[PATCH v8 15/43] block: Re-evaluate backing file handling in reopen

[Max Reitz <mreitz@redhat.com>]

Reopening a node's backing child needs a bit of special handling because
the "backing" child has different defaults than all other children
(among other things).  Adding filter support here is a bit more
difficult than just using the child access functions.  In fact, we often
have to directly use bs->backing because these functions are about the
"backing" child (which may or may not be the COW backing file).

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block.c | 49 ++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 40 insertions(+), 9 deletions(-)

diff --git a/block.c b/block.c
index 96bf8672f1..660386795a 100644
--- a/block.c
+++ b/block.c
@@ -4004,7 +4004,7 @@ static int bdrv_reopen_parse_backing(BDRVReopenState *reopen_state,
                                      Error **errp)
 {
     BlockDriverState *bs = reopen_state->bs;
-    BlockDriverState *overlay_bs, *new_backing_bs;
+    BlockDriverState *overlay_bs, *below_bs, *new_backing_bs;
     QObject *value;
     const char *str;
 
@@ -4043,26 +4043,57 @@ static int bdrv_reopen_parse_backing(BDRVReopenState *reopen_state,
         }
     }
 
+    /*
+     * Ensure that @bs can really handle backing files, because we are
+     * about to give it one (or swap the existing one)
+     */
+    if (bs->drv->is_filter) {
+        /* Filters always have a file or a backing child */
+        if (!bs->backing) {
+            error_setg(errp, "'%s' is a %s filter node that does not support a "
+                       "backing child", bs->node_name, bs->drv->format_name);
+            return -EINVAL;
+        }
+    } else if (!bs->drv->supports_backing) {
+        error_setg(errp, "Driver '%s' of node '%s' does not support backing "
+                   "files", bs->drv->format_name, bs->node_name);
+        return -EINVAL;
+    }
+
     /*
      * Find the "actual" backing file by skipping all links that point
      * to an implicit node, if any (e.g. a commit filter node).
+     * We cannot use any of the bdrv_skip_*() functions here because
+     * those return the first explicit node, while we are looking for
+     * its overlay here.
      */
     overlay_bs = bs;
-    while (backing_bs(overlay_bs) && backing_bs(overlay_bs)->implicit) {
-        overlay_bs = backing_bs(overlay_bs);
+    for (below_bs = bdrv_filter_or_cow_bs(overlay_bs);
+         below_bs && below_bs->implicit;
+         below_bs = bdrv_filter_or_cow_bs(overlay_bs))
+    {
+        overlay_bs = below_bs;
     }
 
     /* If we want to replace the backing file we need some extra checks */
-    if (new_backing_bs != backing_bs(overlay_bs)) {
+    if (new_backing_bs != bdrv_filter_or_cow_bs(overlay_bs)) {
         /* Check for implicit nodes between bs and its backing file */
         if (bs != overlay_bs) {
             error_setg(errp, "Cannot change backing link if '%s' has "
                        "an implicit backing file", bs->node_name);
             return -EPERM;
         }
-        /* Check if the backing link that we want to replace is frozen */
-        if (bdrv_is_backing_chain_frozen(overlay_bs, backing_bs(overlay_bs),
-                                         errp)) {
+        /*
+         * Check if the backing link that we want to replace is frozen.
+         * Note that
+         * bdrv_filter_or_cow_child(overlay_bs) == overlay_bs->backing,
+         * because we know that overlay_bs == bs, and that @bs
+         * either is a filter that uses ->backing or a COW format BDS
+         * with bs->drv->supports_backing == true.
+         */
+        if (bdrv_is_backing_chain_frozen(overlay_bs,
+                                         child_bs(overlay_bs->backing), errp))
+        {
             return -EPERM;
         }
         reopen_state->replace_backing_bs = true;
@@ -4211,7 +4242,7 @@ int bdrv_reopen_prepare(BDRVReopenState *reopen_state, BlockReopenQueue *queue,
      * its metadata. Otherwise the 'backing' option can be omitted.
      */
     if (drv->supports_backing && reopen_state->backing_missing &&
-        (backing_bs(reopen_state->bs) || reopen_state->bs->backing_file[0])) {
+        (reopen_state->bs->backing || reopen_state->bs->backing_file[0])) {
         error_setg(errp, "backing is missing for '%s'",
                    reopen_state->bs->node_name);
         ret = -EINVAL;
@@ -4352,7 +4383,7 @@ void bdrv_reopen_commit(BDRVReopenState *reopen_state)
      * from bdrv_set_backing_hd()) has the new values.
      */
     if (reopen_state->replace_backing_bs) {
-        BlockDriverState *old_backing_bs = backing_bs(bs);
+        BlockDriverState *old_backing_bs = child_bs(bs->backing);
         assert(!old_backing_bs || !old_backing_bs->implicit);
         /* Abort the permission update on the backing bs we're detaching */
         if (old_backing_bs) {
-- 
2.26.2