virtio: handle short buffers in virtio_rng.

If the device fills less than 4 bytes of our random buffer, we'll
BUG_ON.  It's nicer to handle the case where it partially fills the
buffer (the protocol doesn't explicitly bad that).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/char/hw_random/virtio-rng.c

@@ -35,13 +35,13 @@ static DECLARE_COMPLETION(have_data);
 
 static void random_recv_done(struct virtqueue *vq)
 {
-	int len;
+	unsigned int len;
 
 	/* We can get spurious callbacks, e.g. shared IRQs + virtio_pci. */
 	if (!vq->vq_ops->get_buf(vq, &len))
 		return;
 
-	data_left = len / sizeof(random_data[0]);
+	data_left += len;
 	complete(&have_data);
 }
 
@@ -49,7 +49,7 @@ static void register_buffer(void)
 {
 	struct scatterlist sg;
 
-	sg_init_one(&sg, random_data, RANDOM_DATA_SIZE);
+	sg_init_one(&sg, random_data+data_left, RANDOM_DATA_SIZE-data_left);
 	/* There should always be room for one buffer. */
 	if (vq->vq_ops->add_buf(vq, &sg, 0, 1, random_data) != 0)
 		BUG();
@@ -59,24 +59,32 @@ static void register_buffer(void)
 /* At least we don't udelay() in a loop like some other drivers. */
 static int virtio_data_present(struct hwrng *rng, int wait)
 {
-	if (data_left)
+	if (data_left >= sizeof(u32))
 		return 1;
 
+again:
 	if (!wait)
 		return 0;
 
 	wait_for_completion(&have_data);
+
+	/* Not enough?  Re-register. */
+	if (unlikely(data_left < sizeof(u32))) {
+		register_buffer();
+		goto again;
+	}
+
 	return 1;
 }
 
 /* virtio_data_present() must have succeeded before this is called. */
 static int virtio_data_read(struct hwrng *rng, u32 *data)
 {
-	BUG_ON(!data_left);
+	BUG_ON(data_left < sizeof(u32));
+	data_left -= sizeof(u32);
+	*data = random_data[data_left / 4];
 
-	*data = random_data[--data_left];
+	if (data_left < sizeof(u32)) {
-
-	if (!data_left) {
 		init_completion(&have_data);
 		register_buffer();
 	}