When the backup-top node transitions from active to inactive in
bdrv_backup_top_drop(), the BlockCopyState is freed and the filtered
child is removed, so the node effectively becomes unusable.

However, noone told its I/O functions this, so they will happily
continue accessing bs->backing and s->bcs.  Prevent that by aborting
early when s->active is false.

(After the preceding patch, the node should be gone after
bdrv_backup_top_drop(), so this should largely be a theoretical problem.
But still, better to be safe than sorry, and also I think it just makes
sense to check s->active in the I/O functions.)

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/backup-top.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/block/backup-top.c b/block/backup-top.c
index d1253e1aa6..589e8b651d 100644
--- a/block/backup-top.c
+++ b/block/backup-top.c
@@ -45,6 +45,12 @@ static coroutine_fn int backup_top_co_preadv(
         BlockDriverState *bs, uint64_t offset, uint64_t bytes,
         QEMUIOVector *qiov, int flags)
 {
+    BDRVBackupTopState *s = bs->opaque;
+
+    if (!s->active) {
+        return -EIO;
+    }
+
     return bdrv_co_preadv(bs->backing, offset, bytes, qiov, flags);
 }
 
@@ -54,6 +60,10 @@ static coroutine_fn int backup_top_cbw(BlockDriverState *bs, uint64_t offset,
     BDRVBackupTopState *s = bs->opaque;
     uint64_t off, end;
 
+    if (!s->active) {
+        return -EIO;
+    }
+
     if (flags & BDRV_REQ_WRITE_UNCHANGED) {
         return 0;
     }
-- 
2.29.2