1 files changed, 39 insertions, 14 deletions

diff --git a/block.c b/block.c
index 3031413..35a372d 100644
--- a/block.c
+++ b/block.c
@@ -1206,7 +1206,8 @@ static int bdrv_backing_update_filename(BdrvChild *c, BlockDriverState *base,
     }
 
     ret = bdrv_change_backing_file(parent, filename,
-                                   base->drv ? base->drv->format_name : "");
+                                   base->drv ? base->drv->format_name : "",
+                                   false);
     if (ret < 0) {
         error_setg_errno(errp, -ret, "Could not update backing file link");
     }
@@ -4680,8 +4681,8 @@ int bdrv_check(BlockDriverState *bs,
  *            image file header
  * -ENOTSUP - format driver doesn't support changing the backing file
  */
-int bdrv_change_backing_file(BlockDriverState *bs,
-    const char *backing_file, const char *backing_fmt)
+int bdrv_change_backing_file(BlockDriverState *bs, const char *backing_file,
+                             const char *backing_fmt, bool warn)
 {
     BlockDriver *drv = bs->drv;
     int ret;
@@ -4695,6 +4696,12 @@ int bdrv_change_backing_file(BlockDriverState *bs,
         return -EINVAL;
     }
 
+    if (warn && backing_file && !backing_fmt) {
+        warn_report("Deprecated use of backing file without explicit "
+                    "backing format, use of this image requires "
+                    "potentially unsafe format probing");
+    }
+
     if (drv->bdrv_change_backing_file != NULL) {
         ret = drv->bdrv_change_backing_file(bs, backing_file, backing_fmt);
     } else {
@@ -6128,18 +6135,30 @@ void bdrv_img_create(const char *filename, const char *fmt,
         bs = bdrv_open(full_backing, NULL, backing_options, back_flags,
                        &local_err);
         g_free(full_backing);
-        if (!bs && size != -1) {
-            /* Couldn't open BS, but we have a size, so it's nonfatal */
-            warn_reportf_err(local_err,
-                            "Could not verify backing image. "
-                            "This may become an error in future versions.\n");
-            local_err = NULL;
-        } else if (!bs) {
-            /* Couldn't open bs, do not have size */
-            error_append_hint(&local_err,
-                              "Could not open backing image to determine size.\n");
+        if (!bs) {
+            error_append_hint(&local_err, "Could not open backing image.\n");
             goto out;
         } else {
+            if (!backing_fmt) {
+                warn_report("Deprecated use of backing file without explicit "
+                            "backing format (detected format of %s)",
+                            bs->drv->format_name);
+                if (bs->drv != &bdrv_raw) {
+                    /*
+                     * A probe of raw deserves the most attention:
+                     * leaving the backing format out of the image
+                     * will ensure bs->probed is set (ensuring we
+                     * don't accidentally commit into the backing
+                     * file), and allow more spots to warn the users
+                     * to fix their toolchain when opening this image
+                     * later.  For other images, we can safely record
+                     * the format that we probed.
+                     */
+                    backing_fmt = bs->drv->format_name;
+                    qemu_opt_set(opts, BLOCK_OPT_BACKING_FMT, backing_fmt,
+                                 NULL);
+                }
+            }
             if (size == -1) {
                 /* Opened BS, have no size */
                 size = bdrv_getlength(bs);
@@ -6153,7 +6172,12 @@ void bdrv_img_create(const char *filename, const char *fmt,
             }
             bdrv_unref(bs);
         }
-    } /* (backing_file && !(flags & BDRV_O_NO_BACKING)) */
+        /* (backing_file && !(flags & BDRV_O_NO_BACKING)) */
+    } else if (backing_file && !backing_fmt) {
+        warn_report("Deprecated use of unopened backing file without "
+                    "explicit backing format, use of this image requires "
+                    "potentially unsafe format probing");
+    }
 
     if (size == -1) {
         error_setg(errp, "Image creation needs a size parameter");
@@ -6164,6 +6188,7 @@ void bdrv_img_create(const char *filename, const char *fmt,
         printf("Formatting '%s', fmt=%s ", filename, fmt);
         qemu_opts_print(opts, " ");
         puts("");
+        fflush(stdout);
     }
 
     ret = bdrv_create(drv, filename, opts, &local_err);