block: Add support to warn on backing file change without format

For now, this is a mechanical addition; all callers pass false. But
the next patch will use it to improve 'qemu-img rebase -u' when
selecting a backing file with no format.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Message-Id: <20200706203954.341758-10-eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

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 {

block/qcow2.c

@@ -3627,7 +3627,7 @@ qcow2_co_create(BlockdevCreateOptions *create_options, Error **errp)
         }
 
         ret = bdrv_change_backing_file(blk_bs(blk), qcow2_opts->backing_file,
-                                       backing_format);
+                                       backing_format, false);
         if (ret < 0) {
             error_setg_errno(errp, -ret, "Could not assign backing file '%s' "
                              "with format '%s'", qcow2_opts->backing_file,

block/stream.c

@@ -78,7 +78,7 @@ static int stream_prepare(Job *job)
             }
         }
         bdrv_set_backing_hd(bs, base, &local_err);
-        ret = bdrv_change_backing_file(bs, base_id, base_fmt);
+        ret = bdrv_change_backing_file(bs, base_id, base_fmt, false);
         if (local_err) {
             error_report_err(local_err);
             return -EPERM;

blockdev.c

@@ -3416,7 +3416,8 @@ void qmp_change_backing_file(const char *device,
     }
 
     ret = bdrv_change_backing_file(image_bs, backing_file,
-                               image_bs->drv ? image_bs->drv->format_name : "");
+                                   image_bs->drv ? image_bs->drv->format_name : "",
+                                   false);
 
     if (ret < 0) {
         error_setg_errno(errp, -ret, "Could not change backing file to '%s'",

include/block/block.h

@@ -405,8 +405,8 @@ void bdrv_get_geometry(BlockDriverState *bs, uint64_t *nb_sectors_ptr);
 void bdrv_refresh_limits(BlockDriverState *bs, Error **errp);
 int bdrv_commit(BlockDriverState *bs);
 int bdrv_make_empty(BdrvChild *c, Error **errp);
-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);
 void bdrv_register(BlockDriver *bdrv);
 int bdrv_drop_intermediate(BlockDriverState *top, BlockDriverState *base,
                            const char *backing_file_str);

qemu-img.c

@@ -3797,9 +3797,9 @@ static int img_rebase(int argc, char **argv)
      * doesn't change when we switch the backing file.
      */
     if (out_baseimg && *out_baseimg) {
-        ret = bdrv_change_backing_file(bs, out_baseimg, out_basefmt);
+        ret = bdrv_change_backing_file(bs, out_baseimg, out_basefmt, false);
     } else {
-        ret = bdrv_change_backing_file(bs, NULL, NULL);
+        ret = bdrv_change_backing_file(bs, NULL, NULL, false);
     }
 
     if (ret == -ENOSPC) {