1 files changed, 23 insertions, 0 deletions
diff --git a/block.c b/block.c
index cf3534fd5f..064e9d77fa 100644
--- a/block.c
+++ b/block.c
@@ -1537,6 +1537,29 @@ int bdrv_child_try_set_perm(BdrvChild *c, uint64_t perm, uint64_t shared,
     return 0;
 }
 
+#define DEFAULT_PERM_PASSTHROUGH (BLK_PERM_CONSISTENT_READ \
+                                 | BLK_PERM_WRITE \
+                                 | BLK_PERM_WRITE_UNCHANGED \
+                                 | BLK_PERM_RESIZE)
+#define DEFAULT_PERM_UNCHANGED (BLK_PERM_ALL & ~DEFAULT_PERM_PASSTHROUGH)
+
+void bdrv_filter_default_perms(BlockDriverState *bs, BdrvChild *c,
+                               const BdrvChildRole *role,
+                               uint64_t perm, uint64_t shared,
+                               uint64_t *nperm, uint64_t *nshared)
+{
+    if (c == NULL) {
+        *nperm = perm & DEFAULT_PERM_PASSTHROUGH;
+        *nshared = (shared & DEFAULT_PERM_PASSTHROUGH) | DEFAULT_PERM_UNCHANGED;
+        return;
+    }
+
+    *nperm = (perm & DEFAULT_PERM_PASSTHROUGH) |
+             (c->perm & DEFAULT_PERM_UNCHANGED);
+    *nshared = (shared & DEFAULT_PERM_PASSTHROUGH) |
+               (c->shared_perm & DEFAULT_PERM_UNCHANGED);
+}
+
 static void bdrv_replace_child(BdrvChild *child, BlockDriverState *new_bs,
                                bool check_new_perm)
 {