1 files changed, 37 insertions, 16 deletions

diff --git a/block.c b/block.c
index 91859a928d..466d7887cc 100644
--- a/block.c
+++ b/block.c
@@ -312,20 +312,29 @@ fail:
     return -EACCES;
 }
 
-void bdrv_get_full_backing_filename_from_filename(const char *backed,
-                                                  const char *backing,
-                                                  char *dest, size_t sz,
-                                                  Error **errp)
+/*
+ * If @backing is empty, this function returns NULL without setting
+ * @errp.  In all other cases, NULL will only be returned with @errp
+ * set.
+ *
+ * Therefore, a return value of NULL without @errp set means that
+ * there is no backing file; if @errp is set, there is one but its
+ * absolute filename cannot be generated.
+ */
+char *bdrv_get_full_backing_filename_from_filename(const char *backed,
+                                                   const char *backing,
+                                                   Error **errp)
 {
-    if (backing[0] == '\0' || path_has_protocol(backing) ||
-        path_is_absolute(backing))
-    {
-        pstrcpy(dest, sz, backing);
+    if (backing[0] == '\0') {
+        return NULL;
+    } else if (path_has_protocol(backing) || path_is_absolute(backing)) {
+        return g_strdup(backing);
     } else if (backed[0] == '\0' || strstart(backed, "json:", NULL)) {
         error_setg(errp, "Cannot use relative backing file names for '%s'",
                    backed);
+        return NULL;
     } else {
-        path_combine_deprecated(dest, sz, backed, backing);
+        return path_combine(backed, backing);
     }
 }
 
@@ -333,12 +342,24 @@ void bdrv_get_full_backing_filename(BlockDriverState *bs, char *dest, size_t sz,
                                     Error **errp)
 {
     char *backed;
+    char *full_name;
+    Error *local_error = NULL;
 
     bdrv_refresh_filename(bs);
 
     backed = bs->exact_filename[0] ? bs->exact_filename : bs->filename;
-    bdrv_get_full_backing_filename_from_filename(backed, bs->backing_file,
-                                                 dest, sz, errp);
+
+    full_name = bdrv_get_full_backing_filename_from_filename(backed,
+                                                             bs->backing_file,
+                                                             &local_error);
+    if (full_name) {
+        pstrcpy(dest, sz, full_name);
+        g_free(full_name);
+    } else if (local_error) {
+        error_propagate(errp, local_error);
+    } else if (sz > 0) {
+        *dest = '\0';
+    }
 }
 
 void bdrv_register(BlockDriver *bdrv)
@@ -5179,17 +5200,17 @@ void bdrv_img_create(const char *filename, const char *fmt,
     size = qemu_opt_get_size(opts, BLOCK_OPT_SIZE, img_size);
     if (backing_file && !(flags & BDRV_O_NO_BACKING)) {
         BlockDriverState *bs;
-        char *full_backing = g_new0(char, PATH_MAX);
+        char *full_backing;
         int back_flags;
         QDict *backing_options = NULL;
 
-        bdrv_get_full_backing_filename_from_filename(filename, backing_file,
-                                                     full_backing, PATH_MAX,
-                                                     &local_err);
+        full_backing =
+            bdrv_get_full_backing_filename_from_filename(filename, backing_file,
+                                                         &local_err);
         if (local_err) {
-            g_free(full_backing);
             goto out;
         }
+        assert(full_backing);
 
         /* backing files always opened read-only */
         back_flags = flags;