diff --git a/block.c b/block.c
index e0c6c648b1..6a805ff0ea 100644
--- a/block.c
+++ b/block.c
@@ -5849,7 +5849,7 @@ int64_t coroutine_fn bdrv_co_nb_sectors(BlockDriverState *bs)
     if (!drv)
         return -ENOMEDIUM;
 
-    if (drv->has_variable_length) {
+    if (bs->bl.has_variable_length) {
         int ret = bdrv_co_refresh_total_sectors(bs, bs->total_sectors);
         if (ret < 0) {
             return ret;
diff --git a/block/io.c b/block/io.c
index 8974d46941..932aeb5844 100644
--- a/block/io.c
+++ b/block/io.c
@@ -182,6 +182,8 @@ void bdrv_refresh_limits(BlockDriverState *bs, Transaction *tran, Error **errp)
                                 drv->bdrv_aio_preadv ||
                                 drv->bdrv_co_preadv_part) ? 1 : 512;
 
+    bs->bl.has_variable_length = drv->has_variable_length;
+
     /* Take some limits from the children as a default */
     have_limits = false;
     QLIST_FOREACH(c, &bs->children, next) {
@@ -190,6 +192,10 @@ void bdrv_refresh_limits(BlockDriverState *bs, Transaction *tran, Error **errp)
             bdrv_merge_limits(&bs->bl, &c->bs->bl);
             have_limits = true;
         }
+
+        if (c->role & BDRV_CHILD_FILTERED) {
+            bs->bl.has_variable_length |= c->bs->bl.has_variable_length;
+        }
     }
 
     if (!have_limits) {
diff --git a/include/block/block_int-common.h b/include/block/block_int-common.h
index d419017328..a6d271f25d 100644
--- a/include/block/block_int-common.h
+++ b/include/block/block_int-common.h
@@ -855,6 +855,14 @@ typedef struct BlockLimits {
 
     /* maximum number of iovec elements */
     int max_iov;
+
+    /*
+     * true if the length of the underlying file can change, and QEMU
+     * is expected to adjust automatically.  Mostly for CD-ROM drives,
+     * whose length is zero when the tray is empty (they don't need
+     * an explicit monitor command to load the disk inside the guest).
+     */
+    bool has_variable_length;
 } BlockLimits;
 
 typedef struct BdrvOpBlocker BdrvOpBlocker;