[bug report] virtio_net: rework mergeable buffer handling

[Dan Carpenter <dan.carpenter@oracle.com>]

Hello Michael S. Tsirkin,

The patch 6c8e5f3c41c8: "virtio_net: rework mergeable buffer
handling" from Mar 6, 2017, leads to the following static checker
warning:

	drivers/net/virtio_net.c:1042 virtnet_receive()
	error: uninitialized symbol 'ctx'.

drivers/net/virtio_net.c
  1030  static int virtnet_receive(struct receive_queue *rq, int budget)
  1031  {
  1032          struct virtnet_info *vi = rq->vq->vdev->priv;
  1033          unsigned int len, received = 0, bytes = 0;
  1034          void *buf;
  1035          struct virtnet_stats *stats = this_cpu_ptr(vi->stats);
  1036  
  1037          if (vi->mergeable_rx_bufs) {
  1038                  void *ctx;
                              ^^^
  1039  
  1040                  while (received < budget &&
  1041                         (buf = virtqueue_get_buf_ctx(rq->vq, &len, &ctx))) {
                                                                           ^^^^
  1042                          bytes += receive_buf(vi, rq, buf, len, ctx);
                                                                       ^^^

It's possible that this code is correct, but I looked at it and wasn't
immediately convinced.  Returning non-NULL buf is not sufficient to
show that "ctx" is initialized, because if it's vq->indirect then "buf"
is still unintialized.  Also it's possible that receive_buf() checks
vq->indirect through some side effect way that I didn't see so it
doesn't use the uninitialized value...

I feel like if this is a false positive, that means the rules are too
subtle...  :/

  1043                          received++;
  1044                  }
  1045          } else {
  1046                  while (received < budget &&
  1047                         (buf = virtqueue_get_buf(rq->vq, &len)) != NULL) {
  1048                          bytes += receive_buf(vi, rq, buf, len, NULL);
  1049                          received++;
  1050                  }
  1051          }
  1052  

regards,
dan carpenter