qcow2: Make qiov match request size until backing file EOF

If a qcow2 image has a shorter backing file and a read request to
unallocated clusters goes across EOF of the backing file, the backing
file sees a shortened request and the rest is filled with zeros.
However, the original too long qiov was used with the shortened request.

This patch makes the qiov size match the request size, avoiding a
potential buffer overflow in raw-posix.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>

block/qcow2.c

@@ -1020,11 +1020,20 @@ static coroutine_fn int qcow2_co_readv(BlockDriverState *bs, int64_t sector_num,
                 n1 = qcow2_backing_read1(bs->backing_hd, &hd_qiov,
                     sector_num, cur_nr_sectors);
                 if (n1 > 0) {
+                    QEMUIOVector local_qiov;
+
+                    qemu_iovec_init(&local_qiov, hd_qiov.niov);
+                    qemu_iovec_concat(&local_qiov, &hd_qiov, 0,
+                                      n1 * BDRV_SECTOR_SIZE);
+
                     BLKDBG_EVENT(bs->file, BLKDBG_READ_BACKING_AIO);
                     qemu_co_mutex_unlock(&s->lock);
                     ret = bdrv_co_readv(bs->backing_hd, sector_num,
-                                        n1, &hd_qiov);
+                                        n1, &local_qiov);
                     qemu_co_mutex_lock(&s->lock);
+
+                    qemu_iovec_destroy(&local_qiov);
+
                     if (ret < 0) {
                         goto fail;
                     }