block: Fix crash on block_resize on inactive node

In order for block_resize to fail gracefully on an inactive node instead
of crashing with an assertion failure in bdrv_co_write_req_prepare()
(called from bdrv_co_truncate()), we need to check for inactive nodes
also when they are attached as a root node and make sure that
BLK_PERM_RESIZE isn't among the permissions allowed for inactive nodes.
To this effect, don't enumerate the permissions that are incompatible
with inactive nodes any more, but allow only BLK_PERM_CONSISTENT_READ
for them.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20250204211407.381505-7-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -3077,6 +3077,13 @@ bdrv_attach_child_common(BlockDriverState *child_bs,
     assert(child_class->get_parent_desc);
     GLOBAL_STATE_CODE();
 
+    if (bdrv_is_inactive(child_bs) && (perm & ~BLK_PERM_CONSISTENT_READ)) {
+        g_autofree char *perm_names = bdrv_perm_names(perm);
+        error_setg(errp, "Permission '%s' unavailable on inactive node",
+                   perm_names);
+        return NULL;
+    }
+
     new_child = g_new(BdrvChild, 1);
     *new_child = (BdrvChild) {
         .bs             = NULL,

block/block-backend.c

@@ -253,7 +253,7 @@ static bool blk_can_inactivate(BlockBackend *blk)
      * guest.  For block job BBs that satisfy this, we can just allow
      * it.  This is the case for mirror job source, which is required
      * by libvirt non-shared block migration. */
-    if (!(blk->perm & (BLK_PERM_WRITE | BLK_PERM_WRITE_UNCHANGED))) {
+    if (!(blk->perm & ~BLK_PERM_CONSISTENT_READ)) {
         return true;
     }