq *vq, unsigned int in_bytes,
 
 /* Fetch avail_idx from VQ memory only when we really need to know if
  * guest has added some buffers. */
-int
+bool
 vu_queue_empty(VuDev *dev, VuVirtq *vq)
 {
+    if (unlikely(dev->broken) ||
+        unlikely(!vq->vring.avail)) {
+        return true;
+    }
+
     if (vq->shadow_avail_idx != vq->last_avail_idx) {
-        return 0;
+        return false;
     }
 
     return vring_avail_idx(vq) == vq->last_avail_idx;
@@ -1174,7 +1184,8 @@ vring_notify(VuDev *dev, VuVirtq *vq)
 void
 vu_queue_notify(VuDev *dev, VuVirtq *vq)
 {
-    if (unlikely(dev->broken)) {
+    if (unlikely(dev->broken) ||
+        unlikely(!vq->vring.avail)) {
         return;
     }
 
@@ -1291,7 +1302,8 @@ vu_queue_pop(VuDev *dev, VuVirtq *vq, size_t sz)
     struct vring_desc *desc;
     int rc;
 
-    if (unlikely(dev->broken)) {
+    if (unlikely(dev->broken) ||
+        unlikely(!vq->vring.avail)) {
         return NULL;
     }
 
@@ -1445,7 +1457,8 @@ vu_queue_fill(VuDev *dev, VuVirtq *vq,
 {
     struct vring_used_elem uelem;
 
-    if (unlikely(dev->broken)) {
+    if (unlikely(dev->broken) ||
+        unlikely(!vq->vring.avail)) {
         return;
     }
 
@@ -1474,7 +1487,8 @@ vu_queue_flush(VuDev *dev, VuVirtq *vq, unsigned int count)
 {
     uint16_t old, new;
 
-    if (unlikely(dev->broken)) {
+    if (unlikely(dev->broken) ||
+        unlikely(!vq->vring.avail)) {
         return;
     }
 

contrib/libvhost-user/libvhost-user.h

@@ -327,13 +327,13 @@ void vu_queue_set_notification(VuDev *dev, VuVirtq *vq, int enable);
 bool vu_queue_enabled(VuDev *dev, VuVirtq *vq);
 
 /**
- * vu_queue_enabled:
+ * vu_queue_empty:
  * @dev: a VuDev context
  * @vq: a VuVirtq queue
  *
- * Returns: whether the queue is empty.
+ * Returns: true if the queue is empty or not ready.
  */
-int vu_queue_empty(VuDev *dev, VuVirtq *vq);
+bool vu_queue_empty(VuDev *dev, VuVirtq *vq);
 
 /**
  * vu_queue_notify: